MFI Problems
 

About 6 months ago, I decided to get my 72 911T Targa out of mothballs after over 10 yrs of sitting in the garage, tagged and insured (the old "I'll drive it NEXT week" problem).

Well as you all know, turning the key was exciting. I was having engine problems when I parked back then, but with no BBS's etc it was a hard job finding parts, much less getting advice on repairs.

Well to the point. In addition to having to replace the MFI unit (my old 1 had a broken piston) with one I got from Uncle Zak. While the car would now run, it was rough. I now have it running good....BUT.............after driving it for about 15 minutes, it starts to feel like it is running out of gas for 3-5 seconds, then runs great for the next 5 minutes, runs out of gas for 3-5 seconds... and so on. It starts good, sounds good, and accelerates great.
I have done the following but still have the problem:
- checked timing and full tuneup
- replaced ALL the fuel lines
- replaced fuel pump
- cleaned the gas tank and lines thru the frame
- new gas filter
- installed a MSD unit
- installed a CO meter
- installed a temp. gas pressure gauge
- replaced the bar. pressure unit
- replaced the temp. unit
- dry gas in the tank several times
-followed the old "check, measure and adjust" manual for the adjustments

When this happens, the gas pressure gauge still reads 12 lbs and the CO meter is mid gauge. There is little if any response to the gas pedal during these periods. It seems to happen a lot when I start in 1st gear, going uphill, but it happens when I'm running at 60 on flat land.

And, of course, it only happens on the road. Never in the garage where I can see if things are moving properly.

I have come to the conclusion that the problem is in the MFI unit and am ready to give Zak a call for a replacement. But, before going thru all that work of removing, installing, mailing, etc. I'm hoping someone out there knows about this problem and can tell me which screw to turn 1/4 turn CCW to solve it (I can dream, can't I).

Are there any MFI owners in the Maryland area that know these units?

Thanks for any help,
Terry Hastings

Terry,

I see some good checking at work.


I would have been suspicious of the filter screen in the fuel tank but your fuel pressure measurements seem to negate that. Can you read the fuel pressure gauge while driving and experiencing the problem?

This thread has some expansion of CMA:
http://forums.pelicanparts.com/porsche-911-technical-forum/169445-mfi-thermostat-spacers.html

I’m not suggesting the thermostat is involved or not.

Have you measured the fuel flow? Try the “long hose back to the tank” method in one of my posts.

If you think it is a “too lean” situation, a quick method of moving it rich to see is to unscrew the barometric sensor about ¼ turn. That will make it very rich. Use your CO meter to confirm. Don’t drive with the barometric sensor loose, it will unscrew itself and you will loose the shims under the pump.

Best,
Grady

Hi Grady,

I had considered the tank filter, but the engine recovers immediately from these "events".

I have the pressure gauge mounted in the engine comp. On the few times that these events lasted long enough to get the car to the side of the road and set the brake, the pressure was still at 10-12 psi with the engine sputtering.

I replaced the thermostat that came on Zak's unit with the one from my old unit. No change. No change with or without the hose attached to it.
The car starts easy, high on the CO scale and with in 2-3 minutes it's down to the normal range. I have not cleaned either unit, just switched them.

It feels like something is just shutting off the throttle to the MFI and it drops down to idle speed. It seldom stalls out, but runs at idle for a few seconds and then it suddenly responds to the gas pedal like nothing was wrong. (As I am writing this it accures to me that it IS starting to sound like something is being sucked onto the gas tank screen (or blocking the fuel line between the gas pump and the MFI intake???????) under load and then falls away once it is idling for a few seconds. No black smoke when the event ends.

Guess I will drain the tank and check some fuel lines this weekend.

I've installed a 15 psi electric fuel pump on both sides of the NEW stock fuel pump. No change.

The gas flow is good when I turn the pump on with the key. On the road, under load, I don't now how to test it, but I have also installed an inline clear gas filter, without the filter, that is always full during these events. (This would still be true if the line to the MFI was blocked)

Would a low oil level inside the MFI unit cause the rod to hang up? If so, how much oil is suppose to be in the unit?

Thanks
Terry

Terry, you are experiencing fuel starvation.

12 PSI is NOT enough for MFI. The spec on the pump is 1-2 bar, or from 15 to 30 PSI. Also, the flow must be adequate.

This sounds like exactly what I went through with my 72T, as it was sitting for 10 years before I got it in '93. My gas tank was full of rust, the intake screen would gradually get restricted until eventually no flow could be pulled by the fuel pump. Car would die, the restriction would release, car would run good for a couple minutes while the restriction (from the rust particles) built up again. My fix was to pull the tank, get it sealed up, blow out all your lines and clean your screen filters. That dang canister filter ain't cheap either. HTH.

30 psi??????????????
Hi John,

Sorry, for a second there my eyeballs got stuck to the monitor screen.

Everything I have seen says 14+ psi. With the easy driving I do now, I figured 12 was enough. If 15-30 is correct then I going to have to talk to the dealer I got the pump from.

The problem is that the car runs great at all speeds and loads for 15 minutes before it starts acting up, and immediately after.

Can I test it with an adjustable Holly etc electic fuel pump, by bypassing the stock pump? The fuel return lines would still feed back thru the stock pump. I tried using a 15 psi one as an inline boost on both sides of the stock pump to no avail.

I'd hate to spend $400 on a new factory pump to find out that its not the problem.

As you know from above, I have already invested big $$ and major manhours trying to solve this problem. I just want to drive it without having a mechanic and towtruck on standby.

Thanks for the suggestion.
Terry

Sounds like water in gas tank.

K.T.
1973 911 E 2.4 MFI
1965 Devin - 1967 912 Power Plant

I have replaced ALL gas lines and cleaned the tank and tubes thru the frame.
Added dry gas by the quarts (Maybe I should try adding some gas to it??)

Grady has convinced me to check the screenin the tank again.

My brain is numb from the last 2 months on this, so there are no dumb suggestions.

Thanks guys
terry

Is it surging? Did you take the tank out?
Sounds like same problem I had for a year.
Had taken tank out and cleaned but somehow it got polluted gas from gas can left out in rain. Pull tank and dump what's in it.
Swish alcohol around several times to rinse. Blow lines out from front
to rear.
Check rear filter for whiteish looking crap.
Add gallon of clean gas. Start Car.
What About Injectors? Don't blame MFI pump yet.
K.T.
1973 911 E 2.4 MFI
1965 Devin - 1967 912 Power Plant

Terry:

The system working pressure is 1 bar, the relief valve in the pump opens at 2 bar. In any event, 12 psi isn't enough.

Good comments above about making sure the system is free of debris. I used to use an aftermarket pump and I had all kinds of problems-- the pressure was too low, the pump was cavitating, a pre-filter I put in had clogged. The fuel pressure would drop after sustained high speed (like going down the backstraight at Watkins Glen) and the engine would lean out. I would lift off to avoid blowing the engine, and the weak pump would fill up the little plenum at the top of the MFI filter console, and it would run well through the corners, until the next straight. . . frustrating. I subsequently replaced the pump with an in-tank version, set the pressure to 2.5 bar and have had NO problems whatsoever.

First things first: what is the EXACT routing of the supply and return lines from your factory pump? The diagram posted in the factory manual is WRONG, and a corrected version was posted by Grady. Can you post a picture of the connections to your MFI filter console, and the connections on the pump? If you get them backwards, (as specified in the incorrect factory diagram) this could be the source of trouble.

http://forums.pelicanparts.com/showthread.php?s=&threadid=163347&highlight=fuel+t ee

Every item you put in line reduces the flow- that includes the boosting pump and the filter. The flow should be 900-1000 cc's in 30 seconds (that's the "E" flow spec, check the little white book but I'd think the "T" would be the same.)

The pressure is measured in the "loop" between the outlet of the filter (at the bottom of the canister) and the connection on the engine. To verify that you have pressure under load, you can splice in a fuel pressure adapter such as this one, available from Jeg's: part no. 720-2975 and then get a VDO electric sender-- you want one with a 1 to 5 bar scale, and the pressure gauge to match. I got mine from North Hollywood Speedometer. The sender is electric, and you can mount it up front temporarily for the purposes of testing. If you like it it can be permanently installed (which I have done) or you could sell it here.

Anyway, that will allow you to verify that you have the correct pressure under load.

I'm not sure why you are only measuring 12 PSI static. Was it a new factory Bosch pump you installed? Three connections: S (supply) to the outlet fitting with the screen, D (druck) to the pressure inlet on the MFI fuel filter console and R (return) from the pump's bypass. The return line has a tee fitting. One side of the tee is from the R outlet of the pump, with a flow restrictor in-line, the other the restricted line from the MFI filter console. The final connection runs forward to the return fitting on the tank.

When you replaced the lines, did you include the flow restrictor?

Ah hah! Warren posted info about the flow restrictor here. . .

http://forums.pelicanparts.com/showthread.php?s=&threadid=126089&highlight=restri ction+line

Flow restrictor???????????
I may have missed that one.

John, Kevin, Grady,

Keep the suggestions coming.

When I replaced the gas lines, WE did it one line at a time, so I don't remember seeing one. But i think we took the whole Tee unit out as one piece and could not reuse the stock tee because it broke trying to remove the hoses. I will check, but I don't think I kept the old lines. It was replaced with a steel tee. Nothing was placed in line. Is it available from our host, etc? How important is it to solving this problem?

The pump was a rebuilt unit. It was lots cheaper (Ha! Ha!) then a factory new one. I still have the old one, are rebuild kits available? Can I do a temporary bypass of the existing pump, with a high pressure, adjustable electric pump, to see if it solves this problem? I tried an inline one already to no avail.

The in-tank pump idea sounds good. It's always better to push gas then to suck it. How did you handle the return lines?

A NEW TOOL!!!!!!!!!!! There's ALWAYS room for a new tool (or as wife pronouns it, "new toy") . What else is a semi-retired man to spend his money on? (Oh! thats right, I have a Porsche)

Thanks, Terry

pressure
 

i am doing some learning to pep up my 4.3 tbi blazer, and will be MODIFYING THE TBI TO ADJUSTABLE PRESSURE, adding an inexpensive inline gauge recommended , making changes in small increments of pump pressure.(1MM)

We had great results with Kevins car.........HE IS ACTUALLY DRIVING IT NOW!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

TSNAPCRACKLEPOP

Terry,

The factory pump should be fine for your purposes. My in-tank pump is actually inside a fuel cell on my race car, might be difficult to install in a stock tank without signifcant modifications, e.g. you could fit it through the sender hole, but would need a way to anchor it to the floor of the tank. The grisly details are here: http://forums.pelicanparts.com/showthread.php?s=&threadid=115773&highlight=fuel+s tarvation+watkins+glen
The return line is as stock: out of the cell, back to the MFI filter console, over to the engine, back to the console, back to the trunk where there's an adjustable fuel pressure regualtor and the sender for the pressure gauge, then into the tank.


Since the restrictor is in the "R" outlet FROM the pump, I wouldn't think it would be that much of an influence on your problem-- that's all going on in the return circuit.

So here's what I would do. First, make absolutely certain you have the hoses hooked up in accordance with the revised diagram.

What kind of fuel line did you replace the old hoses with? What diameter?

I agree with Grady about checking the screen in the tank. I'd go it one further and CAREFULLY drain the tank, flush it with water and carefully remove the sender and have a look inside with a flashlight. Years of rust, accumulated crud, etc., could have clogged a line, killed your pump, obstructed the silver can filter, etc. Only way to tell is to start at the source.


Once all that's checked then consider the pump. If you use a different aftermarket pump you will need an adjustable fuel pressure regulator also: install this in the return line after the MFI filter console, on the way back to the tank.

Good luck!

Well the weekend is over, so here is an update on my MFI problem.

-The tank and lines are clean (I steamed cleaned it 4 months ago, so no surprice)
- All hoses correct
- Fuel pressure- I installed a calibrated gauge and am getting a steady 14.2 psi at idle thru 4k. I have all new 5/16 heavy duty fuel hose. I do not have the inline fuel restrictor that John mentioned. I added Lucus fuel treatment again.
I took it on the expressway 60-80 mph for 20 miles and ran fine. Turned around to come back and "IT" started again??????????????20 miles, a 1/4 of a mile at a time. I kept looking for a cliff, but my brother won't get out of the car....
Still 14 psi on gauge, while sitting on the side of the road.
Key off, key on and let pump run 5-10 sec. start car (always started instantly)and go again. My brother (he took the wheel because he feared for the Porsche's life) was able to make it back the last 5 miles without stopping, by keeping it in high gear (at 60-65 mph) most of the time with low RPMs.
Got it home in the garage and it ran great (#@%.. WERE'S MY BAT...$#@.. A GUN WILL DO..$%). Still no smoke out the pipes during the drive OR on the many restarts.

Once home I used vice-grips to restrict the return gas line down to various diameters. Pressure ranged from 14-50 psi (50 with it closed) BUT the engine started to run better with some restriction??????

So, what does all this mean? I don't know!!!!!!!!!

All evidence points to the MFI unit. It is the only thing that has not been toyed with other than minor adjustments. That is the only thing that can make the throttle cable feel useless. If the rack is hanging up and not returning to the idle position, then the car will start to starve for fuel. Then when it pops loose again the car will run fine. This would explain why the pedel feels loose and there is no black smoke on the restart or recovery, since no gas was being pumped into the system despite the pedal being stomped on repeatly.

That MFI unit was a used one I got from UncleZak when a piston in mine broke.

Everything else has been replaced.
-new MSD, wires, plugs, cap, rotor, fuel pump, fuel hoses, filters.

Does this make any since to any of you MFI guys. I'ld hate to call Zak about a replacement and waste all that time, money and postage to replace the unit if its not the problem.

I am really tempted to replace the MFI with a 4-bld Holly. If I had access to a spare or junk 2.4/2.7 engine to use for a pattern, I would have already made a manifold for it.

Thanks,
Terry Hastings

STOP! Do not even THINK such things! You will offend the Great Spirit that watches over MFI. You must IMMEDIATELY close your eyes and meditate on the thought, "114.0 mm +/- .2mm."

All right, you still have a lean problem.

If the MFI pump were broken, or way out of calibration, it would exhibit the lean-running symptoms all the time. So one of two things is happening, either:

1) Something is happening to the fuel system after a few miles. The aluminum housing that the MFI fuel filter bolts to actually has a small cavity in the top that holds fuel. When you turn the key before start, the fuel pump fills this cavity with gas. You start the car and off you go, but the fuel pump isn't keeping the cavity all the way full, so it slowly begins to drain. Eventually, the cavity is empty, and the fuel pump is doing all it can to supply the engine, but eventually the engine's need is greater than the fuel pump can supply, and you get the lean stammer.

It has happened to me before. I put a fuel cell in and didn't have the pressure right. Drive a few miles, lean out. Stop car (and unbeknownst to me, the plenum re-fills) pop engine lid, inspect. Find nothing. Car re-starts with spritz of gas from the cold-start system, runs OK for next few miles until it does it again.

Sound familiar?

I bet you a Warsteiner that it's a fuel supply problem, not an MFI problem.

Did you measure the FLOW rate out of the fuel filter? Pull off the hose that comes out the bottom of the silver can. Put a long piece of hose on the fitting, so it extends outside the engine compartment. Get yourself a 1-gallon jug. The flow rate should be 900-1000cc in 30 seconds, which means that the gallon jug should be filled in no more than 4 minutes and 12 seconds. This will also give you an indication of the fuel quality.

Do NOT set yourself on fire during this process like that Bhuddist monk Thic Quang Duc, whos photo appeared on the Rage Against the Machine Album.

or

2) The MFI thermostat is heating up and knocking the mixture all out of whack. It may be that when cold, the mixture is just rich enough to continue running, and when the thermostat moves the control rod, it's leaning it out enough to give you these symptoms.

See the threads here on MFI thermostat for help on that one.

Good luck!

So. that is how it works. I had my old unit out on the bench trying to figure out how it maintain a uniform pressure to the rack.
So it uses the head pressure created by the weight of the gas in the storage. The pump only resupplies that stored gas and supplies the pressure to keep the gas moving into the piston bores. This pressure would not affect the gas volume. OK, I learned something new about the MFI system. That means the volume (flow) of gas to the MFI is more important then the pressure. The 14.2 psi is only a computed requirement to supply the min. amount of gas thru a fixed dia. hose, to keep the storage area full and thus the pressure uniform at all speeds of the rack movement.

I will check the flow tonight if my wife does not mind (today is our anniversity. Let's see ...1 doz roses and NICE card = flow test.....possible..... she hasn't said anything so maybe she forgot.......yeh, fat chance..).

Do you know of anyone (thru PCA etc) familiar with the MFI in the Balto-Wash. area?

Terry

I am facinated by this thread because I went through the exact same symptoms last year with my 72T. As with Zotman72 above, my problem was eventually traced down to crap in the fuel tank (after I had replaced the fuel pump, fuel filter, etc.). Although I think that everyone who is responding above is giving you great knowledge (and they know a lot more than I do about these things) I just have to throw out a second to Zotman72's diagnosis. Your symptoms are identical to what happened with my car. It would start fine and drive for up to 10 minutes, then sputter to a stop. Leave it for a couple minutes it would start fine. I know you said something about cleaning the gas tank, but are you sure its ok?

Perhaps your readings of the pressure gauge rule this out, I don't know, but I felt compelled to share my experience in case it helps...

Terry, guys,

Sorry I haven’t been on since last Thursday.
I spent Friday afternoon with Kurt V in Lohman, MO
and yesterday (Sunday) at George Maybee’s cool Porsche Tractor
and more event north of Denver thanks to KobaltBlau.

It seems some didn’t read the thread listed above.
At the end of page 1, I listed some i
mportant additions/extensions to CMA.
On page 3 I listed the factory technique for fuel flow measurement
and some important improvements that I use. Again, here is the thread:
http://forums.pelicanparts.com/showthread.php?s=&threadid=169445

I agree with John.
Here is the correct version of the plumbing and the WRONG version.
The wrong one was published in Christophorus and the Factory Workshop Manual.
Later Workshop Manual up-dates had the correct version.
The correct version is in Bruce Anderson’s book, p. 84, 2nd Ed.
"
http://forums.pelicanparts.com/uploa...1090862643.jpg "
(C) 1969 Dr. Ing. h.c. F. Porsche A.G.
"
http://forums.pelicanparts.com/uploa...1090862816.jpg "
(C) 1969 Dr. Ing. h.c. F. Porsche A.G.

The correct place to measure the fuel pressure is at the
fitting on the fuel filter console toward the rear of the car.
This is the last, easily accessible location prior to the restrictor.
See page 3 of the thread listed above.


I disagree with Warren.

(probably the only time or my lame memory is failing.)

The restrictor is in the center fitting (toward the engine) on the fuel filter console.
This is #9 in the illustration below and is referred to as
“Overflow valve – Uberstromventil” P/N 901.110.915.00.
The hose-Tee assembly at the rear (with rear pump) is referred to as
“Return line – Rucklaufleitung” P/N 911.356.064.01.
http://forums.pelicanparts.com/uploa...1090862988.jpg
"
http://forums.pelicanparts.com/uploa...1090863049.jpg "
(C) 1969 Dr. Ing. h.c. F. Porsche A.G.
Warren, could the restriction you cite be someone’s “fix” or a Factory service mod?
That restriction is in series with the Overflow valve and would tend to raise the fuel pressure.
If the fuel pressure tested slightly low with a new filter, could
someone have added that restriction down the return hose?
I think it would be uncharacteristic of Factory design
to have the restriction at the Tee.


Terry,
When you cleaned the fuel tank, did you remove the supply
fitting from the tank?

Did you extend the fuel pressure gauge hose so you can
monitor the pressure while driving?

Use the fuel flow test back to the gas tank filler.
This allows you to run the pump for an appropriately long time.

The reason for maintaining 1 bar pressure in the MFI mechanical pump is to
prevent intake cavitation when the mechanical piston draws in fuel.
In that situation, low pressure and hot fuel can flash into vapor and cause a
lean situation and poor running.
Do you have an IR thermometer to measure the temperature of the fuel filter and MFI pump?
I suppose there is a remote possibility of something causing the pump to run
exceedingly hot presenting these symptoms.
After some driving, the body (lower) part of the MFI pump should be close to
the temperature of the oil in the tank and the temperature of the fuel pump (upper)
part should be not much above that of the fuel filter.

Eventually you are going to want a new Bosch electric rotary pump.
After all your effort, it is worth the cost for reliability

Best,
Grady

Grady,

The restrictor I found in the 'Tee' hose assembly wasn't a modification, it was inserted in the hose before the hose was fitted to the 'Tee' and crimped with the Cadmium-plated, steel crimp ring ... so it came that way from whatever supplier provided the hose. My feeling all along has been that the restrictor was intended to raise the pressure in the loop that includes fuel filter console and plenum chamber in the MFI pump ... before flowing back to the tank.

This is one of those situations where spot measurements and testing don't give a complete enough picture ... because the pressure reading that would explain or head us towards an answer ... needs to be taken just before, and as the failure is occuring! It begs for a data acquisition system that is logging fuel pressure at the console several times per minute for a ten-minute+ run!

In a pinch ... a data-logging DMM to a laptop could be rigged to monitor an oil-pressure sender plumbed to the console!

Warren,

I agree. In the absence of a fast responding data acquisition system, I propose a high quality fuel pressure gauge at the end of a long hose so it can be monitored from the cockpit. The hose should be fairly rigid and be able to be bleed of any air/vapor bubbles to give good response time.

Could there be two restrictors, one (Overflow Valve) at the filter console return and one (restrictor) in the Tee? Again, could the restrictor at the Tee be service fix? How many cases have you seen with this restrictor?

Could this restrictor at the Tee be only for the rear fuel pump location? When the pump was in the front there was only the plastic tee to combine the return and safety pressure relief valve in the electric pump.

Best,
Grady

Hi Grady,

First, I put the fuel gauge in the line running from the BOTTOM of the filter to the MFI. I had to use a mechanical gauge, therefore it was mounted in the engine compartment. I am trying to locate an electric gauge with a sending unit. Does this location matter? The pump gave a 14.2 psi throughout the testing.

Second, I orginally removed the tank from the car and had it steam cleaned, dried out and rinsed with drygas before being drained and re-installed in the car the next day.All metal gas lines were rodded out with a wire brush and compressed air cleaned.

The filter was replaced in the fuel filter, but nothing was removed from the filter unit.

I had hoped to do a flow test tonight, from the BOTTOM hose, but may have to wait until tomorrow if the side per your attached thread is suppose to be used.
Yes to the IR thermo.

Terry

Terry,

I think the reason Porsche tests fuel pressure where they recommend is because it is after the MFI pump. If the pressure there is within spec, you know the MFI mechanical piston pumps are seeing proper inlet pressure.

Given all your trials and tribulations, I think it very important to accurately measure the fuel pressure after the MFI pump and while driving.

You still haven’t answered my question about the fuel tank filter screen fitting. Did you remove it for cleaning?

Best,
Grady

Grady,
I can only say that the screen was suppose to be removed and cleaned. I had one of the shop guys helping me, remove everything and clean them. He assures me everything was cleaned.

It makes since to take the psi reading there to ck the min. psi thru the system. I will relocate the gauge. If I don't find an electric guage, I will have to try the long hose method, but am concerned about the affect of air in the system and hose flex.

But, the strange part is that during one of these events there is no resistance to the gas pedal and the car rpms drop to and remains at idle. Its as if the rack is getting hung up in the forward position and the spring is to weak to force it back. Could one of the pistons in the unit be hanging up? This unit, when received by me, had the idle-speed turned and locked in the rich position. I had to take the front off to break it loose, so that means the P.O. may have been having problems with it.
If the pressure cks out, I may have to call Zak up about a replacement unit.

Thanks guys
Terry

Terry,

If the rack is hung up in the forward position and you close the throttles, that is a rich condition.

I don't understand your "there is no resistance to the gas pedal and the car rpms drop to and remains at idle." Is there something mechanical or is there just no power?

Best,
Grady

Grady,

There is no responce to the gas pedal when this happens. You sit there pounding on the pedal and watch your mph drop, then 5-10 seconds later it starts to respond again. If it was anything but the fuel system, then black smoke should pour out of the pipes, from the pedal stomping, on the return to normal conditions. None. It seldom stalls out, but drops down to a rough idle. It starts up immediately, and runs great.

I can not reproduce this problem in the garage. It only happens on the road. Maybe the car just likes it 's life in the garage. After all its spent the last 12 yrs there without having to haul me around.

The only good thing is that it use to happen within 200 yds of starting off, now it's 20 miles. But once it starts, it is on and off every 1/2 mile, until I get to the garage. However this last time we got it back the last 5 miles by keeping the car in high gear and the rpms low. We were able to run at 60-65 mph. During this 5 mile run, the problem occured 5 + times but we were able to drive thru it.

Because of various problems, I have driven this car a total of only 50-70 miles since putting it back on the road, so maybe it is starting to loosen up and will solve this problem on its own.

I did not have this type problem with my original MFI unit, but had to replace it because #6 piston seized up in in it and broke. I replaced it with a used unit from Uncle Zak. That is when this problem started, although there were several other problems that was masking it. The only thing that has NOT been replaced in the fuel/ignition systems since then is this used MFI unit. All of these changes have not had any affect on this problem.

I just wish that someone locally had a "good" MFI unit that I could install to test it.

Well, I did not get to work on the car last night, so the flow test was not done. So it may have to wait until the weekend now.

I am still looking for an electric fuel gauge. Any ideas on who makes them?

Thanks
Terry

Terry,
I'm having the same symptoms with my 71S motor and MFI. It began shortly after my fuel tank was damaged due to me hitting a chunk of wood. The car starts fine and drives along then all of sudden it feels like you’ve taken your foot off the gas, but you haven’t.

I replaced the $57 filter and it helped but it didn’t fix it, boy was it dirty. I finally received my repro tank from Tweeks 4 months after I ordered it $300.00 including shipping. I also bought a new fuel cap (the intake bolt with the filter) $10 our host. My old tank has visible rust inside so I’m hoping this solves the problem.

Unfortunately, I broke my arm playing soccer before I got the chance to put the new tank back in (two weeks ago). I am following your thread with great interest.

I live about 3hr.s from Baltimore. Good to know those around the area.

Terry,

You really want to change out that MFI pump, don't you!

Most of the time it's not the MFI pump at all that is causing the problem. The fact that your car runs well for a while is a strong indication that the problem lies in the fuel delivery system before the pump.

You need an electric fuel pressure gauge.

First, you need an inline fuel pressure gauge adapter with 1/8" NPT pipe thread.

Summit racing sells them, or Jeg's. http://store.summitracing.com/partdetail.asp?part=SUM-G1710&N=120 301668

Next, you need a VDO pressure sender, with a pressure range of 1 to 5 bar. Available from North Hollywood speedometer, www.nhspeedometer.com, or from Summit, http://store.summitracing.com/partdetail.asp?part=VDO-360003D&N=120 301974

This sender measures from 0 to 5 bar with a resistance range of 10-180 ohms.

Next, you need a pressure gauge to mount inside the car. You can use an OIL pressure gauge if it's only temporary (same scale and resistance range, it just says "Oil.") from Summit, http://store.summitracing.com/partdetail.asp?part=VDO-350040D&N=120 400267 301969

OR you can get the same gauge from North Hollywood Speedometer that just says "BAR" on it, that's what I have.

Cut the hose from the MFI pump back to the console and install the pressure adapter with a couple of stainless steel clamps. Screw the pressure sender into the adapter, use some thread sealant. You will need to run two wires to the sender- one, a ground wire, which can be attached to the body of the sender with a large stainless clamp, and the other, a wire running forward to the gauge. Follow the instructions included with the gauge to hook it up, and you'll be all set.

Good luck!

Terry,

The more you explain the symptoms, the more likely it is fuel delivery IMHO.

With no fuel, your pushing on the throttle just gives the engine pure air.
Normally when you push on the “go” pedal, the 911 goes.
With no fuel or ignition it feels like the pedal isn’t
connected to anything that goes.

If it were an ignition problem then, when the problem goes
away, all that unburned fuel must do something.
There would be backfires, black smoke, or something.


Right now it is most important to diagnose the problem.
You don’t want to do a bunch of things and have the problem just disappear.
You would not know what it was or if it will suddenly return
at the most inopportune moment.

I think the first thing to do is measure the fuel flow.
Unplug the CDI first.
You will need a short piece of fuel pipe and something
to plug the fitting at the console.
Use a long hose forward to the fuel filler.
Get a large funnel to put in the filler neck.
Find some sort of calibrated rigid container.
A metal 5-gal gas can would work well.
You can calibrate it by volume or by weight, using bathroom scales.
However you do it, you want to convert to ml/min.
Start the flow with the hose in the funnel.
Switch the hose to the calibrated container and simultaneously start your stopwatch timer.
When the container is close to full, switch the hose back to the funnel and stop the timer.
Let the pump continue to flow fuel.
Record the quantity in the container and return the fuel to the gas tank.
Repeat several times over a 45+ minute period.
The only better way to do this test is to install a flow
meter instrument in the return circuit and go drive the 911 to failure.

If you find a change in flow during these measurements, go find and fix the problem.
If the flow is consistent and within spec; go to Plan B.


Plan B. Measure the fuel pressure while driving.
You will need a connection to the fitting or hose at the fuel filter console.
The fitting closest to the rear of the car is correct.
Since you have all new fuel hose, it will be easy to pull the hose off the banjo fitting,
install a short length of hose and a Tee.
The Tee should connect to a long hose to a gauge in the cockpit for
your helper to monitor and record the pressure measurements. You will need to bleed the air from this line.
Remember, put good fuel clamps on all connections.
Be very careful – even a little gasoline vapor
is exceedingly dangerous.
Go out and drive the 911 to failure.
What were the fuel pressure measurements?
If the fuel pressure went out of spec, fix the problem.
If the fuel pressure remained consistent and in spec; go to Plan C.

Plan C. We will figure that out if necessary.
This should keep you busy this weekend.

Here is one of the gauges I have used.
It is 12 cm diameter across the calibrations.

http://forums.pelicanparts.com/uploa...1090942779.jpg


Here are the Factory gauges P233b.



http://forums.pelicanparts.com/uploa...1090942841.jpg
The upper gauge has a long hose I made (fits in the gap)
to extend it into the cockpit.
The center gauge is as original.

For you DIY types, you can drill and tap the banjo bolt at the console
and use CIS plumbing to a gauge or sensor.
Do not plumb fuel into the cockpit for normal use – too dangerous.

Good luck!

Best,
Grady

John, Grady,
John, I ordered the electric gauge etc from Summit. The "BAR" one is no longer available, so I will be reading the "OIL" pressure. Thanks for the info. One question, is the stock banjo bolt long enough to use with the fitting. If not where can I get that?

Grady, I am picking up a new 5 gal gas can (which I have plenty of uses for) and will fill it up at a station one gallon at a time and mark each gallon level. My wife will use that much cutting the grass up at our PA. farm this weekend, so it will not be wasted. (She loves cutting our 6 acre lawn on her tractor, so she sends me down to the creek trout fishing. Who am I to deprive her of something she loves. It's a hard life but someone has to do it.)

I am hoping to run the flow test tonight, or tomorrow night, before leaving for my farm in PA. for the weekend. But, it has been raining in the evenings and we have a strict rule against using or working with open containers of gas inside the our garage, so it is an outside job.

So, hopefully, if the fish aren't bitting and I get back to Balto. early Sunday, and the weather allows, the flow test will be performed and if really lucky, the gauge will have arrived and get installed. The results will be posted for your review.........and Grady I HOPE you are right. Replacing a $400 fuel pump would be the easy way out.

Terry

Terry,

You don't use the banjo bolt with the fitting.

http://forums.pelicanparts.com/uploa...1091025909.jpg

The hole in the side of the fitting is for the sender. The two barb connections are for the fuel hose. Basically you cut the hose and put one end on each of the fitting, secured with stainless clamps.

You will have to find a way to support the sender inside the engine compartment so it doesn't bang around when you drive.

I recommend that this be a temporary setup until you fix the problem. When you have it fixed, replace the hose.

Also, you sound like a safety-conscious person with your policy of not working with fuel in a closed area. I'll second that by reminding you to be sure the connections are leak-free when you set out to test, we don't want your car burning to the ground. Always carry a fire extinguisher in the car with you and have one handy in the garage when working on the fuel system.

Good luck!

John,
You're right. I looked but did not see. Grady, had my mind dreaming from looking at his neat setups. It's not hard to tell an old racer, just look in his tool box.

I have always consider gas the most dangerous item around a garage and car. It's not a question of "if" it will start a fire, but "when" and "how often". If you are lax around it or at cleaning it up, sooner or later that grinder spark, welder ember or air compressor motor kicking in will make you pay.

Terry

Terry,

What is the pressure range of your new “OIL” fuel pressure gauge?

That long banjo bolt comes with the P-tool. I’m sure it is some standard DIN fitting but I don’t know where to find one. No, the standard banjo bolt won’t fit through two banjos.

Is there a salvage yard with Porsche, MB, VW, Audi, and BMW near you? You might find one in a car.

The other solution is to drill and tap a standard MFI banjo bolt to connect to your new gauge sending unit.


With some shopping, you might find a steel gas can with calibrations. You can also calibrate it just using water – much safer. Remember to dry the can.
If you have a good digital bathroom scales (say 0.1# resolution), you can just record the net weight. Using the density of gasoline (0.68? g/ml), you can calculate the volume of the gas with greater precision.


Good for your “working with gasoline” policy. Gas is probably the most dangerous substance we normally come in contact with. Too many treat it with a caviler attitude – familiarity breeds contempt – with disastrous results.




The screen filter fitting in the gas tank is normally difficult to remove. It requires a large (27 mm?) box wrench in order to not round the hex. I have tried to unscrew it with a tank out of the car and it takes three strong men to hold the tank. It is normally only possible when the tank is firmly mounted in the car and the 911 not likely to tip off stands or a lift from the brute force necessary. Yes, you can use a deep 6-point impact wrench when the tank is out of the car (a normal impact socket will mangle the outlet pipe).

Terry, I’m going to speculate here. Could your helper have not removed the screen but just have “cleaned” it from the fuel gauge sender hole? If that screen wasn’t cleaned properly, there might be the problem.

The original screens on early cars were an exceedingly fine mesh brass screen. Rust particles can slowly accumulate on the wires of the screen and reduce its capacity to flow fuel. While driving, the sloshing of the fuel in the tank will tend to suspend in the gas some of the sediment from the bottom of the tank. The flow of the fuel through the restricted screen will temporarily captivate that sediment against the screen and totally restrict the fuel flow. When you turn off the fuel pump, that sediment is no longer held in place by the suction of the pump and falls away. You can start the car and proceed until the process repeats itself.
The screen can appear normal outline of the wire mesh while being completely restricted.

So, what is proper cleaning?
First, the fitting must be out of the tank. Aerosol carb cleaner, a tooth brush and compressed air are necessary to get it properly clean. It takes some mechanical scrubbing with the right solvent.

There are later replacement screen fittings that are plastic mesh and pass larger particles. These are less susceptible to clogging but pass more crud to the fuel pump to be caught by the (expensive) MFI fuel filter. With proper maintenance, the original brass wire screen is best.


Best,
Grady

What's the deal with the pedal feel. Does the pedal physically move differently with the problem. Can you check the position of the throttle linkage on the engine with the pedal pushed down? Does the pedal stick? I think the pedal should feel the same all the time. It's only moving the throtte plates and the linkage on the injector pump.

-Andy

It seems like I may have a "hot" fuel system issue too. When driving home today, My car was ok as I sat in stop and go traffic for a long time, but I know the heat was just building. Outside temps ~82F. Then, I hit the highway, and uh-oh, power was dropping. I floored it, but it just stumbled. I managed to get to the right shoulder, and the car died. It wouldn't restart, so I waited 5 minutes. The car restarted, and I drove all the way home without problems. I could lay into it, and it made good power. The oil temp went from ~215 down to 195 during this time, and the engine compartment wasn't too hot after sitting.

I rebuilt my fuel lines with my engine rebuild, and I looked for this restrictor, but did not see it. I'm sure my lines were original. Can you buy this separately?? If not, is it possible to put an adjustable in-line relief valve on the return so pressure is maintained. If so, anyone know who sells one??

Thanks in advance!

Oh yeah...what does this restrictor look like??? Is it plastic?

Jay,

Guess what I suspect?

Best,
Grady

I need the restrictor in the return line??? I guess I'm not sure what you suspect, unless you're saying I answered my own question?

Can I buy this anywhere, or should I get creative and try making one? Maybe some tubing pressed into the hose I.D. to reduce flow? I probably can't get 1.5mm I.D. tube with the O.D. to fit inside the hose, but if the length is longer this should get close. Yes...no??

Also, I did have my tank screen/fitting out, cleaned it, scrubbed it, and soaked in in Berryman's Chemtool for about one week, and then cleaned and scrubbed again. It was horribly clogged when I removed it.

I bought my car non-running, so I don't know if this problem existed before.

thanks in advance for any help--

I read earlier posts again, and have one more bit of info. My car had some hoses removed at the filter. Probably a first thought into removing the engine by the P.O. (engine was damaged when I bought it).

Maybe there was a restrictor here? I do remember seeing a black plastic plug with a hole it it sticking out of a fuel line. I thought someone was just plugging this line to prevent fuel spillage (now I think, why would they plug it with something that has a hole in it). Hmmm, I might still have that.

Grady,
Last night I decided to run my engine until it had an "Event" under controlled conditions at the garage. I had noticed that I had been able to predict within a matter of a few miles, and by the temp gauge position, of when the problem would start happening. Dirt in the tank is never that predictable. I started with a 15 mile run at 70 mph until it started acting up again. I then went back to the garage and let it run at 4000 rpm, watching the pressure gauge, for another 20 minutes. The pressure was at 14 psi fine for the first 10 minutes,........THEN it dropped to 8-10 psi for 10 seconds, back to 14 for 2-3 minutes, down to 8-9, back to 12, down to 8, back to 11, never making it back to 14 until I dropped it back to idle. Take it back up to 4000 and in a few minutes it started all over again. The engine started sputtering during the 8-10 ranges. The sound of the fuel pump never changed during this test.

I think my fuel pump has a heat problem. This time I'm going to replace my new rebuilt fuel pump with a new factory pump.

I hope this solves the problem. At the least, it will eliminate one possible sourse. I will keep you updated.

So, once again this BBS has provided direction to one who is lost in the woods.

I give my humble thanks to you and John for insisting I test the fuel pump that I KNEW was ok.

When I started this thread, I felt a little stupid. I didn't even think I would get any replies since this subject has been covered so many times, in so many ways, on this BBS. Twenty yrs ago, there was no source of info available to the P-Car owner except for the dealer (at big $$$) or the local PCA members, that normally only had limited knowledge. How many times in the 80's, while reading some old guy's articles (Grady, does it sound familiar) in the old PCA "Fixin' Up Der Porsche" (or something like that) that I wished I could call the authors for help. Now, a few keystrokes and there they, and many others, are.

That's why, after 12 yrs, I am driving my 72 911T again.

I hope this thread continues to cover MFI problems, because here on the east coast, there is NO local source for MFI work, info, repairs and/or parts. The only way that gap is going to be filled, since there is no reason to believe some "expert" will suddenly move here, is by the MFI owners pooling their knowledge and hands on experiences. The written and spoken word are great, but when it comes to turning the wrench on a NEW project, EVERYONE gets that lead weight feeling in their stomach. I have well over 20 yrs of building and racing cars, a fully equipped garage, and I got that feeling when it came time to replace my MFI unit.

Thanks again,

Terry Hastings