MFI Problems

[thastings]

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

[Grady Clay]

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:
MFI thermostat spacers

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

[thastings]

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

[304065]

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.

[zotman72]

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.

[thastings]

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

[Kevin Taylor]

Sounds like water in gas tank.

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

[thastings]

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

[Kevin Taylor]

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

[304065]

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.

MFI Fuel Starvation...Out of ideas.....help please

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?

[304065]

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

fuel line pics needed

[thastings]

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

[TSNAPCRACKLEPOP]

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

[304065]

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: Fuel Starvation at 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!

[thastings]

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

[304065]

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.

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!

[thastings]

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

[Andy Ring]

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...

[Grady Clay]

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:
MFI thermostat spacers

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.
"
"
(C) 1969 Dr. Ing. h.c. F. Porsche A.G.
"
"
(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.

"
"
(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

[Early_S_Man]

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!