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