72t mfi stalls, won't start after warm up

[joe_72T]

I have a 2.4 MFI car and drive it only in summer months that I have owned for 3 years.

Last fall my car developed a fuel pump leak so I replaced it with another one that had the same part # and looked identical. I then discovered some fuel line leaks near the pump so I replaced those as well. I then stored the car over the winter in my insulated garage with a full tank of gas with fuel stabilizer and a trickle charger (like I do every year)

Yesterday I started the car for the first time this year and she fired up right away with a bit of throttle. After warm up I went for a ride and after about 10 minutes it started to sputter and bog down and quickly stalled. It would not start so I had to get it towed back home (10 kms) This morning she started up the same way and sounded fine with no problems reving freely up to any rpm. Then after about 10-15 minutes she stalled. I started her back up and again and had same problem with a bit of backfiring, lack of power and finally she stalled. Not able to restart.

I will admit that I am not that mechanically inclined but try to do as much as I can myself as I enjoy tinkering with the car. Also, I don't have any special tools to measure fuel pressure etc. I would appreciate any help the forum could offer.

Thank you in advance. Joe.

[RSTarga]

sound like your warm up regulator. Send your pump off to gus at pacific injection to get rebuilt along with your injectors. You don't want to run the car in this super rich mode or you'll wash down the cylinders.

[john walker's workshop]

probably leaned out too much. set it up on a gas analyzer. they like to run on the rich side, like 6%.

[Grady Clay]

Joe,

Most likely the screen filter fitting in the fuel tank is restricted.
This is a very fine mesh screen.
The circulating fuel carries more sediment and finally restricts the screen.
After a bit of time off (particularly when towed), some of the restriction falls off and it runs for a bit again.

Drain the fuel tank.
Unscrew the fitting that supplys fuel to the pump (the small fitting at the drain is the return).
It is best to remove the tank from the car and wash it out throughly.

You will need the alu sealing washers at the fittings and a new gasket for the fuel level sender.
Be prepared to replace more hoses.
Be sure to use the Norma-Schellin pinch-type hose clamps.

If the screen filter fitting is damaged, replace it with the OEM brass type and not the VW-914 plastic type.

There is some common (Home Depot) type material to re-seal the tank to the chassis.

Best,
Grady

[Flieger]

I would guess the in-fuel-tank screen filter is clogged with little scales of rust caused by moisture in the system.

[kramer747]

the worst thing you can do with these old gals is let them sit. Whenever you get a decent day in the winter, take it out and drive!

[Joe Bob]

Hot starting issues are usually tied to a faulty warm up circuit or fuel pressure that drops and is too warm for cold start assistance.

Fuel delivery issues like fuel pumps and dirty screens and filters tend to be symptomatic with balky acceleration. There are more than a few books on the subject, articles on line and such.

An afternoon with an exhaust analyzer can be a good thing. As mentioned, MFI likes to be rich. One of the reasons it didn't last long in the smog era. However, once MFI is set up right, it screams.

[Porboynz]

I presume the car ran well before you layed it up for the winter, after you had replaced the pump and hoses? It does rather sound like fuel starvation. Search the forum for the correct fuel line layout then you can try removing the bypass fuel line from the fuel filter and checking for a good flow into a suitable container once the engine has stalled. Same with the fuel flow to and from the inj pump. (Fire extinguisher to hand, a quick few seconds of ignition on should show a good flow.) That might help you track down where there is a possible fuel blockage. With the fault so easily repeated it should not take you long to logic your way through it. There is another bypass line down on the fuel pump. The cold start circuit can also give problems, causing the engine to run way too rich after it has warmed up. I found this by accident when I once disconnected the lhs heater hose, forgetting it also supplied hot air to shut off the cold start setting on the injection pump. I used a lot of fuel that day, it was like driving a carbed engine with the choke on, it would run OK with a lot of throttle but would not idle.

[Eddyvdw]

Very, very likely to be something with fuel delivery. Follow Grady's advice. I had exactly the same. Next time it stalls simply switch of the engine and wait for 15 minutes. It will start and drive nicely for another 10 minutes until the filter clogs again (and again and again).

Somebody told me that the fuel tank was designed in a time lead was added to the fuel (the time I grew up next to a high traffic road...). The lead prevented corrosion. When you car started to use unleaded fuel (late 80's?), the corrosion of the tank started....

Remove the tank, make the inside rust free (lot's of advice on this forum although I wish I had a radiator shop in my neighborhood...), seal the inside, replace the in-tank filer, re-install the tank (don't forget the washers) and off you go!

As an extra, a picture of the filter in my tank....

[Grady Clay]

Good image.

This filter screen (red circle) is almost completely blocked with rust particles.





In normal operation fuel is ‘sucked’ through the screen by the electric pump.
It then circulates through the MFI filter and MFI pump.
The fuel returns to the tank at the drain fitting (teal arrows).

Since the return fitting is at the bottom of the tank, it ‘kicks up’ all the sediment and puts most suspended in the fuel.
As the contaminated fuel goes through the screen, particles further restrict the flow.
Eventually there isn’t enough fuel to run the engine, even at idle.

Once you turn off the electric pump, there is no longer return fuel to ‘kick up’ sediment.
The rust debris falls out of suspenstion in the fuel.
Some of the flow restricting debris ‘falls off’ the screen as there is no longer ‘suction’.
After a bit, there is enough fuel flow to run the engine again.

Needless to say, this condition tends to progress to worse restriction.

There is a fuel-flow test specified in the MFI CMA process.
This test confirms proper flow through both the screen filter and the much bigger/finer MFI filter.

The flow test is performed at the fitting on the fuel filter concole that is to the rear of the car.
This is the same fitting where you test fuel pressure. (Pressure spec: 0.8 ± 0.2 bar, 11.8 ± 0.3 psi)
The specified minimum flow is 900 to 1000 cc in 30 seconds.

All of this is covered in the Porsche MFI Check, Measure and Adjust (CMA) proceedure.
You will find link to .pdf copy here:
Ultimate MFI resources thread



Most have their clean fuel tank coated internally.
Be sure to remove all the fittings first.
You may need to clean the tank fitting threads afterwards.
(Internally coating CIS tanks causes problems as there is a lot of flow components inside the CIS tank.)

This is a good opportunity to replace all fuel hoses.


Best,
Grady

[Jeff Higgins]

One other thing to check, assuming it's not the screen in the tank - the tank vent. Mine was behaving this way last year, and it turned out that the little metal tube that runs through the left front fender had rusted shut. This has a line on the inside of the trunk that goes to the cannisters that are a part of the evaporative emissions system, then another line to a cannister behind the left headlight.

If this vent system is clogged, the car will run great until it build vacuum inside the gas tank to the point where the electric fuel pump can no longer draw fuel. With a full tank, it will usually run for ten to fifteen minutes before this happens. As the gas level in the tank drops, it will run longer and longer, since it has a greater air volume in it.

Try running it until this happend, then remove the gas cap and re-start it. That will tell you if it's a clogged vent system.

[joe_72T]

Thank you, Thank you and another Thank you to all who are trying to help!

I will take all this info into consideration and try and gedder going soon. I spoke to a mechanic today and he said it may very well be the fuel pump relay. Now that would be too easy wouldn't it?

Joe

[Porboynz]

No fuel relay to blame on a '72 sorry, plus you can hear that MFI fuel pump whirring away easily enough with or without the engine running. If you can hear the pump, its running. It may be pumping vapour but thats why you need to start taking hoses off. Careful, a good pump will flow well.

[Eddyvdw]

Be careful with mechanics who don't have experience with MFI. It's a very specific system. Just remove the fuel level sender and peep carefully with a little led light inside the tank (tank should be almost empty). You quickly notice the condition of tank and filter. Check the flow at the fuel filter in the engine compartment. Take a long hose, connect to the return line at the fuel filter and measure the flow. Should constant and deliver around 900cc/30sec (mine is 750cc/30sec and still ok).

Ed

[tobluforu]

Check all fuel lines, mine did the same thing and one of the lines was collapsing when warm due to old age.