911 2,2L E MFI problem when hot - Help !!!

[frankw]

Hi guys,

I own a 2,2L E Targa that runs just fine (pulls strongly) about 30 miles until it gets hot and then starts to work erratically until it completely stops. I did a CO test when this happens and it seems the mixture becomes leaner and leaner as it gets hotter until the engine is not even able to restart. When this happens, I have to wait by the car during 30 to 45 minutes and it restarts and works perfectly for 30 other miles ! When I drive on the highway at high rev, it can last longer without trouble. On the other hand, mixture seems to be a bit too rich when cold (some black smoke and it works great outright when started even after a cold night).

I changed the fuel pump, the MFI pump has been professionally rebuilt, new fuel filter has been installed, ...

I need help, I'm lost !

Any advice / comments would be greatly appreciated

Frank

[lwa]

Permatune?

[jkh2cpu]

This sounds like slow fuel starvation to me. I'm wondering about the the screen filter in the gas tank. This is a fine mesh bronze screen that passes gas but stops debris. It might need replacement or the tank may need cleaning. I've been there with my 1973 911. It sat for about 10 years before I got it back on the road again. The screen would plug up, the car would run hotter and hotter and then stop. It would run again several minutes later.

I had a similar problem with a plugged carbon canister that is used in the emissions control system. Replacing that solved another fuel starvation problem. The tank was vented through the carbon canister, and once plugged up, it would suck gas until there was too much vacuum in the tank, and it would quit.

It would be interesting to know the fuel pressure when the engine conks out. It would rule out fuel problems.

These old cars are lots of fun if you have the patience to sort it out yourself, or the bux to pay someone else to sort it out ;-)

HTH.

[frankw]

I recently fitted a new gas tank and checked that screen. In my case, I have to wait at least 45 minutes to being able to restart the car.

[tobluforu]

Thermostat may be stuck, or one of the fuel lines is old and constricting?

[356RS]

I agree with HTH on a fuel problem. When cold the thermostat rod is retracted and making the pump rich and the engine runs fine till it's warmed up. So the thermostat is doing it's job. When it's warmed up it's lean. Have you checked your fuel pump flow rate? Should produce 900cc to 1000cc in 30 seconds at the MFI return line going to the fuel filter console. This is more important than fuel pressure.

[frankw]

Thanks for this. I'll check fuel pressure today. I'll keep you informed.

Frank

[frankw]

Hi Guys,

I checked the fuel lines and did the pressure test (about 1l in 30 sec). It was OK. While checking this, I noticed the bypass pressure relief valve fitted on the filter console (return line) wasn't working properly. I removed it and tested the car. It started outright and ran like a dream whithout problem for more than an hour. I'll do a longer test tomorrow but it seems the problem has come to an end.

Do you know what's this valve is meant for ? Is that normal the car run well without it ? Can this valve be the cause of my problem ? I'll would appreciate to understand.

Thanks for your help anyway, great forum !

Frank

[356RS]

The valve in the fuel line connector on the console for returning fuel back to the tank is a simple one way valve. If the valve was shuck closed it would cause an over pressure condition. Was it stuck closed?

[jkh2cpu]

My car came with MFI, but I never liked how it worked, so I went to carbs. As I recall, the pressure delivery to the filter was about 18 psi, but was reduced to 11 psi for the fuel injection pump... I'll quote from the manual, page SF 24...

The roller-cell type fuel pump as a delivery capacity of approximately 110 liters / hr (29 gal. per hr.), up to 125 liters/hr (33 gal per hr.). This delivery capacity is several times the amount of actual fuel consumption and is necessary to keep the fuel temperature in the injection pump as low as possible. The fuel delivery pump delivers the fuel via a fuel filter to the injection pump. The delivery pump is mounted by a bracket to the support member under the fuel tank.

The excessive fuel flows back to the tank via a return line. An overflow valve in the fuel filter will establish a pressure of 0.8 +- 0.2 atm (11.8 +- 3.0 psi) in the fuel system. When the pressure increases above approximately 1 atm (14 psi) some of the fuel is returned to the fuel tank via the the bypass valve installed in the fuel pump. The bypass valve operates independent of the overflow valve.

Does this help? I'm sure the MFI guys can help you with this. You do need all of the parts in the system for the fuel injection to operate properly. You do need the filter (it's a fine filter to remove small debris that could affect the operation of the FI pump and the high-pressure injectors). Also, the fuel regulator in the filter console (or what ever Porsche calls it ;-) ) is necessary for the proper operation of the pump.