'71 MFI starts cold won't start after warm

[clark63]

My '71 911E with MFI will start ok when it is cold. However after it is warm it won't restart. Have to let it sit until it is pretty much cold again. We have spark so pretty sure it is a fuel issue. It keeps running once it is started. Can get it started warm with direct spray of starter fluid. 86k miles on car, MFI has not been rebuilt that I know of. Car runs great otherwise.

Still trying to solve a fuel smell problem with this car. Any suggestions greatly appreciated!

Albert Blue
1971 911E Targa

[r lane]

Go through your check list. Start with ignition and then all of the others, pressure, volume....Have the injectors tested and what is your AFR. Hard for MFI to run lean, but what is happening to the fuel on hot start. after a hot no start, might pull a plug to have a look

[991gts6s]

old wires.. been thru this many times.

[john walker's workshop]

Gotta open the throttle about half way when you crank the starter. It needs air in there.

[clark63]

It is a fuel starvation issue. Recommendations on who rebuilds MFI's?

[Herr Richter]

My 70E was done by Mark Jung in Oregon. He is suppose to be one of best for old MFI.

[stownsen914]

Since it starts when cold and you're not reporting other running issues, I wouldn't immediately suspect the MFI pump. A fuel smell indicates and obvious issue, and I'd want to investigate/solve that before deciding the MFI pump is the culprit. I see you said starter fluid helps, but fuel smell suggests too much fuel perhaps? The cold start / fuel prime mechanisms on the MFI setup can spray fuel at the wrong time sometimes, maybe it's causing an issue? If John Walker's suggestion doesn't fix it, take off the airbox and see what's going on with the fuel priming mechanism.

[clark63]

Thanks. Definitely a fuel starvation issue. The check valve in the fuel filter was not working. Replaced that and we were able to get it to start hot, but after running awhile it started to lose fuel. We are going to replace the fuel filter, the mechanic said he is seeing some debris in the fuel. We replaced all the fuel lines recently (for a second time), perhaps deteriorating fuel lines have fouled things up. I think we need to have the MFI pump rebuilt. It has a brand new fuel pump at the front. The cold start mechanism is working properly.

[proporsche]

this might be a little guessing but i would check your distributor points..if they are opening the right distance
Ivan

[Arne2]

You may want to verify that the fuel lines to the filter console are correct. Years back, some Porsche diagrams for these connections were wrong. I've seen (on my own car when I bought it) were the inlet from the fuel supply pump and the return line from the MFI pump were reversed. Both of these ports on the filter console have identical arrows for inlet, but they are NOT interchangeable. This can cause the type of issues you describe due to insufficient fuel.

The key bit is that the inlet from the fuel supply pump is the one pointing towards the front of the car. The return line from the MFI pump is pointing out the back. The outlet for the return to the tank points to the right side of the car.

[70etarga]

If the supply and return lines are reversed the engine will start and run at low speed but will stumble and cough over about fifty.
At least that's what my 2.2e did when I messed up.
Try using a fuel pressure gauge you can read on the road- I bet this would find the problem.

[Arne2]

Originally Posted by 70etarga

If the supply and return lines are reversed the engine will start and run at low speed but will stumble and cough over about fifty.
At least that's what my 2.2e did when I messed up.

When mine was messed up this way it would it would start and run OK when cold, but when it got warm it would stumble and stall, and not pull a grade at all.

[clark63]

Turns out the brand new Porsche fuel pump from Germany is pushing 3 pounds of pressure instead of 10. 3 pounds is for a carbureted car, MFI is 10. Replacement on order.

[stownsen914]

Make sure the check valve in the fuel filter housing is operational. It's built into the return line fitting in the housing. The system depends on it to maintain pressure. They're old and fail (ask me how I know). It could present as a pump "not making sufficient fuel pressure." With a good check valve, you should get more like 15 psi.

[bkreigsr]

In the 'no start' situation, have you ever unplugged the cold start valve?
...worked for me a couple of times....maybe just dumb luck...

Bill K

[clark63]

Ended up being two problems. The check valve at the fuel filter was not working, it was wide open. The fuel pump was dying, first replacement was the wrong one, for a carbureted car. Got right one for MFI, now starting and running great. Thanks to all who responded!