'71 911E Starting Problem

[73ess]

My '71 911E (all stock) is exhibiting a starting problem. It starts fine when cold, but if I start it for a few minutes, drive it around the block, then shut it off and it sits for a few minutes, it won't restart. I have to wait until it's stone cold again for it to restart.

Are there any sensors on a '71 E that would cause this problem? The MFI pump was newly rebuilt. Maybe a bad fuel pump? But I can hear it buzzing/humming when I turn to the key to the on position.

The motor is the original with 120K-ish original miles. But it's still strong. Engine was recently fully tuned up.

--Steve

[boxster03]

starting issues could be either from nozzles not holding pressure or the cold start enrichment not being disabled with a warm engine. There are also check valves in the fittings at the top of the MFI pump. All have similar operating symptoms.
As a test, I would disconnect the cold start enrichment fuel line, plug the line and disconnect the wire to the cold start solenoid on top of the fuel filter console. Apparently the solenoid valve can also leak.
You should have the nozzles tested
There is an access plate on top of the MFI pump at the thermostat. With a small screwdriver you can push down on the thermostat linkage. When hot (thermostat should be off) there should be no change in running with sight pressure here. This is an easy way to confirm the thermostat is fully off.

[Early_S_Man]

Steve,

Cold-start relay, Thermo-Time switch, and Cold-Start Solenoid are all suspect ... #52, #14, & #12 in the diagram below:

[Grady Clay]

All good advice above. In these situations does it seem to be too rich – “flooded”?

A test for this is to give it full throttle while cranking. If too rich, it at least should fire and try and start.

It would be unusual for the MFI be set so lean that it won’t start without the aid of the cold start circuit. The only situation I can think of is the cold running thermostat is set to never function and the main mixture is very lean.

Another possibility is the fuel tank filter screen could be clogged with rust/debris from inside the tank. This is a typical symptom – starts, drives, dies and won’t restart until it has set for a while.

A way to check for this is to temporarily put a 15+ psi fuel pressure gauge in the fuel hose at the filter console (the hose closest to the rear of the car).

Best,
Grady

[73ess]

Thanks for the advice guys. Well, now it appears that the car won't start at all. Even after sitting overnight. The batteries are fine, the fuel pump hums and the starter is turning over, but the engine won't fire.

[boxster03]

take off the air cleaner and disconnect the cold start solenoid and put a little gas down ea cyl. stack and try seeing if it will start. make sure your mfi belt is on and turning too.

[73ess]

Thanks. What does the cold start solenoid look like and how do I disconnect it?

[boxster03]

the cold start solenoid sits a top the fuel filter on drivers side of car in engine compartment near the lid hinge and has a wire that is gray and white that attachs to the top of it. unplug this wire and it will prevent the fuel from pumping out of the short fuel line that goes into the air cleaner box.

[POKDUB]

I have encountered two hot start problems during my brief car history.

1) where enrichment solenoid flooded the engine and made it hard to start. Overcame by putting in a manual switch to solenoid, rather than its original link to ignition.

2) battery was not 100%. It turned over nicely when cold, but no turnover when warm. New battery, problem solved. Sounds like this is not your issue.