1988, 928 - starts and might run on the highway for an hour +/-. Can stall at highway speeds or when stopping after the highway run. Starts again after waiting for 20-30 minutes and runs aging for an hour or sometimes 2 hours but eventually stalls again - and the process continues.

 

Is this common finding/symptom of vapor lock? MAF failure? Fuel pump?

 

I would start by replacing all three relays....the common 53 versions fuel pump, LH, and ignition. For an engine to stop when running down the road it would not be any "vapor lock" has to be complete shut off of fuel or spark.

 

Next time this happens, stick your head under the car and look at the plastic fuel tank. I'm betting it will be "sucked" in. (If this has been happening for awhile, the tank will probably be permanently "sucked in" at places. You might want to look at the tank before driving the car, so you have a comparision.)

There's an electrical valve that gets a signal when the engine is first started, to pull a vacuum on the charcoal canister system (to clean out the fuel vapors.) If this valve fails in the open position, the fuel system will be subjected to full time engine vacuum. This will suck the tank inward and create enough vacuum that the pump will not be able to draw any fuel. the car will stall....and will not restart until the vacuum inside the tank decreases.

It's common enough that I'd start here, after replacing the relays that Jim is talking about.

 

Also check that the fresh air line from the charcoal canister to atmosphere (the pipe with the flared end that is tucked up behind the rear bumper cover) is open. This line would have to be clogged or there would have to be an internal clog in the charcoal canister to allow the fuel tank to be subjected to significant vacuum. The fresh air line is always open from the charcoal canister to atmosphere. When pressure in the tank rises due to vaporization of fuel the fuel vapors are allowed to escape through the roll-over valve on the tank, through the expansion tank in the right rear wheel arch (at least on the S4 models) and then into the charcoal canister where the fuel vapor is absorbed by the charcoal. Any "non-fuel" vapors (i.e. the air in the tank) are allowed to pass through the canister and out the atmospheric vent line. As the fuel in the tank expands and contracts the fuel vapors are collected in the charcoal canister. Once the engine is running and at operating temperature the canister purge system (a different combination of valves depending on model year) opens at part throttle and engine vacuum draws air through the atmospheric vent pipe connection on the charcoal canister (which is open to atmosphere inside the rear bumper cover) picking up the collected fuel vapors and routing them to the intake manifold to be burned. The canister purge system is shut off at idle and when the engine is cold so as to prevent the fuel/air mixture from becoming too rich.

Mike

 

The crank position sensor can go intermittent, and lose contact when it gets hot.

Another "hot" failure can be the flexplate, but I'm reluctant to mention that! When was it last checked and flexplate pressure released?

 

My guess is fuel pump overheating, then cooling off and resetting the thermal cut off switch.

 

I was thinking this too. Also, is it slow to crank on a restart? That is a sign of excessive pressure on the flexplate, and *GULP* the crankshaft on the thrust bearing.

 

If the vapour lock and relay suggestions prove to be inconclusive I would start to suspect the LH computer module may be on its way out but do eliminate the former first. Friend of mine had similar problems to yours and they dissappeared when he tried my spare LH unit.

Your problem does not sound like a TBF issue to me [the first most know of this is when the car will not crank when trying to start] but nonetheless you should understand this syndrome and ensure you are not the next victim of this blight - you should see this as a separate 928 ownership issue of paramount importance for the long term survival of your motor. A failed LH is "small beer" by comparison.

Regards

Fred

 

He should get a small beer after hearing the good news.