You diagnose the problem...no start.
 

Here's an interesting thing that happened to me this evening. I was driving at freeway speeds and ran out of fuel. The low fuel light had been on but according to the trip meter I thought I had the fuel to get home. I guess I'd been a little heavy on the throttle last week. :p Anyway, I managed to coast off the freeway and into a Chevron where I proceeded to put 20.59 gals into the tank. That's the most fuel I've ever put in the car. When I got back in to start her up it failed to start. After some extensive cranking it would act like it wanted to start but die immediately. I rolled the car over to a parking space and my wife and I walked to a Carl's Jr. to kill a half hour leaving the hood open. I had a theory about why the car wouldn't start based on the way the motronic operates and was testing it. After a half hour we returned to the car and it restarted on the second crank and ran flawlessly. I've got my theory on why this happened. BTW the car has never had any starting/running problems.

What's your theory on why it restarted?

Later,
Kevin

Low on fuel could result in air getting mixed with the petrol resulting in incorrect fuel mixture when you attempt to start. If the car does not start immediatelly you then faul the spark plugs causing more difficult start even if the mixture becomes OK. The prolong initial crancking after the fill caused the air to move out of the the system. Waiting for half an hour also cleared the spark plugs from excessive fauling and the car started normally.

you probably should have turned the key without cranking the engine and waited til you heard the fuel pump stop. i bet it would have started right up.

I forgot to mention it's a 944S2 not that it matters much. It's all pretty much the same control system.

By restarting on second crank I mean the second normal crank as well. The S2 turns over pretty lazy and it takes cha-waa, cha-waa, cha-waa for it to start. I did two of those. It often freaks out valets because of that and I'll return to find the car sitting where I left it, which is quite convienent actually.

When it restarted it did so with no touch on the throttle and settled into a perfect idle like nothing happened.

Later,
Kevin

I'd NEVAHH give any Porsche to a valet, under any circumstances, not in this lifetime, nor in the next.

I couldnt stand to listen to the gears crunching while they figured out the shift pattern (ooooohh the mere thought of seeing my 928 5-speed in the hands of a valet makes me cringe).

regards, P


P

Couple of hints...

I can't hear the fuel pump running in my car at all, never.

I does have to do with "incorrect fuel mixture" but no fouled plugs are involved.

It relates to the car being operated at "freeway speeds" and thus the engine being a full operating temperature. I'm in SoCal so it's not 30 deg F here. It was cool but more like 60F.

Later,
Kevin

well, the car cut out while moving and in gear. the computer sensed an incorrect mixture, among other things, and needed to reset. so, you wait half an hour and voile, computer is reset. i bet you can hear the pump running if you are outside of your car at the passenger side rear quarter. my wife did this with her land rover, bosch electrics and all. i just turned the key, listened for the fuel pump to turn off, turned the key, and it fired right up.

A couple of things combined to cause the no start. First the fuel pump started running out of fuel to pump (obviously). This allowed some free volume in the fuel system which allowed the fuel to boil (vapor lock) quite easily in the fully warm (operating at freeway speeds) engine. This is why the motronic system fuel pumps have check valves in them to hold pressure in the system even after shutdown. Even after adding fuel, the fuel pump is helpless in dealing with the vaporlock condition that had resulted from running out of fuel. A mercifully short wait (30 minutes) with the hood open let things cool down enough. Hit the key, the fuel pump can once again pressurize the rails with fuel, not air, and the car starts like nothing happened.

Later,
Kevin