Hello, I have a 2003 996 C2 that has a curious engine issue during warm up. Here is the problem, which I have been a been able to replicate on 3 occasions over the last couple of weeks. The location is Brisbane, Australia and its currently mid-summer.
If I start the engine from cold and immediately prior to engine start, complete the throttle body re-calibration process, the engine starts, warms up perfectly and drives perfectly. If I park the car and leave it for about 1-2 hours then restart it whilst it is still warm from the earlier run, still completing the throttle body re-calibration, it will start fine, idle fine and then if I drive off after a few minutes with the engine temp at about 70 deg C, it will stall and for about 2 minutes refuse to fire and run for more than a couple of seconds. It usually takes 2-3 throttle body re-calibrations and 5 minutes before it starts and then runs perfectly for the next 30 - 90 minutes as though there was never a problem. I first noticed the issue 2 weeks ago when I started it cold and without doing the throttle body re-calibration as I didn't know about that then. On that occasion, at about 70 degrees, the idle started hunting between 1100 and 600 rpm. It would almost stall before the ECU would catch it and then back up to 1100 rpm until it would die, get caught and then back up to 1100rpm (repeat for a minute). My warm up procedure is quite systematized and curiously, the three times I've had is stall on the road, it has done it in nearly the exact same location about 700m from my house on flat ground. If it was a carburetored engine I'd say it was running out of fuel since it is a complete engine shut down, not just a cylinder or two like with an ignition issue.
The first though was that it was a crank angle sensor however that has been changed and it made no difference. There was a code for a crank angle sensor which is what prompted the replacement. Similarly the MAF was swapped with a known functioning unit and that also made no difference. I have also checked and confirmed that the fuel vapor solenoid valve near the alternator is functioning when I trigger it with a 9v battery. There are no obvious vacuum leaks I can see. Other than it happening on the second run of the day from warm, the only other common factor is that each time I have reversed it down my steep driveway out of the garage however I can't see how that could contribute. It does it equally with a full tank of fuel and nearly on reserve. It does it will the ac unit running or not.
My current thoughts are that it may occur at the same time the engine switches from open loop to closed loop during engine warm up, however, without knowing the timing for that during the warm up ecu map its hard to say. I'm tending to think that if not fuel/ignition mapping related, it may be another ecu controlled function that is triggered during warm up, for instance the other vacuum solenoid that sits near the alternator with the outlet that disappears under the inlet manifold towards the air oil separator. I have not yet cleaned the e-gas throttle body however, it looked pretty clean the other day when I had the air box off looking for vacuum leaks. Interestingly, when my mechanic changed the crank position sensor last week as a result of a logged CAS code, it threw the same code upon start up. That's when they also tried the other MAF sensor. Incidentally, the starter works fine.
Any thoughts and suggestions would be appreciated. The winning correct suggestion that fixes the issue, will receive 1000 good karma credits from both the universe and me.

 

The fuel Vapor solenoid may be sticking open. remove the hose and see if it is pulling vacuum with the electrical connector off.

 

Thanks dgmark. The fuel vapor solenoid tested ok. I took the car back to the mechanic last week and it had logged a code for left bank cam sensor. That was subsequently replaced and (touch wood) it has been perfect since. Odd that the sensor had behaved in the same way on three separate occasions but I'm happy to let it be.