Misfire and abrupt stalling when warm

Sadly getting my car out this spring has been less exciting than usual. My 02 Targa was hibernating in my garage for about 4 months starting early December. It was running perfectly fine at the end of the year and I shut it down and left it after replacing the oil and adding fuel stabilizer. My first problem was that I had accidentally unplugged the trickle charger a few weeks before starting the car so the battery was 100% dead. I managed to charge the battery and it seems to be fine. Finally last week I started the car and took it out for a spin. Started right up and ran beautifully as normal. A little blue smoke on startup and a bit of ticking that settled down after a few seconds. Usual behavior for the spring startup and otherwise it seemed to be running perfectly normal. But on my way home about a km from my house I noticed that it was misfiring slightly after which the engine stopped abruptly. I think it happened immediately after a sharp corner but I don't remember for sure. Given that it had been in hibernation and was otherwise running fine I assumed it was either cracked ignition coils or bad gas. I decided to start the car up again since I didn't think it was a mechanical issue. It started and ran fine for about 30 seconds and then began misfiring and eventually set a code. Misfire on cylinder 4 and multiple misfires. Let the car sit for a few days still thinking it was either bad gas or spark/ignition related. Filled up the tank with new gas on one short trip and it ran fine. Next trip a few days later and it was running fine for about 10 minutes and then started misfiring slightly. Parked the car again. Yesterday I took it out for a good spin with similar results. Ran perfectly well for the first 10 minutes. Then stalled abruptly with almost no warning. It might have misfired a few times before that, but not enough to set a code. Started it up after it stalled and it ran really rough and set misfire on cylinder 6 and multiple misfires. But no other codes. I don't have a durametric so I can only debug with my generic OBD2 app. I'm thinking about running it again today with real-time graphing of the fuel pressure assuming the problem might be a failing fuel pump. My next option is to drive it to my mechanics to hook it up to a durametric and get him to debug. So I think the symptoms are: car starts and runs fine when cold. A short time (10-15 minutes) later it either stalls abruptly or simply runs rough due misfires. Engine restarts easily after stalling, but continues to misfire. From my research on here so far my best guess is that the problem is either the camshaft position sensor or a failing fuel pump. The fact that I am getting misfires and the car starts up again when it has failed is leading me towards the fuel pump overheating and failing. Would the fuel pump age/degrade during hibernation though? Give this problem started immediately after hibernation makes me think it is somehow related. Anyone have any ideas on how to diagnose myself? Thanks for reading. Kevin

You may want to get a Durametric for better diagnostic capability. I agree both the crank position sensor (not cam position sensors) and the fuel pump are strong suspects given the symptoms.

You may also want to do an e-gas throttle re-calibration since it's free and easy.

Take a good look at your 02 sensors switching, sounds like they could be ageing and slow switching. Will cause the engine to lean out and start misfiring then stall. When started cold, the DME ignores 02 sensors and runs in open loop, then as engines warms up it switches to closed loop and adjust fuel mixture according to 02 sensors information. If the 02 sensors are lazy due to age, the mixture can get lean biased and cause this symptom. When warmed up and in closed loop fuel control, the 02 sensors should switch rich/lean 5 times per second. Just my opinion from your scenario, and should be checked out/eliminated from the possibility list.

Pull the cylinder #4 coil and inspect it closely. Although not on a Porsche but on a Toyota I have seen coils short through the primary side and shut the computer down. Would always restart without issue and thankfully it never damaged the ECM on them.

Thanks all. I hadn't considered the e-gas calibration although I was aware of it and had performed the calibration in the past. It is probably a stretch though given the symptoms but it is quick and easy. I'll try that tonight and also watch the O2 sensor information more closely. Sadly it appears that the 996 doesn't report fuel pressure through the standard ODB2 interface, so it looks like I'm stuck without a Durametric.

I agree with Porschetech3. It sounds like O2 sensor(s) issue to me. Misfire when warm only always makes me start there.

Ashai may have found it. I started the car two days ago and it immediately ran rough similar to being out of gas. It didn't set any codes, but I started planning on when I would be able to get it flat-bedded to the shop. Yesterday I performed the e-gas throttle calibration since it was dead simple. Turn the key to the on position but don't start the car. In about 30 seconds I could hear an electric motor near the throttle body. Took the car for a spin along the same route I had tested before and it didn't stall. The engine was making a ticking noise for a while that sounded unusual but thankfully that disappeared. I then ran the car for another 30 minutes on the back roads around my house. No problems at all. I'm not 100% sure that the problem is gone, but so far so good. I'm quite shocked that the throttle calibration could have resulted in that behavior though. Could it be that the throttle was actually starving the engine of air at idle because it was mis-calibrated? I had assumed my problem would either have been due to gas or spark, but I hadn't considered air. Feeling much better. Thanks Ashai! I'll post again in a few weeks to provide an update. Hopefully not sooner...