Hi 928 owners,
just wanted to share this with you guys.
I was driving my 928GTS on the motorway yesterday. Cruising along at a decent speed. All was fine until I noticed that the car was holding back and pulsating a bit. When I pressed on the gas it seemed ok. I noticed smoke waves from the rear window. That is not a good sign. Oil pressure and temp were ok.
I stopped, checked things, found nothing obvious, and since was a few streets away from home I drove back. In the garage the idle was bad, black smoke waves soming form the car, and it smelled.

I let it cool down and decided to investigate today. I connected the diagnose tool and found errors in the LH: O2 sensor, supply voltage, idle switch error. At first the O2 sensor seemed dead, but when I let the engine warm up some longer the O2 responded on my meter and worked in closed loop. MAF sensor showed normal value and responsiveness. The diag tool also indicated that the idle switch worked properly and the LH voltage was on spec.

I decided to stop troubleshooting and disconnect the battery for a while. All seemed ok after starting again later this morning. No more strange behavior, pulsating idle and smoke.

Apparently the LH was running from corrupted parameters somehow, and this caused the hot engine to be over-fuelled badly.

Simple IT-guy statement to this: don't troubleshoot, just reboot.

regards

 

Bizarre......

 

Had a similar problem awhile ago on a long trip, suddenly lost power on the highway. Had some hot idle surging, poor fuel mileage after that. Really bewildered me. I ended up disconnecting the battery by resetting the LH, all is good now. No idle issues, runs great.

 

Black smoke, oil. hmmmmm, ring?

 

Black smoke could be the result of WAY too much fuel. Potential causes that come to mind:

LH getting ready to die. A common failure mode is injectors getting continuously grounded (and therefore are continuously open) and this mode seems consistent with a "reboot" fixing the problem.

Temp-II sensor. Intermittent issue with the sensor or wiring to the LH ECU connector.

 

Dave,
I have a rebuilt LH. We do our own LH modification, and the car's been running with that mod for quite some time.
I checked the LH and EZH temp recording with a diag tool, and they were ok. Injectors seemed ok and when I hit the gas, I noticed proper acceleration without much hesitation. It is the low-gas or idle that showed the problem. Apparently the LH lost track somehow and started to over-fuel by adding a longer injector pulse rate that required.
Anyway, the reset made it come back to its senses

Regards

 

Wires '90 did that one day. I kept the foot to the floor and it cleared itself up after a few minutes. Was weird.

 

Theo--

Had same symptoms a week after a brain replace and was stumped for a bit. Like you, I was blocks from home and limped home to think about it. Did the same thing as you, disconnect battery and let it forget its bad dreams. Reconnected and started it again, no worries since. My concern for reoccurance dimishes as time goes on. Bad part was that this was one of the infrequent times that I had talked K into riding with me in the 928. The seats don't get along with her, plus as the power partner she feels it's her God-given right to drive and she doesn't like to drive the 928. It just adds to her image of the 928 as unreliable as a DD.

 

Possible brief MAF contamination that burned off at shutdown.

 

I had the same thing happen periodically after I installed the SC. As part of the SC install, I relocated the LH ground from the block to a bolt on the manifold. I thought since the bolt passes through the manifold into the block, it should OK. Once I moved the ground back to the original location, that problem ceased.

Could be a loose/bad ground.