993 Control System Anomaly

[earossi]

We have an issue with my 1998 993 that is baffling. Following a rebuild of the 3.6 engine, the engine starts immediately and runs well while COLD. Once the engine has run for 15-20 minutes and has heat soaked the engine will idle rough and will stall if load is suddenly removed from the engine.

Another observed phenomena is that the ECU, for unknown reasons, will not allow an adaptation to occur.

In other words, you can be driving along at 30 mph, in an appropriate gear, and in preparing to stop, you depress the clutch. On about the fifth or sixth stop once the engine reaches temperature, when the clutch is depressed the engine speed will drop to a low enough point where it will stall.

The phenomena is more prevalent when engine speed is less then 2500 rpm when the clutch is depressed to unload the engine. If engine speed is above 3000 rpm or if you just "blip" the throttle while declutching, the engine will not stall. Once the engine reaches idle (assuming it does not stall) the idle is dead solid giving no indication of any instability. Further, the engine can sit there for 30 minutes with a solid idle at about 900 rpm.

If the motor stalls, it will immediately restart with the characteristic "flare" in rpm followed by a return to a good solid idle. A restart NEVER results in a stall. You must load the engine and then unload it to trigger a stall.

The idle stabilization valve was quickly changed out and made no difference in the symptoms mentioned above. We've also changed out all ignition components including wires, plugs, caps and rotors. Both ignition coils have been changed.

Finally, the ECU was removed from the car and sent off to a specialist in Bosch ECU's. Their test of the ECU showed nothing wrong. It should be noted that during the engine rebuild, the ECU was configured to take advantage of a more aggressive cam that was installed in the engine. I have been told that the ECU programming has nothing to do with my current issues.

This is a classic case of an intermittent problem that only materializes on a hot engine.

Though both the MAF and throttle position sensor "test" to be good, we are now going to change them out. Assuming no results from that, we are about out of things to do.

Has anyone ever encountered such an issue of intermittent stalling on a hot engine when load is suddenly removed?

[jhg41977]

Subscribed, wish I had an answer for you.

[jstyer]

What flywheel do you have installed?

[earossi]

Originally Posted by jstyer

What flywheel do you have installed?

I've had the light weight flywheel on the engine for the last two years and have never suffered stalling until after the engine rebuild. There is no doubt that the LWF aggravates my new stalling issue. With a dual mass flywheel installed, my guess is that the inertia of the DMF might prevent the stalling. But, the motor was rock solid with the LWF and I would hate to have to return to the old boat anchor in a DMF that limits the engine's ability to rev freely.

LWF aggravated stalling issues were a problem with early 993's, mostly the 1995 models. However, my car is a 1998 model. And, as I've stated, stalling has never been an issue unless the idle stabilization valve were defective (happened once). Cleaning of the ISV corrected that issue.

When I first experienced the stalling issue following the rebuild, the suspected culprit was the ISV. I re-cleaned it with no impact. So, I then replaced the ISV with a new valve. Installation of the new ISV did NOT alter the stalling issue; so, I've concluded that my current stalling issue has a root cause somewhere else other than with the ISV.