cjbcpa

I have a mechanically stock 951 that I use as a dedicated track car. I've had it for a year and supposedly, it was not tracked heavily if at all during prior ownership. I have some records but not complete.

At my last two DE's the motor would stutter, as if missing, while under heavy load once the car has gotten fully warmed up. The first 10-15 minutes of each session the car would run fine. Once it was hot though, it seems to miss when fully down on the throttle. Backing off on the throttle returns all to normal, but coming out of the next turn on full throttle, stuttering again. Of course, the only way to reproduce is to run it up to full DE temperature. Driving it on the street isn't hard enough.

Any thoughts on the cause or my following attempts to fix? It's not overboosting, I had that symptom last year due to a bad DME which has been replaced.

First, I was thinking of some top end cleaner to eliminate any deposits causing pre-detonation(any suggestions on which vacuum tube to pour it into?).

This would be followed by if needed, replacing spark plugs, ignition wires, coil, rotor and finally fuel injectors. The wires, coil and injectors are certainly original to the car and I have no problem replacing them after 23 years as a precaution.

Have I overlooked anything or barking up the wrong tree?

Thanks.

CB

fast951

Don't forget to check the TPS.

cjbcpa

I think that was swapped out last year as we chased down the overboost problem (not that it couldn't have gone bad again.)

CB

Portly

FWIW, I had a bad "hesitation under load" issue that I just recently tracked down to a cracked rotor. However, my problem was a lot worse - it occurred even when the engine was cold. So it's worth looking at, but could very well be something else.

cjbcpa

Mechanic was going to put the car on a dyno to try and replicate the symptoms, but heard a noise from under the valve cover that he wanted to check out first.

The noise was a broken valve spring so the heads coming off to get at that.

toddk911

95% of the time the above is the problem

AgRosa Brad

I've had a very similar problem. Although it occasionally will happen on a warm day after driving 20-30 minutes. Last year I replaced plugs, cap, rotor. I've got new wires coming. I'll let you know what happens and would ask the same.