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Pete, Technician
Category: Car
Satisfied Customers: 27815
Experience:  17 years automotive training and experience.
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Porsche 928 S4: I have a 1989 Porsche 928 S4. Automatic transmission.

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I have a 1989 Porsche 928 S4. Automatic transmission. I have an extremely mysterious problem that even my really great Porsche mechanic could not solve -- and then he left the dealership and went back to practicing law -- go figure! Anyway, a couple of years ago it started to stall out abruptly when I stopped (at a sign or a light or in a drive through) -- it would start idling rough and the rpm would go up and down like it was trying to find its idle but could not. This always happened in stop and go conditions when I had been using the brakes a bit. NEVER did it when running highway or if only a couple stops with plenty of driving in between. Finally got to the point where I would roll up to the light, get it fully stopped and then shift into neutral so I could hold my foot on the brake and not have it stall. James (the ex mechanic lawyer) was zeroing in on a "vacuum" problem, but subsequent mechanics seemed to think it was electrical. I am clueless! Any ideas? What should I say to the NEXT person that I ask to look at it?

My name is XXXXX XXXXX I am a professional here at Just Answer/Pearl. I noticed that your question was not getting a response and thought I would see if you still need help with this.I apologize for the delay and I hope I can still assist you with this here.

From what you have described here this sounds like a possible fault with the idle speed actuator itself.If this were a vacuum leak usually the idle would be higher then normal and the engine may also run lean with excess air present into the engine.

This would need to be tested further to see if this is the case with the idle actuator at fault and if found at fault usually it would require replacement.My info doesn't list the exact location of this component itself unfortunately but does list this vehicle having this idle speed actuator to control the idle.

Thanks Pete

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I am following up with you to see if you still needed help with your question you had asked.If my help didn't satisfy your question please let me know so we can continue.

Thanks Pete