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1988 944S Backfire on down hill coast

Engine

Vehicle Information: Model(924/944/968): 944S; Year: 1988; Body Type: Coupe; Total Mileage: 129,000;

Short version of a long story: I believe mechanic tried to adjust the idle on my car because it was running rich at idle (later I had to replace the fuel pressure regulator). After this, on short down hill coasts with car in gear, the cars engine would quit. When the throttle was opened with the car in gear it would backfire, then spit and sputter, and finally after a few moments begin to run fine again. Wondering what this jack-leg may have done to my car, I decided to try to adjust the air flow at idle (the long story is how he got the car, it has to do with a daughter, a wreck, insurance companies, ect). The idea was that off nominal air/fuel mixture ratio during a long down hill coast might be causing the plugs to foul. This adjustment brought about significant improvement. The car is almost with out problem, however, after a very long coast the car will briefly spit and sputter before it resumes normal operation. I believe that my first step is to accurately adjust the idle using the jumper at the DME. My factory manuals were stolen, another long story. To set the idle, where do it put my jumper, and what RPM do I set my idle to? Also, any thing else to check if this does not solve the problem?

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