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911SC Irregular Idle with Engine Warm

Fuel System

Vehicle Information: Model: 911SC; Year: 1980; Body Type: Coupe; Total Mileage: 155,000;

After stubling through the the Q&A on this site and searching through my Haynes and Bentley Service Manuals, I have come to the realization that I may have a unique situation. I just had a major service done and the car ran great until it was warmed up. The engine idle would hang at ~2000-2500 RPM. If I loaded the engine up the idel would drop down and behave. I took it back to the mechanic and he said that the decel valve needed to be adjusted and he spent a couple of days working on it but now it will randomly not hold idle when its warmed up. It either wants to stall or the idle will oscilate and increase in magnitude until the idle return stalls the motor. I have not been able to identify this decel valve adjustment capability on my '80 SC nor have I been able to figure out why the motor runs great until it gets up to temperature and then randomly wants to stall. Another item to note with this characteristic of stalling is that the idle is also rough when it wants to stall. I was thinking of just replacing the decel valve but thats a $200 part and I wanted to make sure I was investing in the right area. Also worthy of note is that I had the mechanic fix some oil leaks so he removed the engine and in the process, he replaced all the vacuum lines as most had cracked or become very brittle. He mentioned that the decel valve was probably not functioning properly prior to the service due to the cracked vacuum lines. Please advise as to what might be causing this situation or where I might find more literature on the system to help diagnose. Thanks for your attention to this question.Best,Dal

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