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Engine Surge at all speeds


Vehicle Information: Model: 911SC; Year: 1983; Body Type: Coupe; Total Mileage: 91K;

The car starts fine and goes into a fast idle around 2000 rpm. When the engine warms up the idle drops to around 1100 rpm and will normally settle at around 950 rpm. Some of the time the idle will drop down to 650 and back up. When this occurs the engine runs rough - surging or hesitating a lot (during acceleration and constant speeds). When the idle holds between 950 and 1100 rpm, the engine only surges or hesitates at a constant speed and it is not all that bad. I have owned the car for a year and driven it maybe 2000 miles. I have replaced since purchase - airbox w/pop off valve (blown when purchased), hydraulic chain tensioners, complete tune up including cap and rotor, fuel filter, fuel accumulator (hard starting), fuel pump checkvalve, and new O2 sensor. I resealed the manifolds with new rubber and replaced the manifold to head gaskets. I have checked the fuel pressure and it is good (constant, variable and pressure retention). I checked the plug wires and seem OK. I have carefully read all the other similar problems. I have posted a similar question before but did not provide a clear problem statement. Can you suggest a next step?

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