I have a 1989 ford probe lx 185k miles, and a 2.2 liter engine, (non turbocharged). It idles 1.5k to 3k rpm. When I first start the car it idles at about 1250 rpm, but then climbs to about 2000, or 2500. The only thing that was done to cause this was WE SENT THE FUEL IN BACKWARDS, because we replaced the fuel lines, and gas tank. Other than this it runs like a champ. On the road it runs very nicley. Somtimes (idling) it will fluctuate about 500rpm, and sounds like it is relieving pressure. Like a tsss after the rpm's climb. We tried adjusting it, but it did not work. I have ben having this problem, and looked all around for the answer, but was unable to find it. I do not think that it is a vaccum leak, though I am not sure.
there could be a few things but with the lines reversed, I would suspect a fuel pressure regulator at this time, the car has the symptoms of an idle air control valve but with the fuel line deal I would check the fuel pressure for being out of range and or leakdown
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