I've been having a problem with my 1991 Jetta where it intermittently runs/idles rough. This occurs both when I'm driving and when the car is parked and idling. It seems to me like a vacuum control issue. So far, I've replaced a couple of hoses, the breather hose and the hose from the throttle body. This did not solve the problem. I checked the air flow sensor's strator flap, and it moves freely. According to my Bentley manual, I tested the resistance across the terminals of the potentiometer. The resitance between terminals 3 and 4 was correct, as was the resistance between 2 and 3. When I tested the resistance between terminals 1 and 4 (which the manual says tests the intake air temperature sensor), it measured infinite. Could this be related to my problem? It does seem my problem may be related to the temperature. I live in Pennsylvania, and the temperature has recently become cold (30 - 40 degrees Fahr).

 

did you try cleaning idlecontrol valve