'87 Idle Issue

[llangston1]

MY '87 Carrera has started experiencing some sort of idle speed issue. It starts fine and runs good until I come to a stop light. With my foot off the gas and transmission in neutral the engine idles very smooth just slightly above 800 rpm (first small mark below the 1000 line) for about 20 seconds. Then the idle speed drops below 800 (a full needle width or more) the engine runs rough and the idle fluctuates for several seconds. Then slowly the idle creeps back up to 800 as the engine smooths out and seems to stay there until you pull away and reach the next stop. Then it starts all over again. This just started in the last few months. It doesn't seem to matter if the engine is warm or not. The engine has 99500 miles and the oil usage seems to be slowly increasing. Is this a fouled plug or vacuum leak? Any suggestions from the forum on where to start diagnosing the problem.

[twistoffat]

Been there, had that. The first suspect is a sticking idle control valve. The pin inside sticks and causes the rough idle. When you have the problem at idle give it a sharp tap to see if it corrects the problem. If it does then you know where your problem is. If its not the ICV then the next step is to check for vacuum leaks.