A friend has a 1993 Civic DX with original/stock 1.5L motor. The car gradually began to idle up, and down. It seems to rev up to 3000rpm or so, then down, etc. We checked and the vacuum lines are OK. The motor seems to be in good shape, otherwise. I am thinking it is a sensor of some kind. Any suggestions would be great.....Thanx

 

Check your Idle Air Control Valve (IACV). It's located on the back of you intake manifold. It has one coolant line running into it and one running out of it to the throttle body. Sometimes those go bad and can cause a fluctuating idle.

 

Yep, could be IACV, bad PCV valve, misadjusted timing, etc etc

 

Also, if you have changed your coolant lately. It could be air bubbles in the coolant lines. Try and take off the radiator cap and let the car run with the heater on for a couple of minutes. This will let all of the air bubbles in the system to escape which can solve the problem. If not that then most likely the IAV. Good Luck.

 

also check for vacuum leaks

 

Good idea's!! I will check all of that. Thanx for tha tips!!