What causes fluctuations to the idle when the car is in park?

Answer 1

A vacuum leak is the most likely cause. If you have an aftermarket intake/header/exhaust this could cause mild fluctuations as well. First things first - I'd check all vacuum hoses and any engine components that are used to regulate or are affected by vacuum (PCV valve, etc).

More Information:

If your car revs up and down at idle, there are a few possibilities. My car did this for over 2 years. It could be a vacuum leak or an idle air control valve. The problem that I found with mine was the small valve under the throttle body, it looks a lot like the idle air control valve which is located on the back of the intake manifold. I went to Pick n Pull and got a valve cheap it fixed the problem and the car no longer revs at idle.

I replaced the vacuum hoses on my 92 teg and the car runs perfect now. make sure to get 1/8" vacuum hose from the auto parts store, take each vacuum line out 1 AT A TIME! this way you can measure each piece, cut new line, and reinstall new piece BEFORE going on to the next piece. with this method it is impossible to accidentally switch the vacuum lines.

My 94 Integra was revving up and down from 700 to 2500 rpm while in neutral. It was my O2 sensor and unfortunately with my car the sensor was attached to the throtle body and you must replace the entire thing which is extremely expensive. My mechanic had an idea and for 30 min of his time he 'ghetto rigged' the sensor and body by inserting a screw into it and the problem was fixed, I can also now change the idle with that screw as well. It has been working great for the last few years.