I have a 92 Honda civic dx. And when I started the car the idle keep going up and down only if is on parking or neutral. On drive it won't do that can someone give and idea what to look for.

This link should help:

https://www.askmehelpdesk.com/cars-t...tml#post842969

Start by ensuring there's no air in the cooling system. Fill the radiator, until the coolant comes out the opened bleeder bolt. Next, check for any vacuum leaks. Then, ensure the plastic ring inside the Fast Idle Thermo Valve is fully seated.