I have a 1997 Honda Prelude SH and when the car idle the rpm bounces up and down. Do anyone ever had this problem? If so what did you do to fix it?

 

I had a rich running engine and idle that was surging up and down. I had a Check Engine Light on and it was code P0118 which was Engine Coolant Circuit High Input. I went to check the sensor and it was just UNPLUGGED. I plugged the sensor back in and now it doesn't surge or run rich anymore. Runs like a purring kitten.

 

My check engine came on yesterday and the car temp gauge went to about 3/4 high. I reset the ecu and the engine light went away but the temp was still high. Just drove it again today and it was normal but the idle still boucning around.

 

Sounds like you have an air pocket in your cooling system. Did you recently get a new radiator, or change coolant?

Common characteristics of an air pocket in the cooling system is erratic idle, and intermitantly high engine temps.

The only other thing I can think of that would make your idle go up and down is if your Idle Air Control Valve (IAC) is bad.

 

Try cleaning your EGR and IAC valves...They tend to get pretty dirty

 

can you explain how to do this? EGR and IAC valve cleaning??

 

I had the same problem. It was my fast idle thermo valve