Fluctuating idle and throttle after thermostat replaced

Hello all,

Thanks in advance for your help. I replaced the thermostat on my 97 Honda Prelude, and now the idle is bouncing by about 500 rpms up and down, and the throttle does the same when holding the gas pedal steady. I read that this can happen then air bubbles flow past the coolant sensor, but I’ve flushed and bled the coolant several times and the problem is still there. Any ideas?

You probably still have air entrained in the cooling system. Try this to see if it helps.

Get the engine up to operating temperature. With the engine idling, slightly loosen the upper radiator hose clamp. Take a flat bladed screwdriver and slip it between the upper radiator hose and the radiator hose neck. Allow the engine to idle until all the air is purged from the cooling system. Retighten the hose clamp.


Maybe you knocked a sensor wire loose.