We recently swapped in a SOHC ZC into my Std hatch and the car idled perfectly in the driveway. So we decided to take it for a test drive, and got on it a little, not hard. The car is still open header because I'm waiting on my exhaust to arrive. The idle started fluctuating about 500 rpm up and down.

If we start it up cold and let it set in the driveway, it idles fine until the temp starts to get to a quarter, then it starts fluctuating. I cleaned the IACV before we put the motor in, no vacuum leaks that I can find, bled the coolant, there is no FITV on the motor so thats out.

What else should we try? The car idled perfectly until we test drove it, then it started fluctuating after it warmed up. Any ideas?

Also, no CEL codes!

 

Any ideas?

 

You might have some air in the coolant system still. If the car starts to warm up that means the thermostat opened up and if there are any bubbles in the coolant it will idle search like you are saying it does. I would bleed the system fully and replace any lost antifreeze if possible.

Also make sure, when you bleed the coolant system to make sure to turn the heat on full blast while the car is warming up as it will get the air bubbles out of the heater core as well, if your car has one still that is.