I put my 99 Honda civic EX motor in a 96 civic. I've been driving it every day for a few months, but since day 1, its had an erratic idle at start up. I did some reaserch and found it could be a dirt IACV or air in the cooling system. I cleaned the IACV and it did nothing. Makes sense because it didn't have an erratic idle when the motor was in my last car. I obviously disconnected my cooling lines however when I did the swap. Can someone tell me how to check for/alleviate air in the cooling system? I have no idea.

 

Any CEL codes?

Have you checked for vacuum and intake air leaks?

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Bleeding the cooling system may take 20-30 minutes:

1) Park the car on an inclined driveway with the front end higher than the rear.

2) Push/turn the dash heater lever/**** to MAX heat. Fan need not be on.

3) Follow the directions in the diagram below

 

a bad iacv... same problem with my hatch i use to have

 

i dont know if this engine has one but since you said only at start up have you tried adjusting/ cleaning the fast idle throttle valve (fitv)?

 

I kind of looked into the fitv but not sure if this motor has one. was unable to find one anyways. I swapped the IACV for another one that didn't have an erratic idle either. and still same problem.

there are no CEL codes and i don't think theres any vaccum leaks, thoi'll check again. Intake? you mean the intake manifold gasket? don't think I ever took the intake off so it should be fine. I did take off the exhaust manifold. what would a leaky exhaust manifold cause?

If there was air in the cooling system, wouldn't it find its way out after several months?

 

No FITV on that engine.
Nope. Take the 20-30 minutes to bleed it. Air can cause erratic idle and overheating.