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Have a 1998 honda civic ex with 1.6liter automatic.

Customer drove to shop with head gasket leaking coolant and exhaust gas in antifreeze. Had machine shop rebuild and repair head. Put car back together; yes, battery was pulled out of car. Car starts, doesn't overheat, but idle fluctuates up and down, wasn't doing that before. Re-checked vacuum lines, wires and connections. NO check engine light. Ran across a TSB for an idle learn procedure, seems not to work or apply to this vehicle.

Any ideas? Plus can't locate a memory backup fuse either on vehicle, like dealer suggested we pull to reset idle....

I would look at the cts. For some reason I've seen these roam if there is a sensor issue. The other issue is a vacuum leak or one I'm not proud of admitting, on the rear firewall side of engine you might locate two sensors close enough together that will accept incorrect plug for that sensor. Not sure if it's this particular year or model but it took me more than a few hours to resolve that one. I personally didn't do the work, scanner didn't pick up on it, I stumbled upon it by noticing the wrapping looked stretched and thought it was routed incorrectly and possibly a lead was pulled back in the plug. It may have been attached to the EGR vac amplifier, can't be sure but I was so happy I resolved it and so mad it happened. Of course I blamed the manufacturer for allowing this to happen and am convinced some engineer intentionally did this and is still laughing. If you look up in Identifix, Direct Hit, IATN, etc I would sue the key word roaming, I think it would suit the situation better. Just my personal thought.

OK headache car update...We checked all connections all appear ok..NO crossed wires.. Inspected for vacuum leaks at intake,none....Changed ECT sensor.. Car idles fine cold...But when gets close to operating temp, idle starts surges, up and down. Car won't idle...??????????? 500 rpm to 2100 rpm...

I've had this problem in the past on a few hondas. I can't honestly tell you this will be the cure but we suspected an air pocket in the engine. Our best guess was around the ECT. What we did was use a anti-freeze flush machine and it seems to resolve the problem. I hope that helps you.

This is common on older Hondas. Usually it's the IAC valve stuck and can't bring the idle down. Pull the IAC valve off (sometimes requires removal of throttle body) and see if the valve moves freely. This type of IAC uses a magnet to move a cylinder in a bore to vary the idle air flow. They often get carboned up and sometimes you can clean them. In worst cases replacement is necessary. Sometimes tapping lightly on the IAC while running will get the idle down. If it does, there's your problem.

On the last one I did, the computer was cutting injector pulse to bring the idle down. This created an interesting drivability problem into the mix. Toyotas use the same system and I've had a few on those too. Even Mazda I think.

There are other possibilities, but the IAC is the most common.

Found sticking iac motor...cleaned and freed up with a little bit of oil... running fine.. thanks guys.....