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Fast Idle, Reving

My 1999 Civic (auto trans) idles fast at start up–drives itself until warmed up. Also intermittantly revs on its own at startup after warmed up. Throttle body was cleaned; problems persisted. Idle Air Control Value replaced, problem persists. Car has 173000 miles on it

Check for vacuum leaks.

The computer is controlling the idle, so you need to look at the inputs to the computer. You know, garbage in, garbage out. You probably have a bad sensor, most likely a coolant temp sensor, but it could be a number of others.

I’m assuming the check engine light isn’t on. You might check it for pending codes anyway, could be a clue.

First thing, the engine is supposed to idle faster at first, before it warms up. On my car the idle rpm will be around 1200 at first, then back off to around 800 as it warms up. When a warm engine revs on its own, sometimes this is called “surging”, this is often a fuel/air mixture problem. Vacuum leaks as mentioned above would be the first place to look. Since the IAC and throttle body have been verified good, then the MAF and/or a faulty fuel pressure regulator would be next in line probably. Sometimes the idle adjustment screw will be used to fix an idle problem that is actually caused by something faulty, and this can confuse the heck out of the engine computer, and cause surging. So that’s something to ask a mechanic to check, see if the idle adjusting screw is set to its nominal position.