91 Honda Civic, 1.5l, remains at high idle, 1000-1500 until manually turning off by depressing pedal. Once engine rev'ed over about 1500 does it again. Will not return to low idle on it's own. Does not surge like vac hose off. No engine code. If I disconnect the temp sensor at bottom of radiator (at lower hose housing) will return to low idle but then surges about 500-1000 rpm. If I turn on AC (temp sensor connected) surges and acts like vac hose off, turn off ac and returns to high idle again. I don't care about using the AC, but need to fix idle problem so it will pass emission inspection. Car runs great otherwise without showing any signs of idle problem. Thanks for your help.

Things to check:
1. Igniter
2. Idle Air Control (IAC) Valve-- warm up car, disconnect IAC 2P connector and adjust to 470 rpm, reconnect the 2P connector on the IAC Valve, and remove 7.5 A backup fuse in the under-hood fuse box for 10 seconds. Final idle speed should be around 700 rpm.
3. ECM

91 honda high idle-fixed and passed emmissions inspection. Thanks for your help.

What solved the problem?

Disconnecting sensor, setting idle, and resetting ECU. I also changed the plugs while I was at it.

Here's something not too many people know about Civics. There's a BLU tachometer wire buried in a bundle of wires near the clutch master cylinder reservoir. It has a black rubber cap over it. Carefully pry the protective cap off and attach a tachometer or engine analyzer to set engine idle rpm. This wire comes directly from the ICM.