'89 Civic idling problems

[buddyo5]

My wife was complaining the other day about her Honda Civic's engine racing after starting the engine.I listened to this for about a week when I experienced it first hand. It does'nt do it when the engine is cold. We drove for about 30 minutes,went shopping,and when I restarted the car it idled at around 3750 rpm. Never could get it to lower. Just put it in drive and hoped I did'nt break anything. I noticed another thing it does. Again when starting it will sometimes surge-from 750 to 3000 rpm-over and over.
I've done a little troubleshooting. One thing I've come across is when checking the IAC valve I read an incorrect voltage and the manual says to suspect a possible ECU problem. I looked briefly at the ECU and with the ignition on(motor off of course)there is no red light indication through the little window on the ECU. Does this indicate a faulty ECU? Any help out there?

[RedRacerCivic]

same problem here long time ago, i just messed around with the idle screw and lowered the idle a bit and it stopped the idle going up and down and up and down.

[Perfectionist]

Wow, 3750 is awfully high. Normal cold idle should be around 1800 or so. Even if the IACV and the FIV were totaly open I doubt you could achieve 3750, but I could be wrong. Anyway, an easy way to check the IACV is to unplug it at idle and the idle should drop to a base idle of around 500 to 550 rpm. If it's lower than that, only then do you adjust the idle air screw. Also, during the high idle episode, you could clamp off the air hose going to the FIV to rule out the chance it's leaking.

As for the ECU, nothing will appear in the window if no codes are stored except for a brief single flash when you first turn on the ignition.

In summary, I would look for an intermittent major vacuum leak or even throttle cable binding. Also make sure your coolant is at the proper level and no air is in the system. The TW sensor can fool the ECU into thinking the engine is cold and raise the idle if only air contacts it. Speaking of the TW sensor, you may want to check it out and make sure it's OK. It's basically a resistor that varies with temperature. You can find it just under the distributor on most models.