I have a 98 Civic LX, throwing a CEL for IAT. The car used to lobe hard core at idle, so I bypassed the IACV by blocking the ports so only enough air bypasses; it stays around 770 all day, hot or cold. Well when I am driving the cars rpms will sort of bounce up and down if I am at constant throttle, very low load conditions. It throttles up nice when the pedal is depressed, but if I am in neutral I can't make it run slowly and smoothly thru the rpm range. It will sort of two step around 1400-1900 and then jump up once the pedal is further down. I heard maybe the IAT sensor could be at fault...seems the motor would go in limp mode or run richer, not have a issue under lite load conditions.
Any ideas? Had this happen to you before?

 

The IAT could possibly be the problem and should be an easy fix. Is the sensor connected? If so, try cleaning it.

In what condition are the plugs, wires, distributor cap, and rotor?

 

Just put in new plugs, new IAT sensor, dizzy and rotor are recently replaced. Ran another scan, now the Idle Air Control Valve is throwing a code. I thought this valve was only used at idle??

 

Like isn't the TPS and MAF/MAP in charge of the rest of the conditions? Like 20% constant throttle should run perfect right? I understand if it jumped up and down off throttle but thats not the case.

 

There's more than one OBDII IACV code. Which one is being thrown? You may want to bring the IACV port in the TB back to stock.

 

P1509 Idle Air Control Fault