My son's 93 Eclipse with 2.0L DOHC non-turbo engine has an idle problem. It idles up and down from 1000 to 1500 rpm. When the gas pedal is pressed, it jumps to 3000 rpm and stays there until it feels like coming back down and then starts jumping between 1000 and 1500 rpm again. He has replaced the idle sensor and it didn't help. He did have the battery disconnected all night prior to replacing the sensor, so I guess any bad habit the computer may have learned might have been cleared. He's making the classic mistake of throwing parts at a problem he's already spend $200 for the Idle sensor. I told him it was time to stop and get professional advice and pay that fee rather than an IAC and who knows what else.

Any help would be greatly appreciated, I'm a carbureted car buff and am totally lost.

 

The IAC uses info from other sensors before detemining what idle should be. Turning on the a/c, ECT(engine coolant temerature), if automatic and shifting from park or neutral to drive and even power steering are given to the computer in values in determining idle. So if your ECT sensor is bad then that could mess up idle. I'm not sure by how much. Also if you have an air leak anywhere past the VAF up to the throttle body (VAF is located just past the air filter) that can screw with your idle.