Has anyone seen this problem. My '85 vette idles great when I jumper the ALDL connector for 30+ seconds and unplug the IAC valve. I set my base idle to 550 rpms and idle is solid. As soon as I plug the IAC back in the idle becomes erratic. It bounces from 500 to 800 rpms. These symptoms were the same before and after I cleaned the throttle body. Any ideas or suggestions to solve this idle?

 

Have you tried resetting the IAC by driving it, warmed up at a speed over 35-40 mph? That may help. Isn't 550 kind of high for the base idle?

Do you have any modifications? With a lumpy cam less, or no contribution of idle air from the IAC seems to help prevent stalling. Just a couple of thoughts.

RACE ON!!!

 

As far as I can tell the engine is bone stock. I can set the base idle down to 450 but I really have to baby the throttle to keep the engine running.

Has anyone ever took an AIC apart? Having been an electrical engineer before I went to the pure software side I worked with several models of stepper motors. 4 wire motors like the IAC have no feedback for position. Therefore the ECM has to run it until it reaches the stops, either full in or full out. Then it assumes from design that the stepper motor has some number of steps until it reaches the other end of its travel. If somewhere along the way the motor slips then the count is off. Now the ECM is off until a reset occurs. This happens a lot on old stepper motors. The problem usually compounds itself overtime. I am not familiar with the ECM program but the smart guy would have assumed that the motor will slip overtime and program the ECM as to not care about its position but only increment when needed or decrement when needed.

Any thoughts by anyone?

 

To re set the IAC, push the pintle all the way in. You DID do this when you cleaned it, right? If not, you might have bent the pintle. THEN, you drive it to recalibrate. With the 450 base idle, the IAC doesn't bring it up to normal? If it still idles roughly with a 450 rpm base, I'd say you have a sensor sending erroneous info to the ECM, causing the idle to hunt. If the IAC weren't working, you wouldn't have the problem.

RACE ON!!!

 

Hello all,

According to the GM Training Manual 16009.10-6 GM Fuel Injection Diagnosis, "Each ignition key cycle, the PCM commands the IAC valve to seat itself, (extend) and then to move away form the seat (retract) a calibrated number of steps".

"The position of this valve is dependent on battery voltage, ECT (Engine Coolant Temp sensor), TP (Throttle Position), Engine load(MAP/MAF), Engine RPM, vehicle speed."

You are correct. There is no direct feedback circuit for determining exact position(feedback transducer, differential feed back transducer, nothing). It resets each time, however the PCM does evaluate the "counts" and a scanner can be used to evaluate the command position. I would hook up a good scanner/program and check the value at idle. Values are around 1-40. About mid range is normal, I think. Been a long time since I've looked at that one.

The 90 manual does mention resetting the IAC if it has been connected/disconnected with the engine running.
"In this case, the IAC valve can be reset by doing the following: depress accelerator pedal slightly, start and run engine for five seconds, then turn ignition "OFF" for ten seconds.

You could also have a bad IAC. If you've cleaned the throttle body, made SURE the bypass air ports are clean, throttle shaft play, no "false air", the IAC could be the problem. Not sure how much they cost.

Let us know.

dlmeyers 90 coupe zf6 3 speed shocks

 

All of that may be correct. But I think there are lightyears of differences in the electronics between his '85 and 1990. My remarks are based in my experiences with an '84. I think the '84 and the '85 are more alike than they are different. I started to write of "false air" or a vacuum leak, but it would be present with OR wothout the IAC hooked up. *I* think faulty input to the ECM is causing the ECM to send inappropriate commands. For what it's worth.

RACE ON!!!