Msutoad,

I know a little about GM MEFI (version 3, which is probably older than what you have but the basics are the same). There are a few easy checks you can do yourself.

1) Make sure your throttle valve is returning fully to the stop when you return your throttle lever back to the neutral/idle position. The throttle valve stop should be fully against the stop on the throttle body at idle. If not, you could have a weak spring on the throttle valve, a binding throttle cable, etc.

2) The ECM actually uses both the IAC valve and the ignition timing to control the idle speed. If you have the jumpers to put the ECM in service-mode and an inductive timing light, check the base timing. I think 10° BTDC is normal for the GM 350 with EFI.

3) To test the IAC valve:
a) With the engine at normal operating temperature, note the engine RPM at idle.

b) Stop the engine and disconnect the IAC wiring connector at the IAC valve.

c) Restart your engine and note the engine RPM. The engine speed should be a couple hundred RPM higher with the IAC valve wiring disconnected.

d) With the engine still idling, re-attach the wiring connector to the IAC valve. The engine speed should decrease close to the original RPM noted in step a.

e) If the engine speed varied as described in steps c & d, the IAC valve is probably OK.

f) If the engine speed did not vary, then, using a 12-volt test light, with the engine running and the wiring harness disconnected from the IAC valve, test each of the four terminals in the IAC valve wiring harness. The test light should blink when connected to each one. If so, the IAC valve is probably defective. If one or more terminals do not cause the test light to blink, there may a damaged wire in the wiring harness, a poor connection at the ECM or a faulty ECM.

4) Inspect the distributor cap and rotor. The aluminum contacts inside the cap can get very corroded.

Good Luck.

--KG--