Ozzstar

Need some IAC help with my '93 LT1, automatic transmission. 60,000 miles. Engine runs great...no missing, hesitation or stalling either at idle or under load.

Car is idling smoothly around 950-1000 according to the tachometer with no hunting.

Checked for vacuum leaks including all hoses routed around the intake manifold and the brake booster. No leaks found. Throttle plate and IAC internal port has been cleaned thoroughly.

I removed the IAC valve and found just a little bit of carbon on the pintle, and I cleaned it before reinstalling. The pintle was out 30mm (1 3/16"). I seated that to 28mm as per factory service manual. I then installed the IAC....held accelerator down slightly , started engine and released the gas pedal, let the engine run for 5 seconds...shut off engine for 10 seconds, then restarted. Same idle 1,000 rpms.

Removed the IAC again and found it was back to the 30mm mark. Pushed it in to 28mm, reinstalled, same result....1,000rpm

With the IAC back in, engine off, removed the IAC connector. started car HIGH idle...2,000+ or so. Shut engine, reconnected IAR, started engine, back to 1,000 RPM

Tried the reset 2 more times, same result.

What is your professional advice? DO i have a bad IAC? It appears to push the pintle out but wont retract, or is it pushing out past 30mm but not returning in more than 30mm, or maxing out at 30mm? Thoughts?

I am not a 'throw parts at it, till its fixed guy'. I need to do whatever testing is necessary to find exact evidence leading to the high idle problem.

Thanks - Ozz

JackDidley

Sounds like you need to datalog to see what idle speed is being called for what the ecm is commanding the IAC to do.

Ozzstar

I was told by the company that sells the scanner software I have that it won't work for the OBD1 protocols, just OBD2. I was going to buy a DTC adapter that went from the 12 pin port to the OBD2 16 pin. It would be nice to know what IAC counts the ecm is commanding.

Joe C

so, am I to assume that your only issue is a fast idle? when you say, - explain - did you remove the TB from the engine, or an on-engine cleaning? provide some details on the IAC cleaning. did you use some sort of carb/choke cleaner on the IAC? pretty sure these things don't take kindly to cleaning solvents on the internals.

Cliff Harris

The idle speed is controlled by the ECM using parameters and tables that are programmed into it. The exception to that is if you have a vacuum leak. In that case the ECM does the best it can to maintain the correct AFR.

The IAC is controlled by the ECM and the ECM uses it to control the idle speed.

The IAC does not have a mechanical calibration. The only setting for installing an IAC is to make sure the pintle isn't extended too far. In that case it's possible to break the IAC when the pintle bottoms out in the throttle body passage and you continue to screw the IAC into the bore (happened to me). There is a pair of grooves in the IAC pintle and it rides in a plastic guide with two ridges on it that keeps the pintle from turning as it extends and retracts. It is the plastic guide that gets broken.

The ECM calibrates the IAC at every startup and shutdown. That is the reason for the start for 5 seconds, etc. procedure. Instead of trying to set the IAC to some specific length, just push the pintle into the IAC as far as it will go. Doing the start-stop-start sequence will put it into the correct position.

Ozzstar

Throttle body was removed. The steady 950-1000 rpm idle was occurring, so the first thing I did was remove and clean the throttle body properly before looking into the IAC.

Ozzstar

Thanks for the info I do understand how it works. Now I need a proper way to determine the IAC is at fault.

Cliff Harris

Data logging will tell you what the IAC is doing.

It's not all straightforward and simple. Look for "throttle follower mode". That extends the IAC as the throttle is opened. it helps prevent the engine from stalling when you take your foot off the gas pedal and the throttle blades slam shut.