Thread: IAC Position

Originally Posted by Reptile

In determining a good idle, I've started looking into IAC position. What I guess I'm needing to know...if the IAC is in it's totally retracted postion, will it read on the low side like say, in the 10-20 range, or will it read in the high side?

is it the more it extends out, the lower the number is?
or
is it the more it extends out the higher the number is?

I thought since it was by count it worked in an open closed state... This is a very good question, Brian.

I don't know if this helps or not.

The idle air control valve which regulates the amount of air entering the engine during idle and other times during an engine's run period (such as deceleration). There are actually two idle settings on most fuel injection engines: minimum idle and running idle. Minumum idle is the idle based on the throttle blades alone with the IAC passage closed. Running (or commanded) idle is programmed in the chip.

A GM engine's IAC is measured in steps. For example, 0 steps is all the way closed, and 160 steps is all the way open. So, for instance, if your commanded idle is 600 rpm, and your actual idle is 500 rpm, the IAC will open a certain number of steps to obtain 600 RPM.

Now, remembering that the IAC (at least GM's) works on a range of 0 (fully closed) to 160 (fully open), you want the IAC to be about 40 steps at warm idle. It is also set on a curve based on coolant temperature, with cooler temperatures allowing more steps to help warm up the engine. This is so that it has room to move both back and forth, to account for various loads, etc. This helps to keep the engine from stalling. The IAC also has different step settings for Park/Neutral, as the engine naturally requires less air when it is not under load. Think of the IAC as a smart air choke.

The IAC also provides another function called "throttle following". This function helps keep the engine alive when you floor it and then suddenly let off and close off the throttle valve. It temporarily opens to a certain step (also programmable in the chip) to curve the drop-off in incoming air. Some people disable this function, as they feel it hurts their braking power (as they are still receiving minimal engine power right after they let off the throttle).

The IAC also has a "Keep Alive" function. If it detects a drop in RPM, it will open to help "keep alive" the engine. This is why you want your warm idle setting to be at or below its half open range. For instance, if at idle the IAC was at 140 steps (almost fully open) it wouldn't have much room to open up to keep the engine running.

Originally Posted by Reptile

Some of that does make sense right there. However, I have seen the IAC position at 300 before.

300 seems like its out of the operating range. I'll see if I can dig up any specific info from my GM service manual.

Originally Posted by Reptile

That's what I'm wondering....maybe different scan tools read different IAC positions?

I can scan those regularly. I'm sure it's sort of relative, but let me see if I can give you my idle to WOT range as read by the HPTuners...and I still have to take a look at the manuals.

IAC - idle showed (5 -13) cruise 60 mph 0TPS was (60) 84 MPH 100 TPS was (115)

I have a hole drilled in my TB (stock) blade so this may have the numbers off. Looking more through the scan the IAC seemed to follow the MPH more than anything. I read a few things last night, and it serves multiple purposes so the IAC versus MPH may be just a side effect. Going after the manual right now.

The manual makes no reference to actual counts. It merely says to slowly run the idle up from 600 rpm to 1700 rpm and observe via a scanner whether or not the counts increase with the rpm and drops when you bring the rpms back down.

There are a lot of other steps to testing the IAC, but the above is as specific as it gets for counts.

Originally Posted by Reptile

I think I'm going to assume.....
That the lower the count, that means that the pindle is not extended as much. And when it's higher, that means it's out more to close the gap. It's the only relative thing that I can come up with since it's lower/open more at idle, and higher/closed more at a higher throttle percentage.

I would have to agree with you. With the HPTuners when you do a cylincer balance test, one of the things you manually set is the IAC. When I was doing this test the bigger number I put in for the IAC, the higher the engine idles.
From the manual:

Upon reset (startup): First, the PCM will seat the IAC pintle in the air passage bore. Second, the PCM will retract the pintle a predetermined number of counts to allow for efficient engine startup.