Anybody else have this situation with IAC


New Member
Jul 22, 2001
It seems the range of control on my IAC is out of whack with the needs of my motor. I adjust the IAC and throttle blades so the IAC barely bounces on the bottom of the graph while the engine is hot and the car in neutral. I set idle speed about 950 to 1000. Drop in gear and the idle maintains 950 and the IAC control is right in the middle of the range, right where it needs to be. Now when the engine is cold and it starts up the IAC goes wide open and I have poor starting and very low idle. In fact it has a very hard time staying running at all. There is no way on the throttle body to enlarge the feed hole any further due to the way it is drilled. It just seems as if I need more total CFM. That or I have something somewhere out of whack. I am 99% sure the IAC is fully open as I have visually checked it and also can hear the ariflow as it opens and closes. Plus you can feel it when you put your finger over the hole, of course.

Any thoughts or similar situations out there?
Try this: (from a prior thread)

"Got two minutes?
OK, here's the info you've been waiting for...

With the car warmed up, look in the throttle follower graph. First, set the graph to its minimum setting at your lowest TPS position. So if your TPS reads 15% at idle, then make sure that the line on the throttle follower table is at the bottom of the table @ 15%. Now, adjust your throttle blades until the IAC target shown in the dashboard at the bottom of the screen shows about 20-25. You should now see the cursor hovering slightly above the bottom of the screen. Set the rest of the graph to open the IAC motor as the TPS increases. Last, make sure that the "Maximum TPS for Idle" parameter in the Idle Speed section of the software is set to 1 number higher than your TPS reading at an idle.

What you just did is make it so that the IAC is barely open at idle. It now has its entire operating range to open up for cold idle, startup, extra load on the engine, etc. I suspect it will help your problem.

Good luck. Let us know how it goes!

Craig Smith
FAST Electronics"

Also pay close attention to your voltage @ startup. If its low it could also be part of your problem based on what I've seen.


Maybe you'll have to open the throttle blades so the idle actually sits above setpoint in neutral (50 - 100 RPM), then let the IAC catch the engine when you throw it in gear.

I think I saw that Holley just came out with a 4-bbl TB with an enlarged IAC hole, I suppose it's for this very sort of issue. But I still would wonder if something else is going on (timing trim?).

-Bob Cunningham
JD, have gone through that excercise and what will happen is idle is way too low on start up. Can't keep it running.

Same thing there Bob. If I set the car to idle in N it does not seem to have enough range on the IAC to hold idle in gear, even though I have a 3000 stall converter.
Well a partial fix might be some timing trim- try adding 5-10 degrees or more for a 40 RPM error.

What is your timing at idle?

If you had a carb, I would think that you had a vacuum leak. That shouldn't be an issue here, but just one more thought. What is your TPS position at idle?

-Bob Cunningham
Porting the IAC body and radiused inlet and outlets should be worth a few cfm. I would be interested to know how that works for ya. I'm guessing you are running lots of cam that doesn't provide much vacuum and "that" makes the IAC even less effective.
FWIW- mine is "borderline" on my 4 banger and I've been considering a little porting, myself. Please let us know what you come up with.

EEEK, that timing trim deal....just kidding. I tried that and it doesn't do a thing Bob. My car likes around 25 degrees of advance at idle with the new cam I have in it 258I/268E duration @ .050.

I went around and around checking for vacuum leaks and never found a thing. It does give me the vacuum I figured it would, about 9"-10". TPS position @ idle is 12 and max tps is set for 13. It always closes to 12 as that was the first thing I thought to check, to make sure the computer knew it was supposed to be at idle.

What I think I have to do is start up the motor and climb over the side of the engine and look down the IAC hole to MAKE SURE that sucker is opening up all the way.

JD, I thought about opening up the IAC hole but the amount of room is little to none. Any bigger of a hole and it will be into the main throttle body area. The hole is smaller than the size of your little finger basically, or the size of it depending on your hands.

I have always had this problem and now I am just trying to figure out why it is a pain in my but.

I'll post if I figure it out in the next couple of days.
I'll just throw this out here, not sure if it will help, have you tried changing the Start IAC Position in the FAST? Maybe raising the start IAC postion will help with the cold idle issue.
OK, verified IAC doing its thing and working the way it's supposed to. So I basically revamped all the start up screens and I think I got a couple of things resolved. Basically what I think is going to help the most is I added a ton more afterstart fuel, about 45% on a cold startup, and I also increased cranking fuel when cold to about 23ms, which seems high but that's what it seems to want. I delayed the afterstart enrichment for about 15 counts and it seems to catch and begin smoothing out in a reasonable fashion. After it warmed up I reset the throttle blades to 25 at my target idle of 1020 and I'll see what happens tomorrow on a cold start.

Thanks for all the help guys.
Do yoiu have a spacer of any sort under the TB? I fought that for a while once. The passages under the TB that fed air to the IAC were being restricted by the dividers in a 4 hole spacer.
Craig, I don't have a spacer under the TB. I am so tight under there with my air cleaner, even with the cowl induction hood I had to drill the nitrous spraybars right into the plenum of the intake. Anyway, I finally got it figured out. This made no sense unless I sat there and thought about it for about 24 hours but I had to close my starting IAC position a lot and tweaked the pulse widths up a few notches. The hot/warm start is finer than a frogs hair and I think things are pretty close on the cold start. I don't think I'll ever get the cold start to cold idle smooth with a 258/266 duration cam in there. Its way better than with a carb, thats for sure.
Try logging the iac target position , and you can watch it on start up, go down to see if its in range cold, i would use about 150 for the iac start position cold and bring it down to about 100 warm. Turn up the iac p gain to about 30, and the d gain to about 20 to make the iac react faster, but not too fast to cause surging at idle from over sensitivity.

Try it, see how it goes

Norbs, If I set the throttle blades so the IAC is not locked wide open on cold start up then when warm the motor idles at 1800 RPM. After putzin with this for a bit this past week I have great hot starts, IAC @ about 20 and idle about 1000 or so. Just about right. On cold start up it is starving for air after start up. IAC locks wide open and it idles cold for about 10-15 seconds at about 800 and slowly creeps up. I have the IAC start postion cold at max and in the 150 range hot.

I am convinced the IAC bypass is just too small.