I want to perform a IAC adjustment on my 383 with TPI throttle body.

I have the idle set to 700 rpm in the tune.

The instructions say to set the base idle to 450 rpm and once the process is done the ecm will make the final adjustments to idle speed.

With a modified engine do I still set it to 450 rpm base or shoot for a bit higher rpm?

Just go by iac counts warm. I’d suspect 25-40 engine warmed up would be fine.

I would up it to 50-75 rpm under what you want it to idle at. Then make sure the TPS is within range. I tuned a hot cam car for 850 idle. Idle with the IAC fully seated is 780-800 rpm.

I jumped A&B waited 30 seconds then unplugged the IAC and started the engine. It's idling at 1100.

So I have to back off the throttle stop screw to get it down to 650 or so, correct?

When do I disconnect the A&B jumper?

Ok figured that out.

But the lowest RPM I could get was about 950. Backing off the screw any further had the blades binding closed. I haven't checked idle counts yet.

Disregard previous, I saw you got it while I was responding. Any chance you have a vacuum leak?

unless really worn shaft shouldn’t get a bind iirc you have a 48mm tb
any vacuum leaks ?

If your throttle blades bind at closing, you're already as low as they'll go. Backing the screw out further won't have any effect. Can't. The blades are already stuck.

Might want to fix that problem first, if possible; loosen the blade screws a tad if you can, just enough to let the blades move in the shaft; and work them around until they don't bind anymore, and re-tighten. Might take a bit of 400 grit sandpaper on the blades to get them to fit smooth and tight.

Depending on your cam and whatnot, 700 RPM idle may be too low.

A better way to think of the IAC setting might be, find the idle speed you want, and don't worry about its misbehavior for the time being; then set the minimum air to 200 - 250 RPM below that. So if it turns out you need it to idle at say 800, set the minimum air to 550 - 600. That should bring the idle counts - which is, the amount the ECM has to command the IAC open to maintain the target idle speed - into the right range. You want it open "some", but not "too much". If memory serves it's an 8-bit (256 step) stepper, which is why you use the "counts" as the yardstick: the IAC will then be some fraction, that # ÷ 256, open. So if it's at say 64 counts, that's ¼ of the way open (probably too much). 32 counts would be 1/8 open, probably a better range to be in.

I haven't done anything with this yet but today I was thinking of it and got myself confused.

If I disconnect the IAC and it idles the lowest at 950 rpm, how come when the IAC is connected it will pull the rpm down to 700 at idle?

Probably because you didn't run the IAC all the way closed before you disconnected it. It was partially open at that time, then when you hooked it back up, the ECM commanded it further closed than it was while it was disconnected, to get it to the target idle RPM.

I used a jumper wire between ALDL A&B and waited 30 seconds as per the instructions.

I will check for vacuum leaks and try again. I think I found why I couldn't close the blades any more. The Cruise Control cable is too tight I think.

I disconnected the cruise cable and tried again.

I can't get it to idle lower than 950 rpm.

I jumped ALDL A&B for over a minute then unplugged the IAC to make sure it was seated.

I checked for vacuum leaks with carb spray and couldn't find any.

Data logging in park the lowest IAC count I could get was 90-100.

I am not sure where to go from here.

At this point I am not concerned about the idle speed but the inability to have the IAC function properly.

I am putting the car in winter storage this week. It would be nice to sort this out so I don't have to deal with it next year.

Any suggestions?

Why with the TB closed as far as it would go and no found vacuum leaks would it idle at 950 rpm?

The advice is to close the TB to get the idle counts down. Wouldn't you want to open it so the IAC doesn't have to open as far?

If engine speed won't drop then it's usually because engine is getting too much air. Test if the IAC is the culprit by blocking the IAC passage with your thumb (choke off the air supply). If that's the issue then you've got a stuck IAC valve, dirty passage where IAC can't seal, or you simply need to go through the idle setup procedure (setting throttle stop position, TPS calibration, and IAC setup and adjustment).

I fired it up with everything connected and set as normal. It was idling along.

I blocked the IAC port and it stalled and wouldn't start/run unless the gas pedal was depressed some.

So I should be looking at the IAC valve?

Maybe my procedure to set the IAC and TPS calibration is wrong. This is what I did before:

- jumped A&B with a wire
- turned ignition on and waited about a minute
- unplugged the IAC connector
- turned ignition off
- unplugged the A&B jumper wire
- Started the car and adjusted the TB stop to get as low as idle I could
- Turned off car, plugged IAC back in
- Adjusted the TPS to .54v
- drove around town multiple times

That aside if the car chokes and dies with the IAC blocked, am I correct to assume the IAC is not functioning properly?

Yes, since you blocked the IAC port, the TB plates weren't open enough to allow the engine to idle. Every time we start a TPI, the IAC starts closed and then pulses (counts) on its way to more open to find the set idle speed. With the IAC totally closed (not functional), the engine should barely idle, around 450 RPM with a stockish cam. TPI ECM's usually like 600 RPM idle (?) speed (or something like that), so we start the car, the TB plates allow for 450RPM and the ECM quickly pulses the IAC open until it gets to around 600 RPM. Those are the counts. The ECM assumes the IAC moves when it is pulsed but if the IAC is stuck it doesn't move and the ECM keeps trying to raise the idle. The IAC may stick which makes it very hard for the ECM to regulate idle speed.

There would be nothing in the tune that would mess with the IAC, would there?

I did some more testing. I unscrewed the IAC, jumped A&B on the ALDL and turned the key on. The plunger didn't move at all. But then I did crank and start the engine and the plunger shot out of the IAC. So now I am confused.

I can’t think of what would cause that tune wise. Unless something in the wideband tune is causing it but the command should out the iac. For testing do you have a stock memcal or I forgot if we did non wideband tune you could revert back to

If I remember far back enough it did the almost stalling when coming to a stop right from day one of the 383 swap. I think there were a couple non wideband tunes at the start and the almost stalling was one of my issues. So I don't think it is wideband tune related.

The only reason I brought up the tune, is that with my 1989 Mustang that I have someone remote tuning, we are fighting a IAC/Dashpot issue. On that one the rpms were hanging too high, then now are falling too quickly. lol.

Finally Success!

You all will think I am a idiot..... During the procedure I wasn't actually jumping ALDL A&B.... I was jumping A&F.....what a dope.

Anyways after successfully setting the idle to 650-700 during the procedure the stumble when coming to a stop seems to be gone. I only drove around the block once though.

I then set TPS to .55V

It was weird. I had to open the TB quite a bit to get it to idle at 650-700. But in doing so, the IAC Counts are down to 0-3, and 25 with AC/defroster on. The manual states 5-50 is acceptable. But that is at 450 RPM on a stock motor.

Should I close the TB blades ever so slightly to get a few more IAC counts?

Take it for a spin,?if it runs well , doesn’t did on deceleration. Send it

LOL! I thought I was the only one that made those days-long, head-slapper rabbit-hole mistakes. Glad to hear you got it. You also now have an intimate understanding of the IAC. Lol.

For sure.

I have to wait until tomorrow, but I am hoping my cold start issue is gone also. When I started it cold (less than 60 degrees), it would start good, but then stall out. On second attempt it would stay running. Now that I have the throttle stop screw turned in almost 4 full turns from where it was, the TB blades are open a lot more. That should help with the cold start.