Can't Control airflow with IAC below 130 steps

[Ethan_Bill_Bennett]

Ok so I have been tinkering with the idle on this car for quite some time now. I have the configurations set up to dial in the IAC and Idle air flows but I just can't get the IAC step counts to go below 130ish. If I open the blade just a fraction, the idle trims kick in, the step counts go to zero but the dynamic air stays constant. Almost as if the IAC plunger is not sealing up in the port. If I close up the TB a little, the IAC has plenty of control but the step counts go to about 150ish for idle. I have a FAST 90 mm throttle body. (waste of money) The throttle body is not drilled. I took the Throttle body off to see if the IAC plunger was sealing up but I assume the IAC is not at 0% when I shut off the key. How do I make the IAC go to zero before I shut it off so that I can see if it is sealing up in the port.

[Ethan_Bill_Bennett]

Anyone?

[flame]

If you cause a fairly big vacuum leak the iac will close to bring the rpms down then unplug the iac before you shut the car off

[SSsuperdave123]

Seems like I used the VCM controls to drive it closed with key on, engine off.

[eficalibrator]

A lot of aftermarket throttle bodies (and even the "ported" stock units) sacrifice IAC airflow in the name of total flow through the blade. If you look at how smoothly the flow is directed toward the blade's throat (and NOT the IAC), you'll see that it actually makes it tougher for the air to go through the IAC port. Now compare this to the stock throttle body where there is a significant "step" in front of the IAC inlet area. This was done on purpose to make sure that the IAC functions correctly. Many aftermarket porters and parts designers completely missed this point and sold hardware that make tuning idle control much more difficult.

[Ethan_Bill_Bennett]

Thank you Greg. I ended up having to enlarge the IAC bypass port with a dremel. This helped my issue significantly. It should still be bigger but I don't want to tune the IAC steps vs. effective area table again. It is a Pain in the A**. You are right about the aftermarket TB's. All the ones I looked at were constructed the same way without the IAC in mind. I am sure you can make it work on a 6.2 L with a less agressive cam. But I am running a .650 lift cam with 19 degrees overlap on a 7.0L so my engine is is hungry.

[Ethan_Bill_Bennett]

Since we are on the topic, last two nights I went out to start it, I had it tuned perfectly the day before and the next day it says it isn't getting enough air. It warms up and the step counts go 100 and it just dies. Like the PCM is commanding 100 counts but the IAC isn't actually going to 100. If I open up the TB a little and drive for a while, the dynamic air goes higher and I have to close the TB again... as if the IAC was stuck before and it gets heat soaked enough to where it goes unstuck. So my question is, even though the PCM is commanding a certain step, does it actually know what step the IAC is at? Is it possible I have a sticky IAC?

[eficalibrator]

Cold engines have more friction and require more idle airflow than warm engines.

If the airflow is biased away from the IAC, it can be difficult to get enough flow through the IAC, regardless of how far open the valve may actually be.

Do you have good control of your air-fuel ratio at all temperatures? Do you have sufficient spark adjustment (torque reserve) available at idle?

[Ethan_Bill_Bennett]

I added a full 6 degrees of spark advance in the idle region, from 22 to 28, and I seem to have plenty of torque. I have good AFR control at all temps because I have tuned the MAF and VE tables in those regions with my widebands, via instructions from ur vid. The engine was running perfectly when I shut it down and I know it didn't do any sort of IAC or TPS relearn while it was off because I do that procedure just about every time I work on the idle. Tip in was good, going in gear, forward and reverse, was good, no dipping or overshooting when I tap the throttle, dynamic and desired air flows were right on top of one another, IAC steps were about 63 in P/N, 100 when in gear and rolling, and about 160 when in gear and rolling idle isnt activated yet. It was just perfect. Then I started it back up the next day and The IAC behaved the same way on the scanner but the whole thing went to hell. It warmed up and once it hit 100 steps on the IAC, about 63 degrees celcius, the engine died and wouldn't start unless I held the throttle open. Then when it warmed up I would put it in drive and start rolling, about 2 mph, and the IAC drops to 100 and the dynamic air dropped and the engine died. Only way to make it run was to open the TB a little.

[eficalibrator]

What happens if you unplug the IAC while it's running?

[Ethan_Bill_Bennett]

Don't know yet, I'll try that tonight.