IAC valve - hanging idle

[bhuff30]

I recently replaced the IAC valve in my 97 GT with a new valve. The old one would stick closed on cold starts in the winter, but would work properly if cleaned.

Anyway, the new IAC valve seems extremely overactive. The idle will occasionally hang around 2500rpm for 5-10 seconds before settling. I also notice the engine pulls me along for a few seconds after letting off the gas, clearly because it is trying to idle faster than the current speed. This one annoys me the most. If I push the clutch in at that time, the engine speed will climb to 2000rpm and settle quickly to a normal idle.

I thought the computer just needed to learn the new valve, but I have driven it about 1,000 miles so far. So what do you guys recommend?

Try a different valve?

Disconnect the battery and force the computer to re-learn? (or does the computer not learn these functions?)

I have seen an IAC mod here before where someone recommended putting a restrictor in the IAC path to reduce idle airflow available to the valve, but that seems like such a band-aid fix. Any comments or experience with this mod?

I have a twEECer which I could re-tune the computer with. I have played with the idle settings before, but never found the right variable to reduce this surge. I have only found the dashpot to force the idle to come down faster. Can anyone point me to variable or table names to look at to reduce the peak surge when you let off the gas?

 

[joshjwc9]

Okay, I had the similar problem as well and here is how I fixed it, supposedly it is a combination of a sticking IAC as well as the dashpot for the TPS. I had a sticking IAC, where if I hit it with a screwdriver it would idle back down. I replaced the IAC and reset my battery. Still no luck, so called my tuner who simply widened the dashpot for the TPS reading idle voltage/percentage.

How this helps somewhat.

 

[bhuff30]

I found a setting in caledit for the dashpot preposition. I lowered all the values by 20% and got rid of much of the surge when letting off the gas or pushing in the clutch. My impression is that the dashpot preposition dictates how much to open the IAC when the engine is not at idle. The idea is to flow enough air through the IAC so that if you lift off the gas, the engine braking doesn't slow you too quickly, nor surge and pull you forward as in my case.

I still get an occasional hanging idle at 2500rpm after giving the gas a good rev, but the car is much more drivable now.

 

[trinity_gt]

Are you able to visually compare the old and new IAC valves? It almost sounds like the new one is not the correct one for your engine, perhaps with larger internal passages or differing plunger dynamics etc. Odd that you would need to alter parameters to such an extent for a valve that is supposedly a drop-in replacement.

 

[bhuff30]

The new part obviously isn't a ford OEM part, but they look similar. I didn't take measurements or anything. I would guess it is because the replacement wasn't made by Ford, so it isn't quite an exact fit. Ehh, cheap parts.

 

[joshjwc9]

It was explained to me that it could also be the TPS sensor, the TPS sensor is getting older, there is a chance it isn't seeing the throttle blade completely closed, or the actual blade might not be sealing completely (as if someone used TB cleaner in there). But in my car it also was the TPS. At idle, the TPS was saying the TB was at 16% (or something like that), so the IAC was pumping in extra air to kick the idle down without helping, he widened that dashpot to 17%ish and now the car idles down unless the ambient air/AC/defrost etc. are causing it to idle up.