Occasional High Idle Speed - $8D 383 fully modified TPI

[EagleMark]

EagleMark, You may be correct with the IAC Valve. I finally got a PC connected to the car. The target idle was 600 rpm and the engine was running at 850 - 900. The IAC count was 59. I shut the car off and restarted and the idle went down to 600 and counts were 37. When the IAC hangs, does the counts hang as well? I would think the ECM would pulse the IAC to change and count those pulses, regardles if the valve moves or not. I did not think the IAC gave any feedback of its actual position.

Like Robert said... seems we are finding more and more IAC issues lately or guys are just beginning to see idle issues and notice IAC counts? I think the count is going up because the ECM is commanding more but the valve is not moving or getting stuck in the move?

The one I did hands on last week did the same as yours. I still have the bad IAC here and going to tear it aprt to see if it is stripped? Or if I can see what is happening... I think 30 year old IAC are just wearing out!

[zaut]

I can agree that if the ECM commanded a move, it would assume it was made (no feedback). The count would be what the ECM thinks the IAC is at. However, (please correct me if I am wrong) the counts are not absolute. The ECM counts up and down. What I have seen is low counts meant the IAC was closed or low idle RPM, and high count meant the IAC was open. I would think that I should see a very low count when the IAC was stuck open and the ECM was trying to close it. The data I am missing is what are the "normal" counts for the IAC at any one RPM. Maybe 59 and 37 are very low counts, below what should be normal. I guess I have to make a $50 gamble on a new IAC or keep looking to see if it is anything else. Since my engine is highly modified, I am guessing no one can say what the normal count should be for my configuration. And YES my IAC is 27 years old so it could just be tried. One thing I have noticed since I completed the rebuild was that at cold startup, until a fixed amount of time, the IAC would stay at a fixed number, I think 150. Is that normal? Is there a way to adjust that?

[EagleMark]

There's a max count of usually 145. Then a park count and depending on ECM the IAC will close all the way and go to that position. Well not if the IAC is stripped.

[zaut]

I replaced the IAC last weekend. I am still getting the high idle. I am now wondering if the canister purge valve is sticking on. I will unplug it for a few days to see if that is the problem. Unfortunately I haven't had the high idle when I had the computer hooked up to watch the IAC counts.

I did notice another issue when I had the computer hooked up. I fall out of closed loop occasionally. The O2 ready goes away at the same time. Is this indicating a bad O2 sensor?

On a up note, I finally got the car to the drags Saturday. It run a 13.70 quarter mile with street tires. I was hoping for low 13sec to high 12sec. Since this is my first chance at getting good log data, I am very happy with the 1/4 ET.

I was seeing some knock retard occurring at high MAP in the 3200 to 4000 rpm range right after shifts. I need to make adjustments. But I want to remind EagleMark that the file I put together for using the EST module instead of the memcal knock module works. I would hope you would consider adding that file to somewhere on the site as a reference. I uploaded it again with this post.

[zaut]

Does anyone know what "IAC Command speed open loop offset" does? This bin I started with has that value at 1000 rpm. I am not sure what it is for so I wonder if that may have something to do with my high idle speed. Also how does "IAC Learn" affect things, what does it do?

[JeepsAndGuns]

Where is your o2? Is it in the stock location, is it the original one, do you have headers? I dont know anything about the chevy cars, is it a heated sensor? If you have headers and its not a heated sensor, that might be your problem with it dropping out of closed loop. The o2 is cooling off. You could try wiring up a heated o2 sensor.

[zaut]

I have Edelbrok headers designed to replace the stock manifold. I was thinking that the O2 was cooling off too. Is that common? It happened when the car was idling at normal speeds. Can you recommend a heated replacement? (year, model, etc) I was thinking about doing that anyways because I can get the car in closed loop quicker. I understand there different types of O2 sensors, one I have heard of is a wide band O2 sensor. Not knowing the difference, I have been hesitant to change it. Thanks.