K87ZZ4

I'm experiencing an intermittent rough idle problem. I've been using some scan software to monitor engine data. Your help would be appreciated :yesnod:

Normal operation: In the first several seconds after startup, the IAC Target is 130 and the Present IAC Position is in the 30-70 range. After that, the IAC Target goes to 0 and the Present IAC Position is in the 25-40 range.

Rough idle operationIn the first several seconds after startup, the IAC Target is in the range of 0-2 and the Present IAC Position is constant at 145. It doesn't change after that, the IAC Target stays in the range of 0-2 and the Present IAC Position stays at 145.

The maximum IAC Position or Target values I've seen are 145.

It seems to me that the IAC Position is not affected by the IAC Target, yet it seems to idle properly, anyway. I would kind of expect that the IAC Position would move toward the target and, as things settle down, the position would match the target. But, it doesn't seem to work that way.

So, I'm wondering if the IAC Valve is getting stuck when I have the idle problem? Or, is it maybe the IAC Motor that's not working properly and not moving the IAC Valve to the right position?

I guess this hinges on what the IAC Position really means... is it feedback from the IAC Valve position or feedback from the IAC Motor?

Are the numbers I'm seeing for IAC Target and Present IAC Position normal? Is anyone else seeing the same, or different, values?

BTW, I was monitoring data today and while cruising through a school zone with my foot off the accelerator, I noticed the car was increasing speed slightly (it went from about 25-30 MPH while my foot was OFF the gas). I noticed that the IAC Position was in the 70-80 range (the car had been running for probably only 2-3 minutes), so it seemed that the Idle system was feeding the car enough fuel to actually accelerate.

Later, when I was on a back road, I let off the gas again while cruising along about 25 MPH. The IAC Position was about 15 (and the car was slowing down), but then the IAC Position would "jump" to 32 and I could feel the slight surge. It would spend a few seconds at 15, then bump up to 32 for a few seconds, then back to 15 (back and forth, several times, before I ran out of road ;) Is this behavior normal? Or, might this be another symptom of a problem with the IAC Valve or the IAC Motor or maybe the ECM???

Also, when I accelerate, I notice that the IAC Position increases to 70-90, or even over 100 if I get into it a little harder. I figure that is normal, since it's probably helping to dump fuel in to improve acceleration, but I'm not sure.

Thanks for any help :)

Note: My related topic (Won't start or idle) was getting kind of long (of course, this ended up being kind of long, too), so I tried to simplify it by focusing on the idle system question in this post.

K87ZZ4

A couple of follow-up questions:

I guess I know what the actual IAC Position is, what I don' t think I know is what position the ECM is telling the IAC to go to. Is the IAC at 145
because the ECM is telling it to go to 145? Or is the IAC at 145 and the ECM is telling it to go to something like 30? Anyway I can find out?

And the IAC Target (or IAC Minimum Keep-alive Position, according to DataMaster help info) will stay real low, in the 0-4 range, when it is idling rough, until I start driving, then it just starts bouncing back and forth between 0 and 128 (a couple of times a second), then it will suddenly stay at 0 for 5 seconds, then spike to 128, then go back to 0 for a while, then start bouncing again... it's just very erratic. Maybe it means nothing... but it sure looks weird on a chart! Is this normal? Anyone else see stuff like I'm seeing?

Thanks, again, for any help.

K87ZZ4

Some interesting info about the IAC Target:

Well, I found some sample test DataMaster files and was able to compare some IAC values to mine. It seems that a high IAC Target value (at, or above, 128) pushes the IAC Position down! At least there's a correlation there... when the IAC Target comes up above 128, the IAC Position goes down, but I don't know, for sure, if the IAC Target directly affects (or if it's all that affects it) the IAC Position, but it seems to. Anyone know?

So, when it's idling rough, the IAC Target value never gets high enough to push the IAC Position down to where it's supposed to be; maybe that's why the RPM's don't come up, because the position of the IAC isn't allowing it? What I don't know is... what engine/ECM values affect the IAC Target? Anybody know about that?

When it starts and idles normally, right after it fires up, the IAC Target spikes (to about 180) to push the IAC Position down, then the RPM's come up and, once it hits 1,300-1,500 RPM, the IAC Target again comes up above 128 and gradually pushes the IAC Position down which, I presume, causes the RPM's to gradually come down to about 900 (all this happens in 6-7 seconds).

When it idles rough, there is no spike from the IAC Target... it stays at 128, its (Key On/Engine Off) starting value, during the first second and then drops to 0. During the first 1/2 second, the IAC Position comes down (maybe caused by the IAC Target being at 128?) from 145 to 115, but then climbs back to 145 within 2 seconds of startup after the IAC Target drops to 0.

Also someone else told me that the IAC Position is simply what the ECM is telling the IAC to go to, so that answers that question.

Thanks for any help you can give me...

K87ZZ4

Well, it was starting and idling pretty normally for a few days, but then the problem appeared again. This time (like the first time it happened) th ecar wouldn't start... after it fired up and I let off the starter, it would just die. I tried a few times to get it to start and then just put the pedal down part way and got going because I was running late. That, and I forgot about trying to unplug the MAF again to see if that would make a difference. I'm still confused about why unplugging the MAF seemed to make a difference the first time, while replacing the MAF doesn't seem to have affected the problme.

Anyway, just in case someone else has experience with this bizarre, intermittent, no-start and rough idle problem... please chime in.

Thanks!