What would cause an IAC to drop from 30 steps to ZERO?

[BurbinOR]

And with it comes rough idle, no big deal but something isn't right. I see the counts on my Auto Xray scanner and no rhyme or reason as to why it happens, and is sporadic. This on an 89 Jimmy with the 4.3L V-6.

/forums/images/graemlins/confused.gif

 

[HarryH3]

Is zero fully closed? If so, then you need to look at some of the other data and watch what it does just before the IAC goes to zero. If zero is fully closed, then the computer thinks that one of two possiblities is happening. First, a flaky TPS could be causing the ECM to think that you have stepped on the throttle, so it closes the IAC. Second, for some reason the ECM thinks that the idle has increased, so it closes down the IAC to being the idle back into spec. If it's the second, then finding the cause could be real fun. /forums/images/graemlins/wink.gif

 

[Thunder]

Just a few things to ckeck out
Vacume leak. Stuck/bad EGR. Check the TB base gasket for leak. PVC valve.