I have a 90 k-1500 with 170,000 miles on it. My base idle is between 500-600, my ECM is set to 800. From my understanding, my IAC should maintain my idle at 800, but it hasn't done it yet. I occasionally have an issue with the idle randomly dropping below 500, almost dying, then coming back up to 500, it will continue to do this as long as I am at idle. This is very frustrating beings she is a stick. I have checked out my IAC for the specs listed in my Haynes, tested fine. I thought that might be off, very vague test, so I bought a new one just to test it out with no luck. I am out of ideas, as far as I know there shouldn't be anything else that would mess with how the IAC maintains the idle. Does anyone have any ideas?

Oh, I have no known vacuum leaks, I have checked multiple times.

 

i had a problem simaler to yours on my 92.it turned out to be a bad injector.

 

egr valve, clogged pcv valve, clogged fuel filter, fuel pump going bad, and as stated before, possibly injector issue.