I just bought a 02 blazer with a 4.3, my problem it won't idle all the way down until I've been stopped for a second or two, the idle will drop about 400 rpm to about 750 rpm. Also I can let my foot off the brake and after it starts to roll the idle will jump up about 400 rpm without touching the gas pedal .

 

when i had this problem its was the throttle position sensor. pretty easy to swap, its on the drivers side of the throttle body. two torx bolts and it comes right out

 

When its in park it you can rev it up and it will idle down correct, keeps throwing code 507 IAC idle to high. Replaced IAC no difference.

 

did you give the passage in the throttle body a good cleaning? pretty common for crap to build up in there

 

i used hockey tape around a screwdriver

 

Cleaned throttle body and the hole in the plate. Going to have the computer checked.

 

https://blazerforum.com/forum/2nd-generation-s-series-1995-2005-tech-41/found-my-problem-42968/ check this out. apperntly if your alternater voltage isnt spot on your ecm will get confused. i am having the same issue as you and i think this is the problem. i will put multi meter on mine hopefully tomorrow and post back with results

 

My 1996 MT 2dr Blazer has had this problem too ( engine would rev over 2,000 RPM's). It seemed to get worse when it got cold, and I finally bought a GM Throttle Position Sensor ( 64 dollars ), replaced it in 5 minutes. Since the IAC is the TPS close neighbor, I took a good look at it too. I think the carbon built up in the IAC passage to the throttle body, and was hindering the IAC plunger to fully seat. So I figured I'd take out the IAC ( 2 torx screws and a wire to disconnect ) and clean the plunger on the end with aerosol throttle body and intake cleaner. Then I sprayed the passage connecting the IAC to the Throttle Body, where the IAC plunger seats. To get it good and clean put on a cheap cloth glove, and spray TB cleaner on the pinky finger tip and give the aluminum seat a cleaning ( like you'd use a Qtip to clean your ear ). The cleaning and reassembly of the IAC took about 15 minutes. Between these two things, my truck is running great. Hope this helps.

Jim

 

Problem fixed the new IAC was bad took it back exchanged it now idles about 600 rpm