IAC Issue? Rough idle in park & in drive. Please Help!

[zer0Limitz]

I have a 97 Sierra SLE 5.7L 157k miles. Just fired the truck up today after maybe a month of sitting. I took it for a drive and once I got to a stop light I noticed the truck was running rough while idling (I'm fully stopped pressing brake at traffic light). I almost thought it was about to stall. Press gas, get driving, no problem, seems to have normal engine power. I drove it around for about 20 minutes on the road, same thing every time I stopped and had to idle at a traffic light. When I placed it in park and it was sitting idling it was still running rough like it did when I would idle at traffic lights, sounding like it could stall but never did. I didn't notice any issues while accelerating while driving or pressing on the gas while sitting in park. Also when it was running rough, truck was shaking a little bit, and the RPMs seemed a bit lower than usual, chugging slightly.

From what I've been able to read and watch on YouTube, it is sounding like the idle air control valve might be going bad or sticking? Or the O-ring could be bad and causing a vacuum leak? I am trying to see if anybody has this issue or same situation and if their IAC valve was the culprit.

Any info, advice, past experiences, anything for this is greatly appreciated!

 

[Schurkey]

Connect a scan tool, look at the commanded idle speed vs. the actual idle speed. Look at the IAC action.

While you're in there, look at all the others sensors and outputs.

 

[east302]

Adding to the above for consideration...

-Vacuum leak associated with brake booster
-Engine mount

 

[zer0Limitz]

Connect a scan tool, look at the commanded idle speed vs. the actual idle speed. Look at the IAC action.

While you're in there, look at all the others sensors and outputs.

I will keep this in mind for when I get some more responses and see where it leads me.

Adding to the above for consideration...

-Vacuum leak associated with brake booster
-Engine mount

youre saying brake booster because i was pressing the brake when this was happening at stop lights? and engine mount because i was in sitting in park??

 

[east302]

Yes on the booster and “sort of” on the engine mount. Whenever I had a mount issue, it felt like an idle drop because of the vibration but the idle didn’t actually change.

The IAC valve counts, compared to the commanded versus actual rpm, could help determine if you’re seeing a vacuum leak. I’ve never had to deal with it; maybe Schurkey could elaborate on what to look for.