Yesterday when I was doing some light driving, the engine randomly jump to 3k rpms and held. It did it the 1st time before getting gas then about 15minutes later. I reset the IAC and started it back up a few times. It would run fine for a few seconds then jump back to 3k. After 4-5 tries of starting it up/shutting it down the car finally ran normal for about 5 minutes before doing the same thing. I don't think the car did it after getting on the highway for 15 miles @70 mph. I know it didn't do it again when I got off and drove 6 miles @30 mph. The check engine light never came on. It was about 23 degrees outside.

I noticed after coming to a complete stop, the idle drops to about 6-7 hundred for a couple seconds then goes back to 8-9 hundred.

Things i've tried and will try.
I let it idle at 3k for about 5 seconds to see if it would fix itself but it didn’t. I put it in gear and slowly let the clutch out but that didn’t help either. The check engine light is good. I checked the throttle to see if it was sticking but it wasn't. Last week, I installed a new TPS to fix a surging idle and cleared a low TPS voltage trouble code. I also followed instructions to clean the IAC from shbox. I checked for vacuum leaks around the intake with carb cleaner, no leaks were found. I'll clean the IAC valve again to see if it'll help.

Any help will be greatly appreciated.

Mods are in my sig.

 

Did you check the IAC resistance using Shoebox's procedure?

http://shbox.com/1/iac2.jpg

 

Sleepy_Z:
I had a similar situation. The engine would randomly rev 2 to 2.4k rpm or hold that rpm when the car was stopped and in neutral. I would clean the IAC and engine behavior would be normal for a few days to a few weeks then the cycle would begin again. The TPS was at .5 vdc (TB closed), the TB cable wasn't binding and I had made plumbing changes to the PCV in and out which kept any oil from getting to the intake. I finally gave up on the IAC and plugged the IAC air supply hole in the TB inlet. I adjusted the TB to a warm idle at 800. My winter cold start idle is about 550 which warms to 800 in a few minutes. My inspection emmission numbers (in Jersey) are excellent, the throttle response is fine and I now have some useful low speed engine braking because the IAC can't supply any air when the TB is closed. (works for me, your mileage may vary ... good luck)

 

Some scanner data to see what the IAC, TPS, etc. is doing when this happens might be useful.

 

Injuneer: I'll check the resistance during lunch. I remember seeing that on shoeboxes' website a while back. It didn't even cross my mind to see if there's a way to check it.

NJ-LE: I'll use this as a last resort.

shoebox: looks like I have to get a scanner sooner than later.

Thanks for the responses, I'll post up what I find out.

 

IAC resistance checks were good.

 

Time to data log. Unfortunately, nothing free for OBD-II. AutoTap is a decent product. IF you buy it, get the "GM Enhanced Parameters" - gives you access to way more data, like misfire counts by individual cylinder.

The stock IAC system should be capable of managing the idle. I run an aftermarket ECU, but it can't handle the GM "stepper" IAC motor. So I still run my stock PCM to run the IAC, VSS and fans. As long as its getting the correct data from all the sensors, it can control the idle speed accurately, with no problems,

 

I had a similar situation with our car. I took the TB off and took it apart and cleaned it really well as well as the IAC. It fixed my idle. Might be worth a shot before you spend alot of money.

I also reset it after cleaning.

 

I searched around and found this. I'll have to give it a try since it's not hitting the wallet too bad: http://www.scantool.net/obdlink-sx.html


Hopefully I get around to removing it tonight.

 

Here's some more info after cleaning/resetting IAC and everything is back together.

When giving the car gas @ 1500+ rpms: the TB remains at the same position after releasing the gas. Tapping the gas drops it back to idle right away. TB only sticks when the engine is running.

 

Try cleaning the tb blades,sounds like you have a lot of carbon build up on them.

 

brand new tb

 

Ok didnt know that it was new...Hate to say it but,sounds like you are going to have to replace that IAC sensor...My buddys did the same thing,it would work ok a few times,then it would not work,no matter how many times we cleaned and reset it..

 

If the TB is physically open then changing the tps isn't going to do anything. Check all around the TB for anything that can be interfering with the blades closing.....

 

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Everything has been checked. Only does it when running.