Intermittent Very High Idle And Stalling
 

I have a 2003 Monte with a 3.4. Most of the time it runs just perfect but every now and then it will rev up to 3000+ rpm's and just drive itself. I have to press on the brake really hard just to keep it from taking off. Then out of no where it will just run fine after it has sat a while. Also, sometimes it does the extreme opposite and the idle will fluctuate around 300-500 rpm's and constantly die. After you let it sit for a bit it will be fine. I had the codes checked and it has a MAF code but I don't see how this code would affect idle. I have replaced the IAC valve also thinking this might be it. I am totally stumped. I have read up on these forums a bit and have seen several other posts about people replacing their MAF sensor to no avail and I don't want to waste my money of that isn't the issue. Please if anyone has any help let me know. Thanks.

Okay, reading it I was thinking either the IAT or the MAF, since you did the IAT and you don't want to try the MAF I'm going to open up one of my books and just throw out a bunch of possible causes for the symptoms you describe, this way you can test which you want :)

Problem, Erratic Idle
Causes:
1. Vacuum Leaks.
2. Leaking EGR valve or Plugged PCV valve.
3. Air filter clogged.
4. Leaking Head Gasket. (Perform a cylinder compression check)
5. Worn timing chain or belt.
6. Camshaft lobes worn.
7. Valves burned or otherwise leaking.
8. Ignition system not operating properly.
9. Dirty or clogged injectors.
10. Faulty idle control system.
11. Faulty emissions or engine control systems.

Problem, Excessively High Idle Speed
Causes:
1. Sticking throttle linkage
2. Idle speed incorrect
3. Faulty idle control system
4. Faulty emissions or engine control system.

chibiblacksheep explaned it well. check the idel controll air valve it moghy be the culpret as well.

A faulty MAF will cause idle problems. My MAF started to go bad on my L67, so after the car warmed up, the idle would fluctuate and the car would cut off. If I unplugged the MAF, it idled fine. I recommend replacing the MAF, especially since you're throwing a code for it. I don't know about a MAF making the idle go HIGHER though, I think that has something to do with your throttle position sensor and/or the cable on the throttle body sticking.

kinda same issue
 

when i start up my 03 monte SS. i have the 3.8, my ilde speed is around 1300 rpms, but after driving around it goes to normal. i just changed my IAC valve also. could it be a bad MAF sensor? or would it be something else? i also do have an intake, if that makes a difference.

I thought all cars idled higher when you initially start them up. I had an '03 SS as well, upon start up it would idle at 1,000-1,300, then after a few minutes, or some driving, it would go down to 700-800.

i bought a new IAC valve, and after i installed it, i would start it up, and it would start up around 700-8-- rpms. then othertimes it would start up at like 1000-1300 rpms, that was just kinda throwing me off...

Just wondering if you ever got this problem figured out? I am having pretty much the same problem. Runs fine until it is warmed up then it idles funny, and stalls. It is showing a MAF code, swapped it still did the same thing. Then tried the idle control valve and still have the same issue.

Perhaps check the wiring going to the MAF if it's still showing a MAF code after changing the sensor?

3.8 high idle