I have a 1999 GMC 2500LD with a stock 6.0, LQ4 I believe. I am having idle problems. The truck runs just fine when moving but when I stop it acts like it wants to choke down. It seem to be hunting, for lack of a better work, for the appropriate RPM or something. I had a computer diagnostic and there were no error codes from the computer. Has anyone else had this problem or have any ideas on what it could be. The truck only has 93k miles and has been well maintained, atleast while I've had it. Thank You for any help.

Matt

 

The most common thing on these trucks that causes that is a dirty throttle body. They are notorious for a huge carbon build up around the throttle plate. Remove the intake tube that goes to the throttle body and get some throttle body cleaner. Clean it as good as you possibly can. With the truck NOT running spray some cleaner in there and have plenty of paper towels or clean rags handy to scrub all the carbon out of there paying close attention to the area where the throttle plate and inner wall of the throttle body meet. I just had a Suburban do that and it was so bad that I had to remove the throttle body completely and clean it from both sides. Taking it off makes sure that you get it clean and at the same time you can remove the IAC valve and clean it and where it mounts as well. Let me know how it works.

 

Thanks Shamrock, I will try that this weekend. I hope that it works. I appreciate the help.

 

I can't seem to find the IAC on my truck (having the same trouble with idle) I cleaned the throttle body which really didn't seem all that dirty... I have a 2001 GMC Sierra C3 Denali. It has a Vortec 6000 and I'm aware of there being two different models of that engine but I'm not sure of the differences so mine might or might not be the LQ4... Any help?

 

My problem was actually the transmission. GM has firmware updates for the transmission which I didn't have. My mechanic found a technical bulletin that said that not having these updates could cause problems with the idle. Then the fuel pressure control valve had been broken presumably due to slamming open and shut with the errorneous feedback, or whatever from the tranny, or that's what the mechanic guessed. I know it seems odd but that's what mine was. It got so bad that my ABS light came on, my brakes locked up and I almost hit a telephone pole. Damn computers, who knows what's wrong anymore? I took it to the dealership and had the firmware updated. Then went to AAMCO and had the transmission serviced, now I haven't had any idle problems, no ABS lights, or brake lockups. My truck runs great. I hope this helps.

 

Oh yeah, it's a 4L80E transmission.