First post, been reading the board for a bit. I seemed to have acquried an odd problem. When sitting at a red light, after the engine has warmed up, the idle will drop to 400-350 rpm for about a second, shudder and dim the lights then jump up to 500-550 rpm where it normally idles and then it'll run fine. Sometimes on the way up it will go higher than normal idle and then back to normal, sort of hunting. When sitting in park, after the engine has warmed up, I rev it to any rpm over 1000 and let off the throttle on the way back down to idle it drops below regular idle and slightly bogs, as stated before, and back to normal. This bog will also happen when shifting from park to drive. The odd time while sitting in park it'll do the reverse, go up to 700 rpm, give or take, and back down.

Can someone shed some light on this?

Hope I've explained that clear enough

- Justin

 

What year is it? Maybe it just needs a tune up. I'd start by changing the fuel filter.

 

It's an 03 with the 4.8L. I'll give that a shot.

Excuse the ignorane but what's the procedure for changing the filter?

Also, any other causes to this just incase its not the fuel filter? Not trying to insult your intelligence charger!

- Justin

 

try cleaning the throttle bore...the right cleaner is tougfh to find though...."throttle cleaner"..

be careful not to spray too much in there....just some ,and usae a soft bottle brush.

also,gm has softtware downloads to adress some idle issues

 

If you don't mind taking the throttle body off to clean both sides of the butterfly, that's not a bad idea. I did that to mine and it made a huge difference. You don't have to remove the coolant lines from it, just unbolt it after you clean the front and lay it face down to clean the back.

 

Tune-up and some maintenance as the others have mentioned. Check the plugs, air filter, fuel filter (especially if you have never changed it before), PCV Valve, etc. The fuel filter (if it is the same as mine) is located along the frame rail, on the inside, drivers side, about half way back to the rear wheels. You'll see two metal gas lines and a small pop can looking canister, thats the filter. Its not easy to take out if it has never been done before. Just make sure you open the gas cap and disconnect the battery before you start.

 

One other possible cause (in addition to the many already listed) could be the IAC valve (idle air control). I`ve had these get sticky on a few different vehicles. So far I`ve gotten away with simply running some carb cleaner thru them. I had one that I had to take off and clean with brake parts cleaner to get cleaned up enough to work properly.

 

Thanks for the info! I'll give those a shot and keep you guys posted if anyone's intrested. It may not be for a couple weeks as next week I'm out of town.

 

I would look at spark plugs. Checked mine the other night, and they need changing, gap is out by .020. Where is the IAC located at on the 4.8l? I havent changed the plugs yet, but when i do, i would like to check it out.

 

I`ve never had a problem with my 99 or 05, only other vehicles, so I`ve never looked for it`s exact location....but it has to be right by the throttle body somewhere...the IAC valve is the air bypass around the closed throttle plate to control the idle speed.