Basic troubleshooting for a slight rough idle

**maury** · 05-06-2008, 12:32 PM

I'm attempting to not start throwing parts at this thing. Here's the rundown:
99 k1500, 136k miles - slight stumble at idle. NO CEL codes.
New fuel/air filters. Plugs have about 40k on them. Wires are original.

Am I correct in assuming that problems with EGR and Idle valves, temp sensor and misfires will show up in a CEL? I sure would like to think its vacuum related but haven't found a leak as of yet and its only a minor stumble. She seems to accelerate smoothly. The idle problem seems to have come on all of a sudden and not progressed - but I could be wrong.

Anyone offer a list of possible culprits?
Thanks
maury

**GaryL** · 05-06-2008, 01:22 PM

Could be dirty injectors. You might try running some seafoam through the gas tank. I do that ocassionally and it seems to run smoother once I have finished that tank and put fresh gas in.

**maury** · 05-06-2008, 01:26 PM

Gary,

Forgot to mention I did seafoam the truck just recently with no difference. I try to keep techron in the tank every 4-5 fillups. I had the injectors in my old bimmer (with over 200k) professionally cleaned and balanced. There was moderate improvement but not as much as I would have thought at that mileage. I guess without any codes to work with, I'm at the mercy of changing parts. I'll start with the least $$ first.

**zippy** · 05-06-2008, 02:13 PM

Could it be the Idle Air Control Valve?
It wouldn't cost more than a possible new o-ring to tidy up and have the computer reset the IACV pintle position.

**MrShorty** · 05-06-2008, 02:14 PM

Is the fuel pressure up to spec and steady?

**maury** · 05-06-2008, 02:43 PM

Fuel pressure is within specs. New pump/sending unit assembly and filter. I changed the pump assembly because the gas gauge was possessed. The IACV is something I have to investigate. Anyone have a link to info on this?

**maury** · 05-06-2008, 02:48 PM

.........and would a bad IACV throw a code? I understand it wouldn't if its only the o-ring.

**lmin2** · 05-15-2008, 02:46 PM

try cleaning your idle air sensor..that worked for me..

**Kaedenman** · 05-29-2008, 12:33 PM

I had/have a slight rough ile too. Mine is a 2008, though. They said they found the problem to be a bad 02 sensor. After they changed it, I still notice a slightly rough idle when stopped in drive. My other non GM vehicles never did this.

**bkovacs** · 05-29-2008, 05:46 PM

Originally Posted by Kaedenman

I had/have a slight rough ile too. Mine is a 2008, though. They said they found the problem to be a bad 02 sensor. After they changed it, I still notice a slightly rough idle when stopped in drive. My other non GM vehicles never did this.

if it was the 02 sensor, it would probably throw a code, I would say change your wires! if they are orginal, how about cap and rotor? that would usually cause problems throughout the rpm range though not just at idle, IAC could be the problem