I have a new issue will my 03 Chevy 6.0. When the truck is cold it idles rough. Doesn't actually feel like a misfiring cylinder. In gear at a stop light the idle will drop below 500 rpm and the engine stumbles and then the idle jumps up to about 1,000 rpm and then back to normal for a moment and then it repeats. Once it was up to operating temp it idled fine. Since it only did it when cold most of my research showed tat the intake manifold gasket was the likey the problem. Now this morning the temp outide was -6 degrees and the truck ran rough until it was warm and then it was fine as usually. Then 25 miles down the road I run into traffic and had to stop and the truck did the same rough idle as it was doing when it was cold. It did it about 5 or 6 times and then idled fine again. Warm or cold it runs fine once you are off of idle. Looking for some input.

Sounds like a vacuum leak. Some vehicles have a thermal control vacuum sensor. Go over the vacuum line diagram under the hood. Look for a thermal valve.

enblethen wrote:Sounds like a vacuum leak. Some vehicles have a thermal control vacuum sensor. Go over the vacuum line diagram under the hood. Look for a thermal valve.I am going to check it out more after work. It appears that the intake manifold gasket is a very common problem with these trucks as well. Usually the vaccum leak goes away once they are warm though unless mine is really bad ?

Remove the idle air control valve and thoroughly clean it and the cavity. A sticky valve with respond to the ECM properly but not physically move. So, you can get odd idle behavior like you saw but never trip a check engine light.My FIL had a chevy van that was racing at idle. He could go 45MPH and never press the throttle and had no check engine lights. I replaced the AIC and all was good.KJ

Don't know much about Chevy but it sounds like an ignition coil problem to me. Is this a Coil Over Plug (COP) design? Get a code reader and find out which cylinder has the most misfires. It probably will be like 10 times more misfires then the next closest one, this is a good indication it is the COP (or it could be the plug but less likely).

I received a letter from GM on my '09 6.0 about rough idle quite a few years ago.If my mileage was still under 100,000 and the rough idle happened the letter said to bring it to the dealer.Of course I had over 100,000 on my truck by the time I got the letter.

Rough Idle | Good Sam Community

18 Replies

Thunderbolt
Explorer
Feb 12, 2018
Well if the intake gasket doesn't fix it instead of me pushing it, You could drive up to MN and RAM it for me. :)

dodge guy wrote:
I believe the issue starts with the bow tie they tried to dress it up with!! Sorry, couldn’t resist.

I also agree with a vacuumed leak, possible a sticking EGR if the 6.0 has one.
dodge_guy
Explorer II
Feb 12, 2018
I believe the issue starts with the bow tie they tried to dress it up with!! Sorry, couldn’t resist.

I also agree with a vacuumed leak, possible a sticking EGR if the 6.0 has one.
Thunderbolt
Explorer
Feb 12, 2018
Thanks for the input. I will plan on looking at the knock sensors and harness.

LIKE2BUILD wrote:
Thunderbolt wrote:
I confirmed this morning that my intake gasket is leaking. I started at -4 degrees and once it was off fast idle it started stumbling. I started spraying carb cleaner along the intake and when I got to the driver side rear the engine smoothed out.

Good catch. Sorry I was wrong on the IAC. I had thought the 6.0L had the same separate IAC as other GM's.

While you have the intake off you might look at the knock sensors. My 6.0L only had 2 problems in 10 years of ownership: exhaust manifold bolts and corroded knock sensors. Both of those are extremely common on the LS motors.

Since you'll have the intake removed it's a great time to inspect the knock sensors and replace if they are corroded. The TSB for the sensors shows to make a dam of RTV around the sensor cover with an opening to the rear. Here is a PDF of TSB #02-06-04-023A.

I have 2 additional suggestions:
1) Remove the foam seals at the front and rear of the intake. In my opinion these help to trap moisture and don't really do much other than a slight sound deadening.

2) Replace the knock sensor harness. On my truck I replaced the rear sensor and not long after kept getting intermittent CEL's for the sensor. It turned out the wiring harness also had issues so once again I removed the intake and replaced the harness. That finally solved the problem for good.

KJ
LIKE2BUILD
Explorer
Feb 11, 2018
Thunderbolt wrote:
I confirmed this morning that my intake gasket is leaking. I started at -4 degrees and once it was off fast idle it started stumbling. I started spraying carb cleaner along the intake and when I got to the driver side rear the engine smoothed out.

Good catch. Sorry I was wrong on the IAC. I had thought the 6.0L had the same separate IAC as other GM's.

While you have the intake off you might look at the knock sensors. My 6.0L only had 2 problems in 10 years of ownership: exhaust manifold bolts and corroded knock sensors. Both of those are extremely common on the LS motors.

Since you'll have the intake removed it's a great time to inspect the knock sensors and replace if they are corroded. The TSB for the sensors shows to make a dam of RTV around the sensor cover with an opening to the rear. Here is a PDF of TSB #02-06-04-023A.

I have 2 additional suggestions:
1) Remove the foam seals at the front and rear of the intake. In my opinion these help to trap moisture and don't really do much other than a slight sound deadening.

2) Replace the knock sensor harness. On my truck I replaced the rear sensor and not long after kept getting intermittent CEL's for the sensor. It turned out the wiring harness also had issues so once again I removed the intake and replaced the harness. That finally solved the problem for good.

KJ
Thunderbolt
Explorer
Feb 11, 2018
I confirmed this morning that my intake gasket is leaking. I started at -4 degrees and once it was off fast idle it started stumbling. I started spraying carb cleaner along the intake and when I got to the driver side rear the engine smoothed out.
kw_00
Explorer
Feb 07, 2018
Thunderbolt wrote:
kw/00 wrote:
I had my old one do the same. Intake manifold gaskets were leaking. After I fixed that issue I had the Cpk go out.... that was a easy fix as well! That 6.0 is so built proof that I bought an 09.. my old one had over 200k. So that's my take on it, replace the gaskets with the newer updated ones. Good luck and keep us posted.

I am thinking intake gaskets too from what I am reading. Looks pretty easy to do as well compared to my 5.7 Vortec.

Yes sir and it's a dry intake so easy fix!
campigloo
Explorer
Feb 07, 2018
-6? Move to a warmer climate. Brrrrrrrrrr
Jay_Pat
Explorer
Feb 07, 2018
Paul Clancy wrote:
Jay Pat wrote:
From what I've read, it could be sticktion(?)with one of the injectors.
Arches diesel treatment has been known to eliminate sticktion.
If you can't find it on the shelf at auto parts, I would try any diesel treatment.
Pat

Chevy gmc 6.0 is a gas truck.

Thanks for the correction.
Pat
Paul_Clancy
Explorer
Feb 07, 2018
Jay Pat wrote:
From what I've read, it could be sticktion(?)with one of the injectors.
Arches diesel treatment has been known to eliminate sticktion.
If you can't find it on the shelf at auto parts, I would try any diesel treatment.
Pat

Chevy gmc 6.0 is a gas truck.
Thunderbolt
Explorer
Feb 07, 2018
kw/00 wrote:
I had my old one do the same. Intake manifold gaskets were leaking. After I fixed that issue I had the Cpk go out.... that was a easy fix as well! That 6.0 is so built proof that I bought an 09.. my old one had over 200k. So that's my take on it, replace the gaskets with the newer updated ones. Good luck and keep us posted.

I am thinking intake gaskets too from what I am reading. Looks pretty easy to do as well compared to my 5.7 Vortec.