Rough Idle HELP!

[02YukDenXL]

So I'm going absolutely nuts over this. I have a 2002 GMC Yukon XL Denali (6.0L LQ4) with about 160,000 miles. No mods. Started having a rough idle a couple months ago. I have never had a CEL. No stored codes. Nothing. It has been getting progressively worse. I'll list everything that I've done from the start in the order that I did them in:

Plugs
Wires
Air Filter
Cleaned MAF
Fuel Filter
Cleaned Throttle Body (It was gross)
Seafoam
Replaced intake manifold gaskets
(Replaced knock sensors at the same time)
New transmission fluid & filter

So at this point I still had a rough idle (Or what I thought was a rough idle). After I did the tranny fluid, it almost felt worse. So then I thought that I had a shudder in my transmission that felt like a rough idle. Brought it to a transmission shop and they had a Tech II scan tool and I had random misfires (When I was with the tech he showed me on the Tech II cylinders 7, 2, 6, & 1). So after looking at it, he told me it was probably my ignition coils. I just replaced all 8 last night and the "rough idle" is still there!!!!! Power wise and shifting, overall everything feels smooth and strong. No loss of power.

I'm ready to bring it to a dealer to figure out wtf is going on. I tow a trailer when we camp in the summer and I want this thing running like a top while I'm towing. Thoughts? Comments? Thanks in advance!

 

[4WDTRK]

I am at a loss. Your list sounds pretty exhaustive of all the basic things. Can you throw it back on the scan tool and check if you are still getting random misfires after the coil changes?

 

[TheAutumnWind]

Injectors?

Looks to me that you have addressed all the things that I would check first.

Firing order is 1-8-7-2-6-5-4-3 and those are not all on the same bank. Not sure if there is anything to be gleaned there.

 

[OR VietVet]

Might need to smoke test it to locate a vacuum leak.

Recently had a rig like yours on here that had the rough idle and plenty of power. The oxygen sensors were operating just enough out of range to cause this.

Also need to know if has same random misfire cylinders as before. Did you replace the boots at the same time?

 

[02YukDenXL]

I'm gonna go back to the shop and have them throw it back on the Tech II and see what the deal is. Its definitely still misfiring. I'm lucky I did the coils myself or I would have paid big bucks for nothing.

I'm pretty sure I have 4 O2 sensors. That'd run me $160 in parts and thats if I can even get to them / get them off myself. I'm gonna have that shop diagnose the issue more and I'm not gonna pay them for it since they told me it was my coils and it wasn't my coils.

I'll let you guys know what happens once I go to the shop.

 

[02YukDenXL]

I think it's my Cats. I think they're plugged. I read in my Haynes manual that if you record the pressure in the manifold at idle, then rev the rpms to 2,000, release the throttle and record that instantaneous reading, that the pressure shouldn't drop by more than 1 in-Hg. My readings at idle were 10.3-10.6. Once I rev'd and took my foot off the throttle pressure dropped to 5.6-6.0 in-Hg. I'm bringing it to the shop in the morning and hopefully they can confirm the above. Thanks Raiderron. Your O2 mention may have guided me in the right direction.

 

[ezdaar]

Return those coils since they didn't solve the problem.

Use a Infrared IR Temperature Gun to determine clogged cats.
Temps pre cat will be Significantly higher than post cat if they are clogged.


What brand plug wires are you using? plug wires can be a cause of "random missfire" problems.

 

[02YukDenXL]

So my temps are consistant on either side of boths cats. Driver side was ~580 degrees and passenger was ~550 degrees. Same temps upstream and downstream. So based on what you said ezdaar, its not my cats. Ugh.

I had the misfire before I did plugs. AC Delco Iridiums gapped to .040.

Almost feels like its gotten worse since I did the ignition coils. Coils cant be returned. They're an electronic item.

My mechanic hooked up his scan tool and "reset the parameters" this morning. Didn't do squat.

 

[TheAutumnWind]

So my temps are consistant on either side of boths cats. Driver side was ~580 degrees and passenger was ~550 degrees. Same temps upstream and downstream. So based on what you said ezdaar, its not my cats. Ugh.

I had the misfire before I did plugs. AC Delco Iridiums gapped to .040.

Almost feels like its gotten worse since I did the ignition coils. Coils cant be returned. They're an electronic item.

My mechanic hooked up his scan tool and "reset the parameters" this morning. Didn't do squat.

Not surprised. If it was clogged cats it wouldn't run "smooth and strong" everywhere other than idle.

---------- Post added at 09:32 AM ---------- Previous post was at 09:31 AM ----------

Might need to smoke test it to locate a vacuum leak.

Recently had a rig like yours on here that had the rough idle and plenty of power. The oxygen sensors were operating just enough out of range to cause this.

Also need to know if has same random misfire cylinders as before. Did you replace the boots at the same time?

Vacuum leak is a good idea to check.

 

[OR VietVet]

The oxygen sensors could be failing and still not have damaged the cats. They need to be monitored during a cold start to warm up and driving under load. Yes, the vacuum leak is also a source that could be the cause. Smoke test of engine is needed.