2008 Yukon Denali XL: Rough Idle/Accel after changing spark plugs

[Reid near Seattle]

Hello everyone,
I’m new to this forum. I have a 2008 Yukon Denali XL, with 160,000 miles on it. I decided to change the spark plugs the other day because the original ones were still in it. I pulled out the old plugs, put in the new ones, added some dielectric grease to the boot/caps, and put them all back on. (Yes, I gapped the plug to the correct gap needed, and put in some GNK Iridium spark plugs, as highly recommended by my auto parts store. Anyways, it now is running a bit rough when I come to a stop, and shutters a bit a times while accelerating. No engine lights have come on to say I’m getting a misfire, but I sure wonder... Any thoughts? Should I go and replace the wiring/boots also? I’m feeling dumb because I wanted to save some money by doing this myself, but even though I think I’ve done them correctly, it just doeesn’t sound or feel the same. We were having no issues before I decided to up and change them since the last ones have lasted forever. Help! I created a problem I wasn’t having. I just felt like I needed to replace the old ones... Ugh! Reid near Seattle...

 

[sumo]

Just make sure you properly attached each boot to the plug securely. You may have one that’s not seated all the way properly

 

[Hi-psi]

There are few things you can try. First and easiest as Sumo mentioned, ensure all the wires are firmly secured and seated. Some brands of plug wires won't have that positive "click" like others will when they are fully seated on the plug. Just wiggle, twist, and push them as best you can to ensure they're on all the way.

If the problem persists, try to get a hold of a scan tool. Some of the phone based ones will even work. Look at the live data while the engine is running and focus on the misfire count for the various cylinders. If one (or more) are misfiring, it will allow you to target if it's a specific one or if it's multiple. It can still be misfiring and not set a SEL (Service Engine Light). There is a misfire threshold that if it stays under that count in a specific set of time it won't set a code/light. If you can identify a specific cylinder or cylinders that is the problem, perhaps try swapping one of the old plugs into that cylinder to see if the problem goes away.

If you can't get access to a scan tool, then of course you could always swap the plugs back to the old ones, either all at once, or one by one to see if it resolves the problem. If it does, then you know that one (or more) of the new plugs had some issue. Although rare, manufacturing defects can happen, or it's possible maybe you cracked the porcelain a plug when installing.

 

[swathdiver]

Check your wires and connections. It's always good form to replace the plugs and wires at the same time since the intervals are so long now. Did you gap them to .040" and check them twice and thrice? Is it possible you cracked #8 putting it in? What is the part number of the GNK plugs? Did you mean NGK?

In my racing days I learned that selecting the right plugs is crucial for performance. There was an NGK that we used that gave us an extra 5-8 horsepower but came at the penalty of losing 3 miles to the gallon. The stock plugs worked best in those old muscle cars and from my research, in these LS motors. But as many trucks were made there are opinions. Mine runs smooth with the old 41-985s.

 

[Reid near Seattle]

Check your wires and connections. It's always good form to replace the plugs and wires at the same time since the intervals are so long now. Did you gap them to .040" and check them twice and thrice? Is it possible you cracked #8 putting it in? What is the part number of the GNK plugs? Did you mean NGK?

In my racing days I learned that selecting the right plugs is crucial for performance. There was an NGK that we used that gave us an extra 5-8 horsepower but came at the penalty of losing 3 miles to the gallon. The stock plugs worked best in those old muscle cars and from my research, in these LS motors. But as many trucks were made there are opinions. Mine runs smooth with the old 41-985s.

 

[Reid near Seattle]

Thanks for the suggestions. A few things: Swathdriver, the auto parts store told me that I needed to gap the plugs (Yes, NGK was the correct name...) at 0.044 for a 2008 Yukon Denali XL. If you have different more correct info, please let me know. I have a scan tool, (as Hi-psi suggested) and I’ll go put that on and do some checks too. My wife wondered aloud if engine codes might be coming out, without an check engine light going on. I’ll also go back in and double check the boots to hear for that click, which I believe I did hear/feel as I was putting them on.

Another place I was looking suggested two things to also try, and I want your thoughts: 1) Uplug the battery for like 30 minutes to help reset the computers and clear out the codes, etc, then start it up and drive it around and see what I experience....and look for codes after that. 2) Check to see if I didn’t accidentally bump or mess up any vacuum lines. Vacuum lines...? Are there vacuum lines on that left side of the engine I should be looking around for, or maybe that’s just a thought for another engine. We love our “gas-sucking bliss vehicle (the Yukon Denali XL), and hope to have it for as long a possible. Maybe the 6.2 liter engine doesn’t have much of those on the left side?

Your thoughts anyone? I will go and try those suggestions, and post back here in a few hours.
Thanks!
Reid near Seattle

 

[swathdiver]

The gap for all these Gen IV LS engines is .040", check your owner's manual too. The factory plugs are pre-gapped already and are only to be checked as the tips are fragile and discarded if not set to .040". Don't know what type of NGK you might be running. 41-985, the oem plug, are Iridiums.

Get yourself a subscription to AllDataDIY, and you'll have full access to the vacuum diagrams, trouble codes, almost of the part numbers and everything else needed to work on these. It goes without saying that a good scan tool like the Tech2 is as important as wrenches for working on these things.

 

[sumo]

Your description sounds like a misfire. Sometimes a misfire won’t throw a light until it sees a certain parameter. Depending on what kind of scanner you have, you might be able to see a pending DTC. Also depending on the scanner, you might be able to see engine data and if it does you could see what cylinder is not running right.

 

[Reid near Seattle]

Hi Swathdiver,
You are so right. I just checked another location that lists all the GMC Yukon’s spark plug gaps, and the 2008 6.2 liter is 0.04. Maybe the guy simply read it wrong, and saw that the very next line for 2009, is a 0.044, but not the 2008. So I went in and pulled out each plug, reduced the gap to 0.04, and put them back in. I made sure there was a click/feeling of the boots snapping in place, plus added dielectric grease to help with connections.

I’ve replaced 7 of the 8 plugs. I also unplugged the battery during all of this and put it back together. So now it sounds right, and seems to be fixed.

Thank you to everyone for your suggestions! You guys are great!

ONE LAST favor to ask: DOES anyone have a good way to get at that last spark plug that buried in the back on the left? (I believe that’s the #8 cylinder.) I haven’t searched for a way yet, but the few times I’ve looked around, it seems like others have had the same conundrum too.

 

[swathdiver]

I just checked another location that lists all the GMC Yukon’s spark plug gaps, and the 2008 6.2 liter is 0.04. Maybe the guy simply read it wrong, and saw that the very next line for 2009, is a 0.044,

That would be incorrect too. All Gen IV LS truck engines use the same gap at present. The previous generation began with .060" but they later reduced it for better performance.

#8 is a pain. There are some good youtube videos on it. I bought a 1/4" Tekton ratchet with an adapter and a very short 3/4" extension. Put the socket over the plug first and then connect the rest. I can't get my fat butt up there and my hands are too big, the kid I hired to help was just too small to break it loose. When school lets out I'll get a bigger kid to swap it for me.

I also put anti-seize compound on the threads of the plugs. Aluminum heads and metal plug threads are a pain when corrosion sets in.

Curious Reid, does your Denali have a flat or lumpy lifter valley cover? When was it made? Just curious to know if yours has the AFM hardware as some of the earliest did but was not active by the computer.