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.

 

[sumo]

Swivel extensions work great. You can go through the passenger wheel well. I did mine from the top like so

 

[Reid near Seattle]

Hi Swathdiver,

OK, so I’m a ignorant guy when it comes to some things you just asked me about. I’m not exactly sure what a “flat or lumpy lifter valley cover” is that you’re asking me about. I do know that we purchased our 2008 Yukon Denali XL at 60,000 miles. We did the car fax on it, and it was originally a lease, and then the leasee decided to purchase the car outright. So I’m the second owner, and have put 100,000 miles on it. I have a client that works at a dealership, and he says the engine is ‘rated’ (whatever that means) to go at least 300,000 miles if I take care of it. It’s a Gold pewter color, leather seats, GPS system, heated steering wheel, moon roof, sun roof, etc. I was told it has every option option on it, other than the snow plow option. I’m currently on a business trip, but I can look closer and find out exactly the date and where it was built when I get back to Seattle.

I failed yesterday at getting that back left spark plug out. I got the socket over it, but my 1/4th inch extender is a swivel kind, and became a hinderance, instead of a help. I have bruises (literally...) all over my stomach from engine parts sticking into me =while leaning and laying atop the engine trying to get that evil spark plug replaced. I’ll try again next week. Ha!

 

[bottomline2000]

I could have sworn I read you aren't supposed to gap iridiu. Plugs. Just buy them gapped as needed. The tips are can give and don't take well to being touched.

Sent from my SAMSUNG-SM-T377A using Tapatalk

 

[swathdiver]

Reid, I was wondering if your engine had AFM hardware or not, thought it was an '07 but see that it is an '08 and does not have it. The very early SUVs with the L92 had AFM hardware but was not active in the tune. It was physically removed later in the year and for 2008.

I found that no swivels work on that one and neither did going in from the wheel well with our tools. I just need to someone skinny up there with more heft and muscle than my last helper!

 

[sumo]

I could have sworn I read you aren't supposed to gap iridiu. Plugs. Just buy them gapped as needed. The tips are can give and don't take well to being touched.

Sent from my SAMSUNG-SM-T377A using Tapatalk

You’re not supposed to. The electrode is a fine point and it will easily break.

 

[Rocket Man]

For the #8 plug, SK Tools makes a 3/4" or 1' long x 3/8" drive extension that works perfect when used with a plug socket. It's just long enough that the ratchet works. It's hard to find that short extension but it works perfect. It doesn't seem like it but once you try it you'll be amazed.

 

[Big Mama]

All right on the money. A different take on plugs and wires. As for plugs. The tip on them is very delicate and the gapping process can flatten them. My parts guy says the OE replacement is pre gapped. (I confirmed at least the set I got was right, maybe got lucky) As for wires. I did this same job on the same vehicle and 2 of my plug wires were damaged when I unplugged them. The connectors went back on the new plug fine but I noticed the wires to the connector came off.

 

[Rocket Man]

All right on the money. A different take on plugs and wires. As for plugs. The tip on them is very delicate and the gapping process can flatten them. My parts guy says the OE replacement is pre gapped. (I confirmed at least the set I got was right, maybe got lucky) As for wires. I did this same job on the same vehicle and 2 of my plug wires were damaged when I unplugged them. The connectors went back on the new plug fine but I noticed the wires to the connector came off.

The wires getting damaged happens often. I had a set of new ACDelco wires that I installed and a few days later I needed to change heat range on the plugs. When I removed the wires, 2 of the ends came loose inside the boots even though I made sure to pull by the boot. You need to inspect them visually each time they get pulled off.

 

[Doubeleive]

^^^ like he said you just need a series of adapters and you can get it out just fine with no more hassle than any other plug, in the future when you have a misfire problem a very simple and quick way to determine which plug or coil it might be is to simply unplug 1 wire at a time until you detect no difference in the idle and that will be the faulty one. however that may not have been beneficial in this case if all the plugs were gapped incorrectly. good luck

 

[Rocket Man]

I just looked at my shorty extension and it's an SK 45159. Here's a picture of one.