Weirdness after spark plugs change

[RizwanWali]

So I have a ‘08 Yukon XL Denali (only 74k miles) which had a slight pulsing shakiness in idle like a misfire.

This is what I have done to try to smoothen the idle:

1. Ran a bottle of fuel system cleaner STP brand in the gas tank - no difference.
2. I thoroughly cleaned the intake and throttle body - no difference
3. Changed all spark plugs. Old ones were AC DELCO 41-993 and the new ones I installed were AC DELCO professional iridium 41-110.

I had the battery disconnected for 30 minutes and now idle has become more shaky than before. Also noticed a more pronounced jerk shifting from first into second. I drove it on the highway and did a few pulls but no difference.

Anyone have any idea what could be the problem here? What does the transmission have to do with the plugs? Did I use the wrong plugs?

 

[CaptainAmerica1]

Battery disconnected means you need to relearn the fuel trims. Drive it for a couple days and it should smooth out.

 

[thompsoj22]

I have the same vehicle but am now at 148k, That stumble/miss at idle is just part of the 6.2 in my opinion. There are members that have "silky smooth" idle but i am over trying to attain it. I have a tech2 and have looked at everything that i feel could be a contributor and have found nothing. It isnt consistant, sometimes you cant feel it at all and other times, there it is. I got the vehicle at 125k and to say it was pampered is an understatement, I run only chevron premium, Get 17.5 to 19 mpg at 75 yo 80 mph with ac on. This is without a doubt the most comfortable functional "road limo" we have ever owned! My advice is to stop trying to cure somthing that is inherent to the 6.2. I have thrown a few parts trying to get it to smooth, VVT solenoid, purge solenoid, MAF. Welcome to the forum!

Also im superstitious about cleaning the TB, Be very cautious with the movement of the plate when doing so. IMO if you "super clean" it and open and close the plate recklessly it negatively affects the wiper assy and you end up with "no good deed go's unpunished". ie worse than when you started. just my op

 

[Foggy]

Also, to add, stp sucks....
Use the Techron Concentrate in the black bottle..
The large bottle is about $14, but you can find it on sale a lot.
You'll want to run 2 bottles thru initially due to the large fuel tank.
Then run 1 bottle thru about every 3 -6 months to keep your fuel
sender and injectors clean. It's the bomb...

 

[Doubeleive]

Clean the throttle body and maf, then keep running injector cleaner thru it for a while, I buy it a case at a time it's .99 cents at walmart, 2 bottle per tank of gas.
alternatively you could request a injector cleaning service from the dealer or independent shop, ask how they do it because there is a machine some use to connect at the fuel rail and run it directly into the injectors which would be the best alternative aside from removing and cleaning them individually. I make it a habit to regularly clean the throttle body and maf every 3 months. by doing that there is barely even a few smudges of carbon build up on my cleaning rag. maf cleaner and throttle body cleaner runs about $3-4 each at the auto parts store and will last you a while. use a lint free cleaning cloth for the throttle body, no need to remove it just take off the air intake assembly and clean it while on the motor, the maf you just take it off and spray it. let it air dry a couple minutes then reinstall. easy peasy.

 

[iamdub]

I agree with the above advice.

The not-silky-smooth idle is likely just the normal behavior of these things. I'd also question the new plugs- did you get them for a really good price, like around half of what they are elsewhere? Do you happen to still have the packaging? I ask because low-quality knock-offs are in abundance and you have to really know what you're looking at to spot the imposters. You may have swapped old good plugs for new bad plugs.

 

[RizwanWali]

I agree with the above advice.

The not-silky-smooth idle is likely just the normal behavior of these things. I'd also question the new plugs- did you get them for a really good price, like around half of what they are elsewhere? Do you happen to still have the packaging? I ask because low-quality knock-offs are in abundance and you have to really know what you're looking at to spot the imposters. You may have swapped old good plugs for new bad plugs.

The plugs look original AC DELCO they came in their original packaging and looked brand new.
I must say though I did not change the gap as advised by the salesman.

 

[RizwanWali]

Clean the throttle body and maf, then keep running injector cleaner thru it for a while, I buy it a case at a time it's .99 cents at walmart, 2 bottle per tank of gas.
alternatively you could request a injector cleaning service from the dealer or independent shop, ask how they do it because there is a machine some use to connect at the fuel rail and run it directly into the injectors which would be the best alternative aside from removing and cleaning them individually. I make it a habit to regularly clean the throttle body and maf every 3 months. by doing that there is barely even a few smudges of carbon build up on my cleaning rag. maf cleaner and throttle body cleaner runs about $3-4 each at the auto parts store and will last you a while. use a lint free cleaning cloth for the throttle body, no need to remove it just take off the air intake assembly and clean it while on the motor, the maf you just take it off and spray it. let it air dry a couple minutes then reinstall. easy peasy.

I have cleaned the TB couple weeks ago as I said. I had actually removed it from the vehicle and thoroughly cleaned both sides and the circumference with carburetor cleaner. I also cleaned the chamber that is directly after the TB (which was extremely oily) and made everything look brand spanking new. Also my air intake filter is also clean.
I live in Qatar, there are no Walmart stores here but I will try to find Techron cleaner as suggested by another member.

 

[RizwanWali]

Also, to add, stp sucks....
Use the Techron Concentrate in the black bottle..
The large bottle is about $14, but you can find it on sale a lot.
You'll want to run 2 bottles thru initially due to the large fuel tank.
Then run 1 bottle thru about every 3 -6 months to keep your fuel
sender and injectors clean. It's the bomb...

I will see if I can find this. Thanks

 

[RizwanWali]

I have the same vehicle but am now at 148k, That stumble/miss at idle is just part of the 6.2 in my opinion. There are members that have "silky smooth" idle but i am over trying to attain it. I have a tech2 and have looked at everything that i feel could be a contributor and have found nothing. It isnt consistant, sometimes you cant feel it at all and other times, there it is. I got the vehicle at 125k and to say it was pampered is an understatement, I run only chevron premium, Get 17.5 to 19 mpg at 75 yo 80 mph with ac on. This is without a doubt the most comfortable functional "road limo" we have ever owned! My advice is to stop trying to cure somthing that is inherent to the 6.2. I have thrown a few parts trying to get it to smooth, VVT solenoid, purge solenoid, MAF. Welcome to the forum!

Also im superstitious about cleaning the TB, Be very cautious with the movement of the plate when doing so. IMO if you "super clean" it and open and close the plate recklessly it negatively affects the wiper assy and you end up with "no good deed go's unpunished". ie worse than when you started. just my op

i get less than 10 mpg and that’s all highway driving!

 

[RizwanWali]

Battery disconnected means you need to relearn the fuel trims. Drive it for a couple days and it should smooth out.

i will follow your suggestion... will this also smoothen the gear shift, do you think?

 

[wjburken]

I have cleaned the TB couple weeks ago as I said. I had actually removed it from the vehicle and thoroughly cleaned both sides and the circumference with carburetor cleaner. I also cleaned the chamber that is directly after the TB (which was extremely oily) and made everything look brand spanking new. Also my air intake filter is also clean.
I live in Qatar, there are no Walmart stores here but I will try to find Techron cleaner as suggested by another member.

The chamber right behind your throttle body is your intake manifold. How did you clean this?

Sometimes when you remove and reinstall the throttle body, you have to go through a “relearn” procedure so the vehicle knows the throttle position. This can take a while.

 

[RizwanWali]

The chamber right behind your throttle body is your intake manifold. How did you clean this?

Sometimes when you remove and reinstall the throttle body, you have to go through a “relearn” procedure so the vehicle knows the throttle position. This can take a while.

Yes I just sprayed some carb cleaner on a towel and cleaned the accessible part of the chamber. Basically a foot long section of it right after the TB.

 

[iamdub]

The plugs look original AC DELCO they came in their original packaging and looked brand new.
I must say though I did not change the gap as advised by the salesman.

If you got them in-person from a salesman, then they're likely legit. The half-priced ebay and Amazon specials are the ones you gotta watch out for.

 

[Fless]

The plugs look original AC DELCO they came in their original packaging and looked brand new.
I must say though I did not change the gap as advised by the salesman.

What is the advertised gap, on the box?

Did you check the gaps before installing them?

 

[FrankU]

When I changed the 41-110 spark plugs on the 6.2 engine I was told that the plugs are gaped correctly. Upon checking the gap the new 41-110 plugs were all gaped at a tight .030 verified with 2 tools. They should be gaped at .040 , so much for being gaped properly. Long story but I would check the gap on yours. A tight gap will give you the same problems the OP is expressing.

 

[RizwanWali]

What is the advertised gap, on the box?

Did you check the gaps before installing them?

Thanks for the comment.
I just checked the original box they came in, doesn’t say the gap anywhere. Maybe it’s worth checking out spark plug in cylinder 1 which is easy to get to... I don’t have a gap tool so I’ll probably have to show it to a tech somewhere.

Unless someone here can figure out the gap with a banana next to it for scale lol

 

[RizwanWali]

What is the advertised gap, on the box?

Did you check the gaps before installing them?

The box didn’t state the gap, I double checked. Also I don’t have a gap tool so I took the guy’s word for it being gapped properly.

 

[RizwanWali]

If you got them in-person from a salesman, then they're likely legit. The half-priced ebay and Amazon specials are the ones you gotta watch out for.

The ones I got were AC DELCOs and the set cost me $65 when converted from riyals.

 

[iamdub]

Iridium plugs are supposed to come with the gap preset properly for the application. You're not supposed to change it since most standard gap tools can damage the Iridium tip. Yes, you can adjust it, you have to grab the ground electrode with smooth pliers and exercise LOTS of hand control. The AC Delco 41-110 Iridium plugs should be pre-gapped to .040". Even if they were a little tighter, it wouldn't affect it much at all, nothing like what you're experiencing. SES light on? Checked the intake tube for improper reinstallation/leak?

I recently had my battery disconnected for about a month straight, if not more for an engine refresh. Intake manifold was removed and sprayed out with a garden hose, brake cleaner, high-pressure air, etc. TB blade, TB throat and MAF was all cleaned. It fired right up on the first pop and didn't have to relearn idle or anything. It just loped and idled goofy because of the cam and still on the stock tune.


Have you checked the wires to ensure they're all fully snapped into place?