the trucks come with iridium, i ran into the same problem when i installed plantinum plugs in my truck. Put the AC iridiums in and it'll run like before

 

Where are you seeing this?

 

I was wrong!!!!!!!!!!
Ac Delco Plug # 12621258 IS Plug # 41-110 Iridium !!!
Looked at a new full box this morning.....the above marking were both on it!

I took for granted what the dealer had on the repair ticket.....THEIR wording was in correct!!!

Still, would like to get to the root cause of 2.0 - 2.5 MPG loss!

 

]I have a 2006 sierra 5.3 ltr that is flex fuel I also just changed plugs, wires and reoiled my air cleaner. Your average mpg before and after are identical to mine. I now have 106,000 miles. My truck had 41-985 plugs. Did you ever resolve your loss of mpg?
Issue : Gas mileage loss after having the spark plugs replaced by the dealer, at 94,000 miles! Current mileage 97,500.
( was averaging 18.5 - 19.3 before replacement)

AFTER New plugs were installed, barely can get 16.8...( on the ip reset) as used always for calculating.

Driving conditions.....are calm and cool when tested over a period of 6 weeks.

Dealer replaced Original Spark Plugs w/ AC Platinum PN# 12621258

After doing some research...i am thinking this engine had AC Iridium
PN# 41-110 from factory. Is this correct?
Can the use of Platinum plugs over Iridium create such a significant loss in mileage?
Also had Nitrogen placed in new tires at the same time...
Will reply to all posts if needed.
Steve[/QUOTE]

 

What I would give for that kind of MPGs... I have a stock 2007 Chevy Classic 2500HD with the 6.0 engine. I'm getting anywhere from 8.9 mpg up to 10.0 mpg (max) over the last 35,000 documentated miles...

 

The resolution was, the dealer found Old Plug# 41-985, had them overnighted to dealership to be installed the following day.
After installed, mileage increased to 18.5-19.1 mpg. The truck now has 167,200 miles on it and is running strong with NO problems to report at all.
The original plug wires etc etc are still being used.
***********
Nitrogen is a total rip off to the customer, had the dealer do my tires in July 2010.....about a month or so later tp was checked and all around they were down 10-14 lbs PER tire. I called the dealer to ask if Nitrogen at ALL dealerships would be installed free of charge?? He replied saying NO, and told me to just use air.......SOOO, really about all you end up with is Green Valve Caps.
Am serious....DONT USE NITROGEN, Its a come on for the customer!!!!!!!!!

 

If oiling the air cleaner, by way of spray or liquid...don't overuse.....if MAF Sensor gets coated w/ it......mileage loss may happen.
If in doubt, take it off, spray sensor w/ brake cleaner and blow dry off....re install and go from there.
The dealer blamed many many things, K and N air cleaner, was one of them.
Yes, I still run K and N, just recently cleaned and recharged it 1 month ago.

 

I had a feeling spark plugs were the problem. I used maf cleaner. Wow brake cleaner is a lot cheaper. Thanks for the great info. I hope I can find 41-985 plugs.

 

Amazon has the AC Delco 41-985 for $13 each...