I have a 2005 Silverado w/ the 5.3 Ltr engine ( non flex fuel ).....
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

 

where the new tires the same size and tread style as the old?

 

Welcome to the forum..
how many fill ups have you had since the tune?
Could it be you might be a wee bit more heavy on the gas with the new tune, checking out the performance...
How does the exhaust smell, you really shouldn't be able to smell any gas at the tail pipe...(vehicle warmed up)

 

The tires were the same Make/ Brand as factory, on the 3rd set!
In addition:
I stopped at the dealer, after this issue was noticed .......
A Tech2 was placed on the truck, No issues found... all connections were checked underhood, all were 100% ok.

I run K&N Air filter,have for many years, was recently re-charged, Before the new plugs were installed.
Dealer thought maybe the sensors inside the Air Filter housing were coated with recharge oil! Removed and cleaned with brake cleaner to rid of residue.....air cleaner box was removed and cleaned as well. Filter was cleaned of excess/ too much recharge oil.
All is good here!

 

I drive easy with it for the most part...Not heavy on the gas Before or After the tune it!
Have NOT checked the exhaust for gas smell at all, but will do!

Main ????? : Are factory plugs Iridium OR Platinum??????
I am still leaning toward the plugs being the issue!!!!!

 

3,500 miles since plugs changed.
Many full fill ups, either of 2 main gas stations used, same octane etc etc !
NO gas smell at exhaust!

NOTE:
My neighbor has the same Make/ Model/ Year truck with lower miles.
An Original Plug was just removed;
AC Delco 41-985 ( small #s 12571164) Iridium???????

Lets all figure this one out, is it a dealers way of going cheaper on parts, with no care as to what the end result is?

 

I think 41-110 replaced 41-985(shown as part# in manual). I know the 110 is Iridium, but I thought they both were.

From the manual:
Spark Plugs/GM Part#/AC Part#
4300 V6/25162556/41–932
4800 V8; 5300 V8; 6000 V8/12571164/41–985
8100 V8/12578277/41–983

 

So what you are saying is......the dealer installed the wrong plug?
Meaning there WAS 41-985's in it....and THEY Installed the Platinum PN# 12621258 !!!!!

Yes, the 41-985 may have been replaced with the 41-110's, BUT, this doesnt mean they are being used from the Dealer.

Just pulled a plug from a 07 Tahoe.....currently has the SAME issue as Loss of mileage.......but worse!!!
Dealer installed AC Platinum.....12621258......the same as they did my truck!
Lets go from here !!

 

You might want to do a little google'n. Quick search looks like GM part# 12621258 is Iridium.

I think GM part# 12621258 might be AC Delco part# 41-110.

See here: http://www.rmsautoparts.com/proddetail.php?prod=41-110

 

PN# 12621258 is a Platinum Plug!!!
PN# 41-985 is original.....replaced with PN# 41-110 BOTH Iridium plugs!!!!
From what i am gathering here..........have not googled yet , but will before contacting the dealer.

Still Main ????? :
Iridium VS Platinum ????

any further info is more then welcome, please!
Thanks all for the help, have a great week ahead!

Read next page..... INFO found out earlier today!