Iridium Plugs issue

I recently put the new Iridioum plugs in my 1999 2500 Silverado with 6.0L engine.
It ran like crap!
GM recommended the Iridiums over the platinums, reguardless of the fact that my original plugs were platinum gapped at .060 whereas the new iridiums are gapped at .040.
My truck started to hesitate badly, felt like it was misfiring, and my mileage dropped from about 12 mpg to under 10.

I spent a week trying to figure out what was wrong with my truck. I replaced the fuel filter, tested all the coil packs and was just about ready to dump a full tank og gas and replace the fuel pump when I decided to re-exam the plugs.

I replace the plugs AGAIN with autolites platinum gapped at .060 and the truck ran beautifully, my mileage is back up.

We have a few older GM's, 99's, 2000's in our fleet at work with high miles and the plugs were replaced with the iridiums. They have the same apparent problem but the mechanics regapped the Iridiums at .060, not knowing delco and Gm say not to regap them.
I have noticed on the newer GM's their emissions sticker doesnt specify spark plug gap. Mine and the ones at work specify .060.

Did GM respec their engines so the iridiums work in them and forgot that maybe the older 6.0L engines might not run correctly with them? Anyone else put iridiums in their 6.0L with an emissions sticker that specifies .060 gap in their plugs and notice a hesitation, misfire or mileage problem after?
Am I the only one here noticed the 99's seem to have an issue with that?

Thanks for your feedback. AL

Perfect timing for your post.

I was just about to get the iridium plugs. I have 75000 with the factory plugs still running good.

Was debating over the autolites platium and the iridiums. Looks like you have made my decision. Thanks!

Do you think the problem is the gap or what? What did the plugs look like after u pulled them?

Were these Delco Iridiums?

I put the NGK TR55IX plugs in my 5.3L. Didn't really notice a whole bunch difference from the A/C Delco plugs that were in there before. It will be interesting when it comes time to change them again whether or not I will purchase them. I may just go with the Platinum ones and see if i notice a difference. I even swapped in NGK High performance plug wires but, that didn't seem to do much either...

Just my :2cents: worth.

PS -- My engine called for .060 gap and I was worried about the TR55's being at .055 but, it doesn't seem to bother the engine.

I have the Bosch 4 plugs....yah, I know that some of you might say that they are only for european cars...but I know several guys that use them.....and I know a lot of guys on these and other forums use them.......I love the extra power that they give me, and yes you can feel the difference from the 'cheap' delco ones.......actually a big difference.......I recently checked the spark plugs after about about 65,000 kilometres, and they looked like brand new....there was just a little bit of of grayish colour at the tip..........bosch plugs are really good and I would recommend them in a jiffy......

I have the Bosch 4 plugs....yah, I know that some of you might say that they are only for european cars...but I know several guys that use them.....and I know a lot of guys on these and other forums use them.......I love the extra power that they give me, and yes you can feel the difference from the 'cheap' delco ones.......actually a big difference.......I recently checked the spark plugs after about about 65,000 kilometres, and they looked like brand new....there was just a little bit of of grayish colour at the tip..........bosch plugs are really good and I would recommend them in a jiffy......


Not sure how Chevys will respond to the Bosch plugs but i have worked on Jap imports for 20 years and i can tell you a bosch in a jap engine is junk.

I just used the plain NGK TR55's and have no problems at all. They were like $8 a box of 4

I took the bosch plugs out of my truck in favor for the AC iridums. They have given me no problems and did not change any of the power of the truck. no regaps required.

i put the ac delco #41-985,iridium in my 6.0L, and i do not like them. seems like my milage decreased about 1.5mpg. does anyone know the ac delco number for the platiums at .060" gap ?
i think the .040" gap on the #41-985 is too small.
thanks.crawfm
2002,2500HD,CC,6.0L

Delco doesnt recommend the platinum with the .060 gap, they have a plug called rapid fire gapped at .060. the number for it is .12 Rapid-Fire. And I too noticed about a 2 mpg fuel loss with the iridiums at .040. I put Autolite's platinum in which are gapped at .060 and mileage went back up. AL

Yea, they were Delco's...and they aren't cheap!!!!