'02 Tahoe 5.3 - New Ac Plugs = Worse Gas Mileage

UGA91 · October 30, 2007

Hello all,

I just put in a set of new plugs in my '02 Tahoe w/85k mi. I searched this site and decided on going with the new AC Delco Irridum plugs as this is what dealers are calling for as well as most parts stores. It originally had the AC/NGK plugs that no one seems to carry.

Since putting them in, the idle slightly soothed out slightly and there is maybe a little more pep in the step, however, the last 4 tanks of gas show significant drop off in gas mileage.

Up until now, it's been rock solid consistent @ 14.3 - 14.7 mpg with my normal driving. Now with new plugs, no other changes including my driving I'm only getting 13.3 - 13.7MPG. You'd think new $60 of plugs would be an improvement to economy.

What gives? Anyone else experience this problem?

RyanZ71 · October 31, 2007

Hello all,
I just put in a set of new plugs in my '02 Tahoe w/85k mi. I searched this site and decided on going with the new AC Delco Irridum plugs as this is what dealers are calling for as well as most parts stores. It originally had the AC/NGK plugs that no one seems to carry.

Since putting them in, the idle slightly soothed out slightly and there is maybe a little more pep in the step, however, the last 4 tanks of gas show significant drop off in gas mileage.

Up until now, it's been rock solid consistent @ 14.3 - 14.7 mpg with my normal driving. Now with new plugs, no other changes including my driving I'm only getting 13.3 - 13.7MPG. You'd think new $60 of plugs would be an improvement to economy.

What gives? Anyone else experience this problem?

Did you disconnect the battery for 30 min or so?

Also do you get Winter Blend gas where you live?

silverad03 · October 31, 2007

Did you also check the gap on the plugs when you put them in?

UGA91 · October 31, 2007

*did not disconnect the battery after install......will give this a try.

*not sure about winter blend on gas (Atlanta market) but after 5 1/2 years this has been the most consistent vehicle I've ever owned regarding gas mileage. I do not believe it was the gas.

*I checked the gap on the new plugs as well as the old. Based on my guage, the old ones were .02 off as well as the new ones (.58 & .38). Mine is a very cheap guage and since both new/old were off by exact same amount, i didn't adjust them.

Thanks for the replys so far

C & A s Dad · October 31, 2007

Two things: When you were checking the gap is it possible that you damaged the electrode? This thinks are very sensative to being damaged from a gauge. Next, did you change the wires or simply re-use the old ones. If you used the old wires, this is likely where you problem is. Even if they look good, you likely damaged them internally when you pulled them off.

Bish · October 31, 2007

Two things: When you were checking the gap is it possible that you damaged the electrode? This thinks are very sensative to being damaged from a gauge. Next, did you change the wires or simply re-use the old ones. If you used the old wires, this is likely where you problem is. Even if they look good, you likely damaged them internally when you pulled them off.

Ditto on the plug wires. They are fairly easy to damage when removing the boots. Especially if you haven't removed them in many miles. When installing new ones, I use a glob of dielectric grease to keep the connection from corroding and to make sure the boot comes off nice and easy if necessary.

UGA91 · October 31, 2007

I was gentle on the plugs when checking the gap, don' t think the electrodes were damaged.

I figured since it was running strong, it wouldn't be the wires. I think I'll try to reset the computer first. If that doesn't take care of it, will try the wires next.

Thanks again!

gmcjoe · October 31, 2007

Did you also check the gap on the plugs when you put them in?

Thats what I was going to ask

Did you disconnect the battery for 30 min or so?
Also do you get Winter Blend gas where you live?

Why would you need to disconnect the battery after changing plugs?

RyanZ71 · November 1, 2007

Why would you need to disconnect the battery after changing plugs?

Rule of thumb that was handed down to me by several GM mechanics and several others on the forums years ago when I first got my 99 and had done plug replacements. Something to do with resetting the computer. Whatever it does, it helped. You can go into all that garbage about "That has nothing to do with the spark plugs" but for me, it made a difference.

gmcjoe · November 1, 2007

Rule of thumb that was handed down to me by several GM mechanics and several others on the forums years ago when I first got my 99 and had done plug replacements. Something to do with resetting the computer. Whatever it does, it helped. You can go into all that garbage about "That has nothing to do with the spark plugs" but for me, it made a difference.

Interesting, this is the first I've ever heard any mention of that on the forums or from my engineer/service buddies.

If it works for you

redvett · November 1, 2007

When you installed the plugs did you put anti seize on the threads and torque the plugs to 15ftlbs? If not you may not be getting the correct heat transfer resulting in poor performance. Resetting the PCM isn't needed for a tuneup.

UGA91 · November 9, 2007

Update:

I disconnected the battery as suggested and ran through a full tank this past 1 1/2 weeks with the same type of driving (maybe a little harder than normal as it seemed to have a little more umph). Jumped back up to 15.1 mpg for 335 miles.

I did not change the wires out, but only 'reset' the computer. Not sure if this is always neccessary or only when going from old gap size to new ones, but I guess I'm a believer.

Thanks for all the input.

Wayne