Changed Spark Plugs-MPG Dropped

[rocknrun]

A couple of weeks ago I changed the spark plugs in my recently acquired 04 KJ, which has 89,000 miles on it. The plugs I pulled out were NGK spark plugs and in bad shape; the center electrodes were really worn down, barely sticking out from the ceramic. Put in Autolite Double Platinums and was looking forward to a MPG improvement, instead my MPG has dropped by about 2 MPG. Any ideas?

 

[LibertyTC]

That is interesting. Why not try another set of stock NGK's and see if it goes back up?
In reality I doubt that new plugs should cause this, maybe a heavier foot, less air pressure in tires? Heavier Jeep?
I run the stock NGK's and change them every 18,000 miles. Those coil packs are hot & tend to eat the electrodes causing wider gap quick.

 

[Porkchop]

Maybe running the A/C more

 

[Dave]

I don't see how changing the plugs can give you 2mpg less than with old worn plugs. Must be a coincidence with something else?? A lot of little things affect fuel economy.

I am no expert but that's my thoughts.

Dave

 

[cplchris]

only thing i could think of if it were the new plugs causing the drop would be an incorrect gap on the new plugs

 

[tboneil24]

Only thing I can think of is you have a cracked plug. I would think that would throw a misfire code though. Do you have a check engine light?

 

[rocknrun]

I think Dave and porkchop might be right...probably coincidence. Libertytc, I think I will pick up a set of NGKs and just for giggles try them out and see what happens. The old plugs I pulled out looked so bad I just knew I would get a few more miles out of a tankful so I was disappointed. I drive about 800 miles a week, knew when I bought it the KJ it wouldn't be real thrifty on gas; I'd been Jeepless for several years since I retired the XJ though and wanted my butt out of a Buick (last GM product I'll ever buy) and back into a Jeep :waytogo:

 

[67Customs]

Over torque the new plugs?

 

[retmil46]

only thing i could think of if it were the new plugs causing the drop would be an incorrect gap on the new plugs

X2. Did you check the gap on the new plugs prior to installing them?

A couple years back, my father had a local mechanic change the plugs on his '98 F150 with the 5.4 V8. Thing ran like crap after he got it back. When I dug into it to check it out, not only had the guy used the wrong plugs - he'd installed the plugs for a natural gas engine!!! - out of the 8 plugs, 3 were gapped for a natural gas engine, 3 were gapped for the V6 engine, and only 2 were gapped remotely close to what his engine called for.:freak3:

 

[rocknrun]

X2. Did you check the gap on the new plugs prior to installing them?

A couple years back, my father had a local mechanic change the plugs on his '98 F150 with the 5.4 V8. Thing ran like crap after he got it back. When I dug into it to check it out, not only had the guy used the wrong plugs - he'd installed the plugs for a natural gas engine!!! - out of the 8 plugs, 3 were gapped for a natural gas engine, 3 were gapped for the V6 engine, and only 2 were gapped remotely close to what his engine called for.:freak3:

Yep, checked the gap on each of them. Hope your dad never took his truck back to that mechanic!

 

[Banditsteve]

I think that subconciously you are driving with a slightly heavier foot to see if it feels any better with the new plugs in. New plugs will only improve the mpg if they are set correctly. I think in a couple of weeks you will see an improvement when you forget about them. Steve :smokin:

 

[retmil46]

Yep, checked the gap on each of them. Hope your dad never took his truck back to that mechanic!

Be forewarned - my father lives just down the road from you in Mooresville - so that same mechanic is in YOUR neck of the woods as well!!