OEM Spark Plugs Out of Spec.

[Mustang_Cougar]

My gas mileage has recently taken a bit of hit on my '05 Pontiac Vibe AWD w/57K miles, so amongst other things, I decided to install some new plugs and a new PCV. Based upon good reports on Corolla, Matrix & Vibe forums, I went w/the NGK Iridium IX and they came pre-gapped between .042-.044 (about perfect for my application). Anyway, I measured the original OEM plugs to just see if any of them were really out of tolerance and here's what I got:

Cyl.
1: .030 (?)
2: .037
3: .040
4: .038

It's been my experience that the gap expands over time due to erosion; but these plugs all come in below the .044 that is the OEM spec. And I'm really at a loss to explain the plug that came out of cylinder one (.030). Any insight or comments as to why my car would have left the factory w/such gapping would be greatly appreciated.

BTW: Yes, I used two different gapping tools to check the tolerances and no engine work has ever been done on this vehicle (I'm the original owner).

Mustang_Cougar

 

[Colt45ws]

What type of gapping tool did you use? The coin ones or a wire type? Ive found the coin types to be inaccurate and difficult to get a consistent reading.

 

[MarkM66]

That one was dropped before installed.

How bad has your milage dropped?

I may be wrong, but I doubt those plugs are the culprit.

 

[punisher]

Originally Posted By: MarkM66
I may be wrong, .........


You're not.

 

[Eddie]

I really doubt the mileage suffered from the plugs unless you were getting as misfire code. The gap should always we checked with a good wire gage. Feeler gages and circular gages are no good for this. You may have broken off the tips on the fragile iridium plugs!!!

 

[brianl703]

Originally Posted By: Mustang_Cougar
Any insight or comments as to why my car would have left the factory w/such gapping would be greatly appreciated.


Was it built in a UAW plant?

 

[cchase]

I can't imagine you'd get that much (if any) erosion on iridium/platinum plugs in 57k in the first place.

 

[Mustang_Cougar]

Originally Posted By: Colt45ws
What type of gapping tool did you use?

On the old plugs, I used a wire gauge first and then, because they were so out of spec, I confirmed w/an old coin type. The new plugs were gently checked w/only the wire gauge.

Originally Posted By: MarkM66
How bad has your milage dropped? I may be wrong, but I doubt those plugs are the culprit.

Over the last year, and accounting for both summer/winter gas blends, I've consistently lost about 3mpg vs previous years. Same commute, tire pressure regularly checked, no codes, clean air filter, clean MAF sensor, no change in oil grade, etc.

Swapping out the plugs and PCV were just the latest in the "start w/the cheap things to fix it" routine. The old plugs looked fine, albeit the strange factory gapping; the PCV passed the rattle test too (still replaced it though, as it was only $6).

Originally Posted By: brianl703
Was it built in a UAW plant?

Yes (NUMMI, Freemont, CA).

Originally Posted By: cchase
I can't imagine you'd get that much (if any) erosion on iridium/platinum plugs in 57k in the first place.

Yes, I agree...they're supposed to be 100K mile plugs and they looked great. The only problem was the under spec, factory-set gapping. As I already had the new, properly gapped IX's and the old ones may have been damaged when I double/triple/quadruple checked their gaps, I installed those.

Anyway, I'll be tracking my gas mileage as usual, so if I find any positive difference, I'll post the numbers.

 

[LTVibe]

Originally Posted By: Mustang_Cougar

Originally Posted By: brianl703
Was it built in a UAW plant?

Yes (NUMMI, Freemont, CA).

But the engine was built in Toyota's non-union plant in Buffalo, WV.

 

[The_Eric]

O2 sensor.

 

[CharBaby]

Originally Posted By: The_Eric
O2 sensor.


Yeah, I'd have to agree, especially if the sensors have never been changed out. But also, I want to blame the fuel that we're buying these days. I'm not happy with the MPG of any of my vehicle lately and I have complained in other posts. In one vehicle in particular, I'm going to clean the throttle body and the IAC valve as I have already changed out the O2 and A/F ratio sensors.

 

[morris]

been working on cars/trucks since 1965. i have found that when the center electrode rounds off the performance drops very fast. the trucks i run call for a .035 gap, i have found that .040 works well also

 

[Rix]

NO!!! to the o2 sensors unless they're checked, and found to be not operating properly. (in which case it SHOULD have a code)

 

[The_Eric]

Originally Posted By: Rix
NO!!! to the o2 sensors unless they're checked, and found to be not operating properly. (in which case it SHOULD have a code)


I said O2 sensors, with the assumption that their function would be checked. Also, an O2 sensor whose performance has degraded WILL NOT always kick a code, but WILL cause a decrease in mileage and performance. Seen it dozens of times.

 

[mechtech2]

A 'coin' type gauge is tapered , so you read it at the edge of the taper, where it meets the gap edge.
Tricky!

so you upped your gap to where it should be?
You should gt better ignition of the mixture when cruising now.
Lean and less dense likes the larger gap.
Actually, we need as large a gap as will not misfire. Misfires are under load, and that is our limiting factor.
Factory gap recommendations leave a nice margin for wear and component deterioration.

Nice work