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Are Your Spark Plugs Gapped Incorrectly?

152826 Views 545 Replies 129 Participants Last post by  wasney
I'm creating this thread to increase exposure of what appears to be a very common issue (affecting 100% of Cruzes so far). This stemmed from the following thread:

My original experiment was to increase spark plug gap over what they are gapped from the factory and test for measurable gains in fuel economy. What I didn't realize was that I had actually gapped them to the correct spec, and they were gapped much too small from the factory.

AllData specifies a spark plug gap of .033-.037 for the 1.4L Turbo motor. Reports so far have come back with .024, .025, .026, and .029 as measured spark plug gaps from the factory iridium plugs. These should have been pre-gapped from the distributor, but clearly weren't.

Coinneach checked his spark plug gap on his Cruze LS with the 1.8L N/A motor and found a .020 spark plug gap, which is absurd for a N/A motor and is smaller than he or I have ever seen before in any engine. He increased that to .035 and had the following to say when I asked if he noticed a difference:

Like a whole new beast. It's not quite as zippy as the 1.4T in the Eco that I drove when I was shopping, but it's a *censored* of a lot snappier now.

I currently don't know what AllData specifies for the Cruze LS, but if someone can get that information into this thread, it would be of great benefit to everyone. Hopefully, someone will get a tutorial/video made soon. There are significant performance and fuel economy gains to be had by correcting the spark plug gaps on these cars.
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...ah, but the "real" question should be: "...what does GM specify the gap should be?"
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Well I just pulled my plugs they were all around. 25 I bumped them up to. 33 and this weekend I'm indexing them.

Sent from my Droid
...what process are you using to achieve that indexing: 'stacked gaskets' or simple 'twist-to-fit'?
Inserts I'm buying a kit.
JEGS Spark Plug Indexing Kit - JEGS

Sent from my Droid
...excellent, you're doing it the "correct" way!

FWIW you'll probably notice about 1-5% increase in hp (see #4) with the extra 2-5% coming courtesy of the turbo's higher operating CR:

The old MOPAR dual-plug HEMI™ engines "love" indexing.
...that's *why* I said we need to ask GM what the gap SHOULD be, NOT just go by what they seemingly incorrectly have printed in the Owners Manuals.

...ah, but the "real" question should be: "...what does GM specify the gap should be?"
.028 on a N/A engine would initially make me question the quality of the other ignition system components...particularly coils. I'd find it hard to believe though that the coils are that poor quality. I would guess that there was trade-offs made between emissions requirements and combustion efficiency as it relates to gapping. Just my guess...
...or, it could be that:

(A) the OEM plug maker simply "goofed" and isn't providing plugs gapped to GM's specifications?!?


(B) GM(USA) doesn't KNOW what the proper gap should be for the GM(Opel)-designed 1.4LT engine they're using?!?
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...from Technical Data 10-3 of that 2008 Canadian Saturn Owners Manual: 1.8L L4 1 Automatic / Manual 0.035 in (0.90 mm)
...Having worked through GM system engineering processes and having experience on the factory floors themselves, I can say with confidence that an oversight like you are suggesting is not even a remote possibility.
...ex-GMHE engineer here.

...and, Cruze steering wheels shouldn't have fallen off, nor PRNDL shifters incorrectly register, and transmissions shouldn't shift erratically, and Sonics shouldn't have been made/shipped/and sold that didn't have rear brake shoes, but they have, and continue to do so. much for GM Quality Control being 100% effective.
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Reactions: 3 be honest, it simply sounds like the "new" Global GM hasn't got a clue about: (a) their products, (b) the specifications of those products, (c) the incoming QC of farmed out parts for their products, or (d) the outgoing QC of their final products.
...and, then there's the obvious discrepancies in their PUBLISHED spark plug gap numbers!

...sorta like how the 2011 Cruze Manuals states it's OK to flat-tow them behind an RV when it's not! Who does their "editing" these days?
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...facts can be specified, measured, quantified, and verified.

...opinions can be defended, argued and/or ignored.
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Man and 7AARCUDA are quite the tagteam - ever think of joining the WWF? I mean let's make smartass comments to everyone who posts something that doesn't align with your mode of thinking, that is extremely mature. that your opinion or your fact?

...sorry, but the "kid" in me made me type that (but, the "adult" in me grinned as I did it)! "in spec" spark plug gap ensures: (a) intended engine performance, ie: power and economy and (b) compliance with EPA emissions regulations. "in spec" spark plug specification is it's operational gap, ie: what it starts out with (gap widens with duration and wear).

...too narrow gap and less than full ignition-coil energy (watt-seconds or Joules) is delivered to the air-fuel mixture, resulting in under performance and more than intended / permitted emissions.

...similarly, too wide gap and less than full ignition-coil energy is delivered to the air-fuel mixture because the arc periodically fails to establish, resulting in mis-firing and poor performance and even more emissions; and, with increased CR, possible "blow-out" of the arc.

...the "question" is: what gap does GM-Powertrain intend this engine to have (for intended performance & emissions), which, seemingly, they haven't (yet) got right in their own publications (conflicting values) and in their delivered vehicles (conflicting values).
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...not nit-picking, but it looks like the 0.021" gap percentage should be 0.9% rather than 0.09%

.019: 1.8%□□
.020: 9.8%□□□□□□□□□□
.021: 0.9%
.022: 5.4%□□□□□
.023: 3.6%□□□□
.024: 25.0%□□□□□□□□□□□□□□□□□□□□□□□□□
.025: 32.1%□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□□
.026: 9.8%□□□□□□□□□□
.027: 2.7%□□□
.028: 5.4%□□□□□
.029: 3.6%□□□□
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Sounds like you might have to do this yourself, or find another dealer who won't lie to you. I would also have asked to speak to the tech directly, not the service adviser.
...I got the same "...numb/dumb speak..." when I asked at my dealership: "...oh, they're OK because we couldn't replicate the problem you described." Nothing whatsoever was explicity stated as to whether even ONE plug had been pulled to check/verify what the gap was. Argh!

...and, people wonder *why* I'm not a "happy-camper" GM-fan?!?!?
Hey, GM =/= GM dealerships.
...true, but "mothership" GM seemingly does nothing about their bad/poor dealerships, so they're guilty by "responsibilty" in my book, thus the GM "captain" can go down with his ship as far as I'm concerned (wink,wink).
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Reactions: 1 a "compromise" solution, you might ask GM for a "...corrected REPRINTED page(s)...", containing the new "correct" information(s), so that you can insert it/them into your Owners Manual.
...that works out to 84.1% being 0.025"±0.002" or, alternatively, between: 0.023-0.025-0.027 inches, which imples 15.9% are out-of-spec either (a) from NGK or (b) banged up during GM installation?!?
Many thanks to Stacy and her involvement in this issue.
...yes, indeed, "Thank You," Stacy ! ! ! !
...sorry to see your efforts come to nothing (so far), as I know that you're doing the right thing, regardless of Global-GM's lip-serviced feet-dragging!'s come full circle, around again, to the point I made in my first post on this subject:

...ah, but the "real" question should be: "...what does GM specify the gap should be?"
...maybe this guy within GM-Powertrain might have be able to find an answer:

Tom Read
Technology Communications, Powertrain
Mobile: 248.496.0852
GM Renaissance Center
Global HQ | 100 Renaissance Center, 482-A38-D16, C64 | Detroit, Mich.

GM Powertrain Engineering HQ,
Bldg. C | 895 Joslyn Ave., 483-710-201, 2L08 | Pontiac, Mich. 48340
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