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Are Your Spark Plugs Gapped Incorrectly?

152825 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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Cuda, the gap specs aren't the problem; it's that our cars are being delivered with incorrectly gapped plugs.
My question (and really everyone's should be) is are the plugs gapped wrong or is GMs listed spec wrong?
Considering the increase in economy and performance we've uniformly seen after regapping, I'd say the former.
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You do this one step at a time, patiently and slowly. I believe it's the smaller of the common spark plug sockets to get it out. You'll also need a torx bit, but I don't remember what size it was. Don't adjust gap with one of those coin looking things; you'll need proper feeler gauges. You might otherwise damage the plug as it's an iridium tip. Once you get the engine cover off, the coil bar is right in the center with the two bolts that need removing, and you can wiggle it right out (again, patiently and slowly). Check inside the boots to make sure the springs didn't bind, and proceed to the spark plugs.
It's the 5/8" spark plug socket and T-40 torx bit.
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How much does that cost

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Adjusting the gap, assuming you have the tools, costs nothing but maybe 15 minutes of your time. The gap tool shouldn't cost more than $3 or so at Pep Boys, Autozone, etc.
Actually it's a T-27 torx (same size that you use for your rear brake drum screw removal).
Beg to differ. You can certainly use a 27, but 40 is more snug and doesn't slop around like a 27 or 30. Yes, even on the drums.
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I just called NGK and they said that this plug was made specific for the 1.4 and the correct gap is indeed .027. He also stated that the ignition system wasn't designed for a larger gap due to resistance and the variable timing. I have a few calls in to local dealers also.
So we have the plug manufacturer saying one number and the car manufacturer saying something else. That's not encouraging.
Then, have your dealer's service department on the line while you do this so NGK can explain to them why they're wrong and should instead be gapping the plugs to .027 instead of what AllData and GM's service manual states. If you could get GM's customer service on the line, you might have some fun as well.
If your idea of fun is two engineers and a CSR all screaming epithets at each other, sure.
For grins, this morning I replaced my factory NGKs with new Autolite iridiums. I gapped them to .030*, went for a longish drive, refueled, ran some errands and went to work, then came home. Performance and fuel economy loss *CONFIRMED*; 1st and 2nd feel slow as February molasses now and I'm getting 30mpg indicated city**. Bumping them back to .035 first thing tomorrow. Once my Trifecta is installed and tweaked, I may play with the gap more and get some objective numbers via datalogging.

*: Little Old Autozone Lady Cashier told me, "They're already gapped, no need to worry about that." Ha. Also, HA. Yeah, they were gapped: .045.
**: I know, kri moar about only getting 30 city in a car rated for 25.
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GM didn't specify a .025 spec anywhere. That was Vince's idea. GM paid NGK to spec to .028 on all motors. GM's service manual states .033-.037 for the 1.4T and .028 for the 1.8N/A.
What's really funny (as in weird, not lol) about that is, I just looked in my owner's manual and it calls out .028 for both engines. Someone, maybe multiple someones, dun goof'd.
Well the manual states 0.028" spark plug gap. NGK set them to 0.027" gap. Looks like they are close to spec to me.
The owner's manual states .028. The service manual states .033-.037. And looking back over the thread, you can see that a lot of us have plugs that weren't set to .028 in the first place.
People shouldn't be having toxic coolant leaks into their cabin, the struts should never have started popping, the drum brakes should not have been "adjusted" to zero, the paint behind the rear doors should never have started chipping, the clutches up till November 2011 should never have been failing...

I and many others here can go on, and on, and on...
In any technical endeavour, there's engineering (what *should* happen), manufacturing (what *does* happen), and tech writers (what happened?).
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I've been internetting for a long time and BBSed for several years before that, and never have I seen so much animosity over such a simple issue. This topic is devolving into a religious war - on one side we have people, call them the Thomases, who heard a different version of the Word and had to find out for themselves; on the other we have those who believe the Word as handed down (in multiple conflicting versions, no less) and absolutely will not tolerate any dissent.

They're just spark plugs, people.
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