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Are Your Spark Plugs Gapped Incorrectly?

152673 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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Got this from a buddy of mine on Click to enlarge.



What I want to know is why AllData specifies a smaller spark plug gap for the 1.8L than it does for the 1.4L Turbo. It's usually the other way around.

That said, I can't see anything wrong with gapping the 1.8L motor bigger than the spec. There definitely won't be any danger of spark blowout with that motor running a .035 gap.

Edit: the images got screwed up for some reason.

In any case, AllData and GM both call for .033-.037 on the 1.4L Turbo, and .028 on the 1.8L N/A motor. I would say everyone can safely run .035 regardless of motor.
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...ah, but the "real" question should be: "...what does GM specify the gap should be?"
Same thing. That has already been checked and verified in the thread I linked in the initial post, only they went a bit further and added another decimal point of value.
My question (and really everyone's should be) is are the plugs gapped wrong or is GMs listed spec wrong?
My guess is that since everyone's findings are all over the place and are nowhere near accurate that GM's and Alldata's listed specs are correct. Seriously, .020 on the 1.8L? That's laughable.
How the ef do i do this, and where did i put my dad's spark plug ratchet thing.
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.
I've only just used a coin thing. What do you suggest we use for gapping them correctly..?
Something that looks similar to this:

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going to be doing this later today as soon as i get home from work
Please report back with what you find as far as the original gaps go.
The cost of the tool is my question. Thats the tool linked above? Or just a coin gapper? It looks like itd be more than $3.

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Coin gappers aren't recommended with iridium plugs. You can try it, but there's a chance you could break the tip.

Not sure how much the other tools are.

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You sure you didn't do my engine? That's exactly what I found on mine. I regapped to a hair over 0.030". Good use for a traditional feeler gauge, and practice for the upcoming valve adjustment on our Honda.
Sounds like you should up those a bit to .035. :)

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Did this to mine yesterday. 1.4L Looks like all were right about .25-.26. I took them all up to .35. So far I wasn't in shock and awe after the adjustment but I didn't expect to be either. Car runs smooth fuel mileage seems the same based on my normal 80miles round trip to work each day.
Wait till you fill up at the pump after the tank is through. I'd expect a 1-2mpg boost.
going to check mine to see what they are, if they are at the .2 range i will be going to buy ngk spark plugs and putting new ones in my car! hopefully i can check it this weekend
No need for NGK spark plugs. The ones in the Cruze are excellent and are Iridium plugs as well. You'd see zero gains going with an NGK plug over what's in there already, but you'd see a hole in your wallet instead.
ok just got doing this to my cruze with the 1.4l turbo... man glade i did... one was gaped at not even .19 and the others not even close to .25... gaped them all at about .36... went and fuelled up.. will report back later in the week what my mpg are.. i've been getting about 25 to 27 mpg on a full tank....again glad i did this.. thanks for the heads up
.19!!! Wow. It's disturbing to find that these are gapped so far off what they should be, and so inconsistently at that!

Did you notice any difference in how the car drove?
Wait....mine has factory NGK plugs installed. Do they not all come with NGK anyhow? I am confused.
Are they NGKs from the factory? I never bothered to actually check what brand they were. If they are, even better!
Here.. if you guys did not know.

GM's specified gaps(directly from SI):

1.4L - 0.0335–0.0374 in
1.8L - 0.028 in
Yep, we've confirmed this. Thanks for confirming it again (being honest, no sarcasm). What confuses me is why they recommended such a small gap for the N/A motor and a larger gap for the Turbo motor, lol. Coineeach seems to be loving his .035 gap on his LS though.
I would assume the larger gap for more volume and dense air from turbo. Smaller for just atmospheric pressure. Like you said preivously, larger gap ensures all the mixture is burned.
Right, it's just that historically, they've used smaller gaps on forced induction motors to prevent spark blowout. The spark plug gap for my N/A 95 Regal was .060 to give people a comparison.
I gapped mine to 35 and see no difference in mileage or power over 600 miles even though I really wanted to. plugs were gapped at 25 stock. be careful gapping these plugs, you can bend or break the electrode easily. use pliers to pull the ground strap up and then bend them down on the ground. a normal gap tool will ruin your plugs.
I went over this in a bit of detail a few posts (or pages) back, regarding which tools to use and why. A coin-type tool is not how people should be gapping iridium tipped plugs. Feeler gauges need to be used or a specific spark plug gapping tool.

Try putting your gaps back to .025 and see if you see a difference again. I know I felt a difference for sure, but it was mostly at lower RPMs, which the auto transmission won't really let you get into a whole lot.
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.
Agreed here. I ended up using my T27 a few minutes ago, but it wasn't as tight as I like it to be. Fortunately, these aren't on there very tight from the factory and can be removed even with a bit that goes into the end of a screwdriver.

I gapped my plugs to .040 and went out for a drive, scanning for KR and misfire events with Torque. Not a single issue. I'll be scanning the whole way to work tomorrow. I think I'm content at .040. That's only .003 larger than the GM/Alldata spec. Engine is smooth across the powerband and feels great.

I'll report my fuel economy when I fill up this week.
What will you be able to do about this problem.

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Write a case for it, file a complaint with engineering, find you a dealer and set up an appointment for you to take it in and have the dealer do it. Maybe with enough reports and complaints, GM will wake up and realize they're not gapping the plugs correctly when they're delivering the cars to the dealers.

My bad! You are right in that T-40 IS the correct size for the M6 X75 bolt.
As for my ride to work today it did feel like the car had a smoother low end pick-up but couldn't tell if the gas mileage was affected as the expressways were all fk'd up this morning.
I also can't tell if gas mileage improved yet. If it does, it will probably be in the range of 1-2mpg. I did also notice a smoother low end acceleration.
I had to copy this over here as well:

Got bored during my lunch break and decided to check mine. All 4 varied between .024 and .026. I set them at .035. It literally took 10 minutes.

On the way back to work I did notice the low RPM hesitation is gone.
If anyone reports hesitation at lower RPMs, looks like we have a solution!
Is this correction doing much for those with Cruzes with AT?
There should be a definite increase in fuel economy. It may not be much (1-2mpg), but it's something nonetheless. AT Cruzes may not notice it as much, but there will also be a slight increase in performance.
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