Vehicle runs rough after spark plug change

[Jay]

Hello everyone, hope you’re doing fine 😀

I’m Jay, an auto tech student (currently in my engines class). I had the chance to roll my car into a bay during class to change spark plugs, do compression test, and cylinder leak down test on my engine.

My car is a 2007 Mazda CX-7, 2.3 L 4-cylinder, FWD with a factory turbo charged engine and 155 000 miles on it. Also my spark plug gap spec is .029-.031

And after the whole process everything is perfect, except my engine runs rough/shakey ( shakes a bit more than usually that i can notice it at idle from the driver seat ). All the details are down below if you’d like to read, thanks for your help 🙂

1. I pulled my old plugs out, which turned out to be the original, factory FoMoCo (Ford motor company) spark plugs, back when Ford owned/cooped with Mazda. they were in pretty good shape, no rich dark deposit, only has a white deposit on them which i don’t know what that is (old carbon?)

2. After doing my tests and all was good, i replaced those plugs with NGK’s laser iridium plugs (~14$ per plug) with .032 gap. I knew it was .001 above spec but that’s what all part shops like oreily’s autozone and NAPA had specifically for my car, it only pulled up .032 plugs on their websites. So i said oh well i won’t fight that.

3. Torqued the plugs to spec, Put my coils back in exact order, made sure 2 times the wires were properly connected and i even torqued the hold down bolts on the coils and made sure no dust or dirt got into the spark plug wholes the entire time.

4. Car started, No problems what so ever (no misfire nor a mil) and drove it for 1 week now.

5. Only thing that i noticed since day 1, is that the car runs rough, the engine shakes at idle more that it ever did before the spark plug change.

I wonder why this is happening, I was really careful on every step. I have two things in mind, either that gap difference is affecting performance, or the fact that the plugs are new, the engine is actually firing correctly now? maybe stronger combustion?

EDIT: Forgot to mention that the NGK plugs had the same id number as the fomoco one

Topic

[none]

Two things come to mind. These days, OE plugs are about the only thing you should ever install in your engine. Motorcraft plugs should have been replaced with Motorcraft plugs. The more immediate thing to consider is a vacuum leak. You might have left a small vacuum hose disconnected somewhere or a gasket might not have been seated correctly somewhere. Get a scan tool and look at your fuel trims. If they’re showing high positive corrections, rev the engine up to around 1500-2000 RPM’s and look for the fuel trims to return closer to a zero percentage correction. If you can diagnose it as a vacuum leak and correct that leak, then you can try leaving your NGK’s in to see how they behave.

[Glenn]

I concur with no_common_sense regarding starting with the plugs. I once put an expensive set of Bosch plugs in a GM. It ran horribly until I replaced them with AC Delco plugs. Just about drove me crazy. Good luck…

[Mike]

You installed plugs that were gapped beyond the maximum recommendation. The first step would be to remove and gap the plugs, aiming for the low end of the recommended gap range. That way, the plugs will stay in spec longer as the gap increases with age.

Make sure the new plugs are an exact cross-reference replacement for the original FoMoCo plugs. Same reach and heat range. Personally, I don’t put much credence into the modern myth of only certain plug brands working well with certain engines. I’ve never had the issue crop up. I put Bosch Platinum plugs into everything I work on, (provided Bosch makes an exact replacement for the stock factory plug), and have always had consistently great results with them. As long as the specs match, the engine shouldn’t care what name is on the plug, provided you aren’t using some weird or low-quality brand that is better suited to a lawn mower.

It’s possible that a plug boot isn’t seated properly. You said you double checked, but boots can be tricky at times, giving you the impression they’re fully seated when they aren’t. Make sure you distinctly hear the metalic “click” of the boot connector mating with the plug terminal. It can sometimes take a surprising amount of grunt to overcome the air bubble that can become trapped within the boot when pushing it onto the plug. A very light smear of silicone dielectric grease inside the boot sometimes helps when seating boots. The grease also helps the next time you pull the boots off the plugs, which might be after several years and many miles of service, when the boots could otherwise have baked onto the plugs if not for the lube.

That last point brings up another possibility. Perhaps one of the old boots/wires was damaged during removal.

[Jay]

Thanks for your reply, I will try to test for vaccum leak, but i doubt it because i always keep an eye on vaccum.

Another thing I forgot to mention, the plugs between fomoco and ngk have the SAME id number

[Jay]

Will try to gap the plugs, although i never did that before and i heard not to do that on iridium plugs, but i will research how to.

And the new ngk plugs have the same id number as the fomoco ones, but im not sure about the heat range and all that.

And good point about coils not seating properly or not being plugged in all the way. Because i do recognize one coil on cylinder number 4, that was sticking out more than the rest, i tried to adjust it, remove put it back on, but it stayed like that, so i thought it’s normal since i’ve seen that on some other car as well. Yet again i didn’t over fiddle with that one. I will check this asap

Appreciate your help

[Fernando cunha]

Why do I get the feeling that there’s cheating here.
Is like been in classroom doing exam.
The guy next to me is asking for answers.
You said your a auto tech student (currently in my engines class)
why not ask your teacher or mentor.
Honestly!
If you don’t trust your teacher for answers
God help the auto industry these kids our the future.

[Christopher Shankle]

the L3T engines are notorious for carbon buildup on intake valves due to direct injection fuel delivery. Find somewhere with either BG or Terraclean fuel service products and have it run through the intake system, then go drive the crap out of the car. If this doesn’t fix it, then the injectors are also known to get carbon “Caps” built up on the tips making the fuel scatter wrong. Lot’s of problems with those cx-7’s.

[Chris]

To gap you want to take the end and just bend it inward slightly to close the gap. Make sure it is still centered and get a gaping tool from whatever parts store. They only cost like $2 dollars. It sounds to me this is probable cause. Another thing to do is look for a cable or hose that may be disconnected. Does it have a check engine light?

[Aaron]

Good point CordiaMad!

[Author]

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