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Discussion Starter · #1 ·
Yesterday, I noticed a misfire under boost. I've only had the car 2 weeks, and it certainly wasn't doing that when I bought it.

I had just recently put a new set of plugs in, first time working with this kind of spark plug, and I've been working on cars long enough to know that the last thing I messed with is the first thing I should look at if something suddenly seems to be not working right, so I bought another spark plug (because it really felt like a single cylinder misfire under boost to me) and went looking for problems.

It was really hard to see for my old eyes, but I was almost certain I saw a tiny chip out of the pad on the electrode for the number 3 plug. I went ahead and put the new plug in the number 3 cylinder and put everything back together, and wow. Not only is that dead misfire under boost gone, I am 100% positive the engine is running considerably stronger than it did two weeks ago when I bought it. I will assume that difference is simply due to the old plugs being worn, and the gap on them being too wide.

End of the day, the car is even more of a ripper than I thought it was when I purchased it, so not a bad deal there.

The lesson, when things aren't working right, the first place to look is at the last thing you screwed around with!
 

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2014 ST3. Garrett GTX2860R Gen 2. FBO
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Nice find! It's a bit of a shot in the dark when you don't have a code reader or anything to help narrow you down to one cylinder.
 

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Discussion Starter · #3 ·
Nice find! It's a bit of a shot in the dark when you don't have a code reader or anything to help narrow you down to one cylinder.
I do have a generic OBDII code reader, but it wasn't showing me anything helpful. I believe the plug was firing well enough when not in boost that the computer wasn't seeing a misfire, no pending codes, nothing to give me direction, just what it felt like to me, and my collective experience to point me at the spark being blown out under boost.

From there, I knew that one of the plugs had a bit too much of a gap on it, so I tapped it a couple of times, and closed it a bit too much, so then I endeavored to open the gap back up a touch. Something I've done a million times on old school plugs without a bit of trouble, but I kept reading how easy it was to damage these plugs, and how sensitive this engine is to plug damage, so that gave me a good nudge in the right direction.

Added bonus, I ordered a full set of 4 new plugs as soon as I noticed the misfire, but they won't be here until next week. Now, these plugs are working fine, so that new set will go on the shelf until it is time to change these out. Always nice to be a step ahead.
 

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2014 ST3. Garrett GTX2860R Gen 2. FBO
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You're right about the plugs being fragile. I recommend picking up one of these tools, and using feeler gauges for gap measurements



The only thing those coin tools are good for is opening gap back up with the hole that's stamped into them.
 

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Discussion Starter · #6 ·
You're right about the plugs being fragile. I recommend picking up one of these tools, and using feeler gauges for gap measurements



The only thing those coin tools are good for is opening gap back up with the hole that's stamped into them.
Yup, mistake number one. And actually, the hole in my coin tool isn't tapered like the one's I've had in the past, it is the full thickness of the tool. I'm near 100% certain that is how I chipped that little contact pad on the ground strap. I used the coin took, very gently, just to measure the gap the second time, not to adjust it. Will be ordering a proper tool for gapping these plugs before I do the job again.
 

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Discussion Starter · #7 ·
I picked up my st a few months ago and just replaced the plugs. I checked the gap and they something like .03" (oem plugs). Car runs much smoother now
Mine, likewise, were all right around .030 with one being honestly a bit closer to .032. I made sure the new set were all at .028, a small difference, but judging by the way the car runs now, a pretty important difference.
 
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