[SOLVED] Bucking at WOT and melted ground electrode on Cyl 4

First a little background: I'm the second owner (bought Aug '22 with 155k miles) of 2012 MS3 with Cobb Stage 2, tuned, HPFP. Turbo blew within first month and was replaced by a mechanic I trust. Since I've owned it the car has been driven very gently - just used on short (10-20 mile) trips as a means of transportation.

It's been running fine until I let it sit for about a month while I was on vacation. On my very first drive it began bucking in 2nd and 3rd gear. CEL came on so I pulled over and pulled a code P0304 (misfire on cylinder 4). Cleared code, started again and let it idle, then continued on my trip without further issue. The bucking has not returned EXCEPT in 5th or 6th gears. It happens every time while at low RPM and going WOT in order to pass a car on a two lane road. No further CEL or codes being stored.

So I pulled the plugs (Denso Iridium ITV22 gapped too large at .030") and 1-3 look ok. #4 however is fouled and no longer has a ground electrode. Not good. I performed a compression test and get 180/180/180/170. Cylinder 4 has a lot of carbon buildup, other 3 cylinders look good (see pics for comparison of #3 to #4)

Honestly I am not looking to put a lot of effort or dollars into this vehicle. Just looking to use it as a daily (gentle) driver as long as I can. I have ordered replacement NGK LTR7IX-11 (6510) and will gap them to .027". Obviously this doesn't address root cause, but if it allows me to get another year or so out of it I'm fine with that.

What happens if that's all I do?

Are there any other relatively simple things I should do before putting the new plugs in getting it up and running?

Thanks in advance!
Dave

 

I think you found the source of the missfire

Who tuned the car

Are the spark plugs you ordered stock heat range?

The spark plug link you posted was an Amazon link be careful there are a lot of counterfeit spark plugs on Amazon

 

Definitely the missing ground electrode causing a misfire, also stop going wot at low rpm. Don't boost until past 3k rpm, your asking for ZZB going wot at low rpm

 

Don't use Amazon ngk plugs. They are fakes. Get them through a parts store or one of our Approved Vendors.

 

Thanks all for the quick responses! I will return these plugs and buy from a reputable source, plus follow advice regarding no low RPM to WOT

Do you think the blown plug was the likely root cause and not just a symptom of a bigger issue?

What should I do about the existing carbon build-up that's in cyl 4? I saw some YouTube video's on removing the valve cover and cleaning the intake valves, but won't have time for that or anything larger for a while. Is it necessary and will I be ok to drive it for a while with just the plug replacement?

Thanks again!
-Dave

 

Check this thread out for the carbon. https://mazdaspeeds.org/index.php?threads/another-piston-soak-success.16255/

 

Problem was resolved with 4 new NGK plugs, purchased at Autozone. Bucking/stumbling is gone.

Thanks again!
-Dave

 

Very lucky the electrode didn't score the cylinder wall!