Misfire & messed up spark plugs

[Schwamm]

My car was driving fine this morning until I stopped after about 5 minutes and idled when it suddenly started misfiring heavily. The car barely stayed running. I pulled a p0300 random misfire code, and swapped in an older, used set of plugs and it ran fine again. I'm going to get a brand new set of plugs, but the plugs in the car looked terrible for the mileage, and I'm wondering what could cause this.

These are my plugs in order, with cylinder #1 on the left:

Plug #1 has 8k miles, the others have 22k miles. The first plug was swapped more recently because I noticed it looked bad when changing valve stem seals.

Close up of the first plug:

Plug #1 looks destroyed. The others look pretty bad given the low mileage. These are NGK Iridium plugs, ZFR6FIX-11, gapped to .28. My car runs well, and the only issue is oil consumption; it uses 1-1.5 quarts per 1000 miles on the highway, roughly, and I've never figured out why. Not sure if that could have an impact on the spark plugs. What could cause this? I don't want to keep swapping plugs every 10k miles while ignoring a bigger problem.

 

[ap123]

do a compression test.
What was the gap when you removed them?

#1 looks like it knocked.
#2 looks good, camera seams a little out of focus
#3 looks like its burning a little bit oil
#4 looks good

one thing to note, burning oil can lower the knock threshold. But it sounds like your consumption is small it should not matter

i would suggest you try zfr6r or bkr6e see if it changes anything. also they're alot cheaper and they're life time is about 30k miles.
Also run some sea foam through it, carbon deposits can cause funk things to happen too.

 

[Schwamm]

Thanks for the reply.

My last compression test was a few months back and all cylinders were between 188-196. Spark plug gap was initially .28 on all but is now .26, .32, .28, and .30 (1-4). I'm not sure how #1 got smaller, but maybe it was incorrectly gapped initially. Would that cause the plug to wear like that?

I think I consume too much oil. I wouldn't call it small to lose a quart every 2 tanks of gas. The last time I pulled the exhaust manifold the #1 cylinder was black around the exhaust outlet. I didn't know oil consumption could cause a knock like that but it makes sense. I do also probably need a new cat because mine smells like rotten eggs and is probably causing a lot of back pressure. Not sure if that could be related.

Ill try the sea-foam and I've already bought replacement ngk iridium plugs, but what makes the ones you recommended different? Anything else I can do in the meantime to troubleshoot?

 

[hornsfan10609]

ZFR6F are the stock plugs, that's what I would run

 

[ap123]

just run the plugs horsfan recommended(whatever the stock OEM recommendation is)
What i was really trying to get at is that Iridium and Platanium plugs are actually not great for boosted cars. Normal copper plugs like the ones i mentioned are a lot better because they're better conductors of heat. Which is crucial for boosted engines

Your really shouldn't need to gap them down that much for stock boost, something like 0.032-0.035 should be plenty(just a guess im sure they have the actual recommended spec on here some where). Rule of thumb you wanna run as large of a gap as you can but as small as you need to. Too small of a gap can cause the engine to misbehave as well.

for reference ~0.029 is what i run on 20psi, and thats a pretty conservative gap.

1/2 quart a tank is not that unheard of. I mean you're right at low boost it is more then what one would expect. S/C Millenia PVC valve might help a little, probably wont. A catch can might be beneficial.

 

[hornsfan10609]

Mazda spec for the plug gap is .028-.030"