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This is the work of the previous owner, or shop. Somehow they had installed a wrong spark plug for what reason I don't know.
5 spark plugs were NGK's while one was Denso which I had problems with, on Cyl 2. Unfortunately the plug hole is stripped bad.
Going to helicoil it and go from there. If it fails, next option is to replace the head.
Love your SC's. Don't do this kind of crap.
Somehow, for 70k+ miles I have driven this not knowing it was like this, there were no running issues. Only has been recent that I've been getting a minor misfire under heavy acceleration.
1995 SC300 5-Speed, everything underhood is original with 324k miles.
A Denso SK20R11 (which is what that looks like) is a drop in plug for this motor. It's a iridium long life plug good for 100K. Now how it got to that condition I have a solid guess but neither you nor I can prove it. I say someone installed it with a impact gun or driver of some kind.
If I can help in any way even though I'm 2 hours away, let me know.
May I suggest a pass with heavily doped tap (same pitch and dia of the plug) with repeat pull outs and fresh grease.
Why: At least 1/2 the threads look good (unless while extracting the damaged tip end destroyed the good threads of the head. And if the new plug gets torqued in and you dont see any leak then it would be a cheap fix.
At 3xx,xxx miles the valve guides are probably shot, you should have the head taken off and refreshed by a shop and fixed at the same time, get the guides replaced and a valve job. shes put in her work for 300k time for a major tune up if you plan on keeping the car long term
Regardless of the stripped thread, the cylinder head is in need of a rebuild, given the high mileage; I am assuming it hasn't been rebuilt in the past 100,000 miles. If you remove the head from the car it can be repaired with a Timesert rather than a helicoil. A Time-sert repair is more durable, it is a permanent repair; helicoil repairs sometimes strip/pull out although they are easier to install. I used a Time-sert repair kit to replace a stripped headbolt thread on my Toyota truck's 22R long block; it's been running great for the past 5,000 miles.
Spark Plug Troubles
