I was coming home from my friends house last night after playing video games till the wee hours. When I was about 2 miles from my home I herd a loud POP then a popping noise that got quicker as the RPM’s climbed. I thought, OK, old car, I have abused it plenty, I through a rod through the block, no big deal, sti lower end time. When I got home and popped the hood to see the carnage I was shocked to find that all was well, except that one of the spark plug boots was hanging out of the hole. I was shocked even more when there was no plug in the hole!

I was staring blankly at it in amazement when out of the corner of my eye I see the spark plug peeking out from under the ABS block. So, I fished out the plug spun it back it (the threads didn’t feel grabby or any thing weird, and it tightened up just fine). I started the car and it sounded fine, so I drove it down the road to the school about ¼ mile from my house, where I normally test my cars. The car felt fine, so when I was pulling out of the parking lot there was one car about ½ mile down the road coming in the same direction I was going to travel so I pulled out slowly, got lined up and slammed the gas hard. It pulled strong through the first three gears so I let off, slowed down and pulled into my drive.

After I parked my car I noticed the car behind me pulled into my drive way. It wasn’t until he turned his lights on that I realized it was a cop. I rolled down my window and put both my hands on the wheel in plain site.
He asked what I was up to, I told him testing my car and explained the story.
He asked why I pulled into this driveway, I told him I lived here.
He asked if I had any weapons, I told him no very respectfully.
He asked why I had no interior, I told him because I was planning on welding in a roll cage soon.
He got my license went back to his curser, I felt like a dork getting pulled over in my driveway.
He came back and asked me if I was ‘some street racer’ I told him “no sir, I do all my racing on the track, that is why I am adding a roll cage”. He said fair enough and went away. No asking for registration or proof of insurance, no warning about speeding, or wreck less driving. He pulled me over because I seemed suspicious and he didn’t make up any bogus charge because I wasn’t, I respect that.

So my issue is Why did the plug come out?
None of the threads felt mangled putting it back in.
It tightened back in just fine so the threads on the head aren’t stripped.
There is black soot all on the ceramic of the plug and on the plug boot so that would indicate a leak over time right?
Pops speculates that I didn’t tighten it enough last time, or just through heating and cooling it wiggled its self loose.
Any one have any insight?

 

The spark plugs are supposed to be torqued to specification but using a larger torque wrench without a u-joint adapter is difficult. Any time the angle changes from perpendicular, the torque reading will no longer be 100% accurate. So, yes, it might be a case of the spark plug just loosening up over heat and cooldown cycles, or the crush gasket not being crushed down enough during initial tightening, or something like that.

On a 2.5 RS, you're lucky you were able to recover the spark plug and drive home okay. I would have guessed that the spark plug would drop to the ground or some section of the chassis where you would have to dislodge it. On a WRX, of course, the coil packs and integrated spark plug boot are secured by a capscrew so that even a finger-tight spark plug will be retained.

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0==WW==0
"…axles of evil…" - george w. bush

 

Who was the last person to change out the plugs?

Could have been a simple case of oversight, and not tightening down the plug. I would hate to think that a shop would have done that, but you never know.

I would check the rest of the plugs if I were you, just to be safe.

 

It was me, that’s why I was so amazed that it happened.
I have never had an issue like this before, but then again this is the first car I have had with aluminum heads so I was paranoid about stripping the threads. I went with the plug manufactures suggested "1/2 turn after seating" suggestion for tightening, because of the before mentioned torque wrench issues.

Today when I went some where and realized that this was a long term issue.
My car about 6 months back started to make a ticking noise. I just assumed that it was valve train noise that most cars start making when they get old. But today the ticking noise was gone, that’s when I realized that the ticking was combustion chamber gasses escaping around the plug!!