Important to remember when changing plugs!

[codfizzle]

OK so something's been happening to my car lately; and I'm not sure if it's a common thing in Legacies or what, but it's something simple that has caused me several headaches and can easily be prevented/remedied.

I was having a really strange cyclical sound coming out of my engine bay at highway speeds, which almost sounded like a slapping piston. About a week ago I was changing my spark plugs and I discivered what the problem was. The #3 plug was not seated all the way down in the hole, because the crush washer had jammed itself on the threads of the plug at an angle, so when I torqued down the plug I was only seating it to the point where part of the washer was touching the cylinder head. So, since the plug (even though I thought it was tight) was only screwed in half way, there was compression leaking out around the plug; hence the slapping sound.

So I was thinking about this after I changed my plugs, to the point where I went out and double checked the new plugs I had just put in, to make sure the crush washers hadn't jammed on those too, and guess what? 3 out of the 4 plugs were jammed on the crush washer when I pulled them out!

So I repositioned the washers on the plugs, smeared some dielectric grease on there to hold the washers in place while installing the plugs, and put them back in. But that was not the end of the problem. After I fixed all the washers on all the plugs, I went for a highway test drive. The sound was almost gone, but I could still hear a little bit of a slapping sound over 80 mph. So I started thinking, "what could cause the plugs to not seat properly, even though the washers were no longer jamming them up?"

I took a plug out and looked down the hole, and realized there was a TON of dust and crap packed down in the hole right where the crush washer is supposed to seat to the cylinder head. So the plugs weren't seating to the actual metal of the cylinder head, they were seating down to that compressed layer of crap down in the hole. So the seal still wasn't tight like it should have been, and compression was still leaking out around the plug.

Then I read in my Subaru manual that every time you take a plug out, you're supposed to unscrew the plug a few turns, and blow the hole out REALLY WELL with compressed air before taking the plug all the way out or putting it back down tight. I went one step further than this since the compressed air step had been neglected so many times before by the previous owner. I unscrewed the plugs a few turns, blew it all out with compressed air, sprayed it all out with brake cleaner, blew it all out clean again, and then re-greased the washers on the plugs and carefully reseated them.

Finally the plugs are seated the way they are supposed to be, and the slapping sound is gone. My idle is also smoother. I don't know if anybody else has had a similar problem, but if you experience any of the syptoms that I experienced, check your plugs, and make sure the plug holes are cleaned out properly and the plugs are seated properly. I had no idea such serious problems could so easily come from something as simple as my plugs not being seated quite right.

[ericem]

I do seem to have a odd sound at highway speeds, but I figured it was my tranny. I do recall finding some washers stuck when removing my old plugs, and would not be surprised if the same thing happened with the new ones. Didn't blow out the dust either!

[biggreen96]

Good catch. So many of us have these tiny nagging problems we can never seem to get rid of and we are so quick to blame and replace $200 sensors when it's just these little problems that all stack up.

[atroby91]

i too thought it was my tranny. i'll look into it when it gets warm