Lately I've noticed a bit of roughness between 4000-5500 rpm, somewhat worse under load, and set aside some time this weekend to inspect the plugs, coils and connector boots, check for oil leaks in the plug wells and decide whether a slightly early tune-up is in order. The plugs were fair for their age (~44,000 mi), not great, so in a few weeks I'll go ahead with the tune-up and replace the valve cover gasket as well--the #4 boot was oil-fouled and replacing it cured some of the roughness. My worry is the #2 and #5 plugs would not come out. The plugs broke free easily enough then became increasingly difficult to turn. I stopped trying around 30 N-m or so (the tightening torque is 25 N-m). I tried working them back and forth a few times and gained a little ground, but still met considerable resistance. With no spares on hand I couldn't risk snapping an insulator or, worse, damaging the heads, so I re-torqued them to be dealt with another day. I'm hopeful this is just some anti-seize compound galled up in the threads--some other plugs also felt easy/hard/easy as they came out, but not to this degree. I was working on a cold engine so I'm also hopeful that if I tackle these two plugs first with the engine hot, they'll spin right out. Any other tips on how to deal with balky plugs without giving myself a heart attack (I likes my cylinder heads, I does, and plans to not hurt 'em)? Also, if the tune-up doesn't cure it completely, what else should I poke at to smooth out the upper RPM range? Engine is an M54 2.5L with about 207,000 miles, in otherwise perfect running order, no obvious cracked hoses or vacuum lines, but I haven't thoroughly checked all that in a while (will do so during the tune-up).
If the two plugs don't come out easily with a warm motor, then loosen them what you can and spray some WD-40 of similar solvent down the sparkplg holes and allow it to penetrate the threads. It may take a couple sprays. Then work the plug back and forth and they usually come out. With 207K the valve stem seals may be allowing more oil down the stems which can result in a lot of carbon on the plugs. New plugs could eliminate all the issues.
That and oil fouling were the two trouble signs I wanted to check. No sign of either on the ones I could inspect, so that's 2/3rds good so far. The car has Bavarian Autosport high-performance coils and while I'm happy with them, it's become apparent the hotter spark cuts plug life by 40-50%. (Unsurprising, really.) I also plan to run a bottle or two of Techron through the system since I can't recall when I last did that. Fuel filter was changed around 200K. I'm trying to think of all the "old school" diagnostics so I don't give in to the impulse to buy a code reader. Yet.
I revisited this problem today at last. I attacked the plugs immediately after a 20-minute drive, starting with the two stuck ones, and they all came out with ease. In fact, since they were last torqued into a cold engine, a couple of days ago #2 worked itself loose to the point of allowing combustion blow-by! (That was a decidedly odd sound.) I admit this had a lot to do with moving the work up two weeks in my schedule. Anyway, all is now well, and I am waiting for the engine to cool to something close to room temperature so I can change the head cover gasket. EDIT: Finished around dinner time and went out for a test drive. It's much, much smoother and sounds fantastic. At this point, the faint vibration I still feel in the shifter at mid RPMs could well be due to the age of the tranny mounts, so I''m not gonna sweat it. ForcedInduction, thanks for your thoughts on the plugs even though good ol' heat saved the day.
