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Old 03-27-2018, 10:47 AM   #28
equ
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Join Date: Aug 2005
Carmudgeonly Ride: 981S, 340i
Posts: 9,580
Update. The idle shake came back. CEL (misfire cyl.3) was gone for a while but recently came back and stayed on. Idle shake got a bit worse. So off to the mechanic...

Lo & behold, spark plug #3 was bad. They gave me a high price of labor for all six instead of just one, so I just got that one done. He wanted $120 for the diagnosis & labor of the bad one, fair. But $300 for doing all six. What?

In retrospect... I was basically "in there" for all six when I was doing the coils. Having never done plugs, and with no one around who had the "feel" for it, I did not feel confident I had the mechanical feel and I got paranoid about engine threads. I have many destroyed threads in my history, don't need to add a BMW N55 to that list. I do have a torque wrench for wheels, not sure if it would apply to plugs, and I think I'd need to get a socket for plugs. Next time...

The other lesson was that I "over-relied" on the service history. Plugs were done at 41k at an indie just before my purchase. By 45k, the car already had idle shake. So I suspected coils etc. I did the coils just before 50k, here we are at 52k and finally reaching the culprit. So I lived with a bad plug for more than a year and 8k miles. The symptoms seemed to be contained to idle rpm, car still got 27mpg on the highway, but now that the plug is done, the butt dyno says +10% in mid-range. Hope the shakes and misfires didn't do any mechanical damage. Just tired the engine mounts, perhaps.
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