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I have been dealing with a very tough-to-track-down misfire on what I believe is cylinder No. 2. I took the rav4 to the dealer and had it diagnosed, and they came back with recommending a head gasket replacement. They think it is blown. My question is WHY do they think that? I have NO excess...

Good afternoon all. I have one final issue to solve with my Rav... There will occasionally be a very quick engine jerk that seems to be random. The rpm will quickly drop and come back up. It occurs when in drive, 2, L, neutral. It happens when driving and when completely stopped at a light...