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Intermittent cylinder mis-fire

I have a 2000 Mercury Sable. I have a long standing intermittent problem with cylinder 5 missing and the engine running rough. My mechanic moved the injector to cyl 6 and the coil to cyl 4, but the problem remains with cyl 5. It is intermittent and has been going on for years, but has gotten more frequent as time has passed. He believes it is a valve problem. I thought it sounded like an intermittent wire, but he doesn’t think it acts like that. He removed the valve cover but said everything looks OK at the top of the head. Would a bad valve cause a cylinder to miss intermittently?

Run a compression test on the engine. It will tell you if its a valve or something wrong with the cylinder.