the worse you can do is break plug wire if its really on there which has happened to me. try rotating the wire or give the boot a good a yank. i would put some dielectric grease on the boots so it doesnt freeze up. I do this on all my gas cars everytime i do a tune up and not a problem.

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)