How do you know (visually)when to repair or replace a distributor cap and/or rotor? Can you clean/service either without replacing them? I realize they're relatively cheap, but I don't know how old the ones on my new-to-me used 1992 190E are and I don't want to spend $70 if I don't have to. The issue my car has is it seems down on power. Spark plugs and wires and compression check all show good.