Don, don't assume the spark plug wires are fine. If they are the original wires they may need replacing. Recommend checking them, or have a local indie check if they're still adequate. Your 1994 E320 cabriolet should get about 20 miles per gallon.

When you replace the spark plugs apply a thin coating of Never-Seize on the threads and make sure they are gapped per the Owner's Manual specification.

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Fred Hoelzle