Mine is a 01 E430. For a while it just sat in the garage but during the past 1.5 years, I have been driving it mostly on long trips. I'm now at 110k miles with OEM spark plugs and the only change in performance I see is a very slight drop in MPG - from 24 to about 22. So I'm wondering if I should just keep on going or replace plugs. Is it possible for the plugs to be fouled and not show a clear deterioration in vehicle performance? If I don't have to change them, I don't want to change them.