Make sure that your wife fills the tank with premium gas (93 Octane). The 98 E320 requires premium gas due to the high compression engine. If lower octane fuel is used then "pinging" may result, in this case a bottle of octane boost may help.

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Ray
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles