Other than emission-related component failure, the easy things to look for are:

1. Have you recently fueled up with 87-grade octane?

2. Did you close the gas cap firmly (three twists followed by that ratchety "click")?

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car