Q: check enginelight is on and the gas range fluctuates often even after i changed the gas cap and had a smoke test done

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my check engine light came on. took it in for repairs and based on the code, they replaced the gas cap. the light stayed off for a few days. I filled the tank and drove to work, all was fine. on my way home, as I started climbing a hill, I noticed the temp guage went up a little (almost to the 100 degree mark) then the light came back on. took it back in and the same code came up so they did a smoke test that did not detect any leaks. the light stayed off until I filled the tank up again and the exact same scenario occurred on the same hill. I took it to the MB dealer and they could not find any leaks either said that the light is not affecting the condition/driving of the car - but who wants to drive a car not knowing what is wrong with it. I filled it up last night and decided to watch the gas range. it fluctuated often and went from 507 miles at one point to under 400 miles in a 27 mile trip. and when sitting still, the range continued to drop every 12 seconds

Codes can’t be the "basis" for replacing ANY part on a car. Codes ONLY provide clues to mechanics as to the origin of a fault and the ONLY use of these clues is they permit identification of the parts and systems that must be ACTUALLY tested and diagnosed. If a mechanic ever tells, you, "I just read Code Xxxxx, let’s replace "x part", run for the exits (literally). There may have been absolutely nothing wrong with the gas cap and if a non-OEM cap was installed, you may be worse off now because non-OEM caps, of sub-standard design and quality, will often themselves be a cause of EVAP system codes and for that reason non-OEM caps are not recommended. To best advise you, I would need to see the exact code as that code will be associated with quite a few items but if the Code happens to be P0455, or "close", there are lots of places where you could have a vapor leak, such as fuel pump gasket, or pinhole rust spot in top of tank that won’t show on smoke test. Furthermore, there are component faults, that have nothing to do with leaks, that will set Code P0455. As far as the gas gauge fluctuation, if the only code that has set relates to an EVAP system malfunction, the gauge fluctuation likely stems from a problem with the sending, or receiving unit (dash gauge) and/or the interconnecting wiring. The bottom line is if you request a check engine light diagnostic - Code P0455, the responding certified mechanic will get this resolved without further delay. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.

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