Forester- Knock Sensor

hamullerhamuller Member Posts: 11
edited May 2014 in Subaru
I have a 2000 Forester 5 sp. When it was new I would get 30+ mpg and was delighted. In the last several years the mileage has slipped despite tuneups, etc. Recently the check engine light went on. My mechanic replaced the knock sensor. The car seems to run better and I was quite surprised on a recent 200 mile road trip to get 30.2 mpg.

Can anyone explaine this? Can the faulty knock sensor account for this?


  • bigfrank3bigfrank3 Member Posts: 426
    Sure. When the knock sensor detects detonation, or even if it thinks it does, it reduces the engine timing (via the computer) which will reduce performance and fuel mileage. A bad knock sensor that was reporting to the computer that the timing needed to be retarded would cause you to run in this reduced performance/fuel mileage mode to "protect" your engine, but apparently it didn't need protecting. Spark advance is important for maximum performance and mileage.

    This is the problem with a lower-than-required octane fuel on both a normal engine or a turbo. If the computer gets info that it needs to retard the timing it detunes the engine. In your case it seems it was a sensor issue not the fuel.
  • ateixeiraateixeira Member Posts: 72,587
    Kudos for a great explanation. :shades:
  • hamullerhamuller Member Posts: 11
    Many thanks for the explanation. hamuller
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