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P0303 - next step?

crawfe1crawfe1 Member Posts: 2
Hi. 2006 Forester 160K miles. All of a sudden, very rough idle, good at higher speed. Blinking CC; CEL on or blink. Initial diag: P0303, compression <<100 on #3. I have to use a new dealer so no track record for trust. Does the low compression rule out simple problem like wire, plug, coil, etc.? What would you check next? New to Subaru; Thanks!

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    ateixeiraateixeira Member Posts: 72,587
    Gotta be a head gasket I'd think...
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    xwesxxwesx Member Posts: 16,833
    Low compression could be many things, including a sticking valve, timing, rings, head gasket, etc. It probably isn't a "simple problem" like the wire or plug. More testing is needed to pinpoint the problem, starting with a leakdown test.
    2018 Subaru Crosstrek, 2014 Audi Q7 TDI, 2013 Subaru Forester, 1969 Chevrolet C20, 1969 Ford Econoline 100, 1976 Ford F250
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    crawfe1crawfe1 Member Posts: 2
    Thanks. I'm curious why it's better at driving speeds?
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    xwesxxwesx Member Posts: 16,833
    Interesting. I glazed over that part as I read the low compression information. I had a '96 Outback that suddenly was horribly rough during idle, but (like yours) was fine at higher speeds. The problem with that one was a knock sensor. But, the knock sensor failed, which did post as a trouble code (knock sensor out of spec). It ran especially poor when the engine was cold.
    2018 Subaru Crosstrek, 2014 Audi Q7 TDI, 2013 Subaru Forester, 1969 Chevrolet C20, 1969 Ford Econoline 100, 1976 Ford F250
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