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Low compression on #6

Discussion in 'SVT Shelby GT500' started by EvilSnake, Sep 2, 2019.

  1. EvilSnake

    EvilSnake Member Established Member

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    Car is a (recently) built 5.8L, stock cams, 3.2 Kenne Bell. All the goodies. 878/832 at the wheels.

    Noticed the car sounded like it was lightly missing when the engine was under load... just did a compression test and #6 is especially low at 100psi.

    complete compression test results are as follows...
    1 = 172
    2 = 168
    3 = 168
    4 = 165
    5 = 155
    6 = 100
    7 = 140
    8 = 153

    I'm not exactly sure what could be the issue. Maybe valves???
     
  2. Catmonkey

    Catmonkey I Void Warranties! Premium Member Established Member

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    It's low enough to where I'd probably pull the engine and the driver's side head to check it out. It could be a head gasket. #7 seems to be an outlier too.
     
  3. Weather Man

    Weather Man Persistance Is A Bitch Premium Member Established Member

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    Pull the head and inspect cylinder walls. Hopefully just a rebuild on the head,
     
  4. blubyu87gt

    blubyu87gt Well-Known Member Established Member

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    I'd do a leak-down test before pulling it apart. At least to know exactly where to look when you get it apart.

    Sent from my SM-N950U using Tapatalk
     
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  5. EvilSnake

    EvilSnake Member Established Member

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    If it was a head gasket wouldn’t I be seeing oil in the coolant?

    Im wondering if this isn’t valve related.
     
  6. biminiLX

    biminiLX never stock Established Member

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    What year cams?
    How has it been driven since rebuilt? Datalogging?
    That #6 cylinder definitely needs to be evaluated.
    Bore scope?
    -J
     
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  7. EvilSnake

    EvilSnake Member Established Member

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    It’s a 2011.
    Car hasn’t been driven very hard. Never taken to the track. In fact when the issue presented itself I was on a road trip just doing highway speeds. Of course when I passed people I geared down and had some fun but I wasn’t anywhere near redline.
     
  8. me32

    me32 BEASTLY SHELBY GT500 TVS Established Member

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    What did the plugs look like?
     
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  9. Poisonous West

    Poisonous West Member Established Member

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    Who built your motor?
     
  10. EvilSnake

    EvilSnake Member Established Member

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    The plugs are mint and the motor was built by Performance Plus in Saskatoon
     
  11. DSG2003Mach1

    DSG2003Mach1 Well-Known Member Premium Member Established Member

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    you dont always get water in the oil or oil in the water with a head gasket, hell my lightning was leaking coolant out the side of the motor when it let go
     
  12. merkyworks

    merkyworks Well-Known Member Established Member

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    Leak down test will really tell you more, fingers crossed it could be as simple as a leaking valve seat.

    Edit:
    Looked at your compression numbers again and really your entire driver side bank is questionable, its 11% to 72% down compared to cylinder #1.

    1 =172 = 0%
    2 = 168 = 2%
    3 = 168 = 2%
    4 = 165 = 4%
    5 = 155 = 11%
    6 = 100 = 72%
    7 = 140 = 23%
    8 = 153 = 12%

    Hopefully it could be confined to the head and its something like a head gasket, cam timing off 1 tooth or bad valve seats.
     
    Last edited: Sep 3, 2019

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