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Compression check

Discussion in '365 GT4 2+2/400/412' started by 180 Out, Sep 8, 2012.

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  1. 180 Out

    180 Out Formula 3

    Jan 4, 2012
    1,286
    San Leandro, CA
    Full Name:
    Bill Henley
    #1 180 Out, Sep 8, 2012
    Last edited by a moderator: Sep 7, 2017
    Here's a how-to photo set for a simple task: a compression check. First you disconnect the coil, so the engine doesn't fire up, and the Bosch connector for the air flow plate position switch so the fuel pumps don't run. It's on the driver side. Then you prop open the throttle butterfly so air can enter the intake freely. I used a big socket. You can also hook up a battery charger so the battery doesn't run down (no picture).
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  2. 180 Out

    180 Out Formula 3

    Jan 4, 2012
    1,286
    San Leandro, CA
    Full Name:
    Bill Henley
    #2 180 Out, Sep 8, 2012
    Last edited by a moderator: Sep 7, 2017
  3. 180 Out

    180 Out Formula 3

    Jan 4, 2012
    1,286
    San Leandro, CA
    Full Name:
    Bill Henley
    #3 180 Out, Sep 8, 2012
    Last edited by a moderator: Sep 7, 2017
    Clip the gauge onto the end of the hose, get in the car, and crank the starter until the gauge maxes out. About four revolutions will do. I got 150 psi on nearly all 12 cylinders, but one was 135 and one was 160. I don't know the cause of the weak cylinder. Later I will go back and squirt some oil in the hole and do what's called a wet compression test. If the oil improves the result that will indicate the rings are not sealing as well in the weak cylinder as in the other ones.
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  4. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    26,822
    30°30'40" N 97°35'41" W (Texas)
    Full Name:
    Steve Magnusson
    #4 Steve Magnusson, Sep 8, 2012
    This will not disable the fuel pumps during starter motor cranking (in fact, it will also make the fuel pumps run whenever the key is "on" regardless of what the engine is doing) -- best to remove either the fuel pump fuses or the fuel pump relays to disable the fuel pumps.
     
  5. Doug R

    Doug R Karting

    Oct 11, 2005
    208
    Yellow Springs Ohio
    #5 Doug R, Sep 8, 2012
    Also your clip appears damaged and the metal wire retaining clamp is missing. These clips get brittle and need TLC when disconnecting, the metal wire needs carefully "released" from the studs.
     
  6. 180 Out

    180 Out Formula 3

    Jan 4, 2012
    1,286
    San Leandro, CA
    Full Name:
    Bill Henley
    #6 180 Out, Sep 8, 2012
    That's funny! I've been doing it this way every time I wanted to disable the pumps while working on the car! I'd swear I got this method from Ferrarichat. Thanks for setting me straight.
     
  7. bundas

    bundas F1 Veteran
    Owner

    Dec 31, 2005
    7,090
    lexington ky usa
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    mitchell barnes
    #7 bundas, Sep 8, 2012
    me, i remove all plugs. then do a leak down, add oil and run test again. any comments?
     
  8. jes-13

    jes-13 Karting

    Jun 9, 2006
    56
    California
    Full Name:
    John
    #8 jes-13, Sep 8, 2012
    I agree... if you only remove 1 plug at a time, a head gasket leak to an adjacent cylinder could be masked... pull all the plugs.
     
  9. blkprlz

    blkprlz Formula 3

    Mar 24, 2007
    2,169
    Tampa bay
    Full Name:
    Bruce
    #9 blkprlz, Sep 10, 2012
    I often hear that compression tests only give you a general idea of where the engine's at & if you have 10% or more difference between the cylinders, the leak down test is where it's at. Leak down gauges/tester will give you the %'s if that's what you're looking for but I think the simplest is just hooking up shop air to the compression tester (with schrader valve removed) & the piston at TDC to force air past the problem area...& just listen for a hissing sound from the various places (intake, exhaust, oil, & coolant).

    Be sure those plug wells are clean so no dirt falls into the cylinders.
     

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