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Compression Check procedure-gctid829028

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    Compression Check procedure-gctid829028

    Gonna change the spark plugs, so I think I'll check the compression. (But not a leak-down; I only have a compression tester.)

    I'll run the engine for a few minutes first.

    Do I remove all the sparkplugs, or just check one cylinder at a time, with plugs in the others?

    Is it necessary to clamp the fuel line to prevent fuel flow?
    1998 Capri 1950CL
    3 Liter MerCruiser
    Furuno 1622 Radar, Garmin echoMAP44dv, Garmin 300 AIS receiver, Uniden Cl 2 VHF with Hailer,
    2 batteries with Combiner, Joystick Wakeboard Tower

    All plugs out, throttle left wide open and spark to distributor disabled. With throttle wide open and plugs out there will be no vacuum to deliver fuel. No need to disconnect the fuel system.
    1989 Avanti 3450 Sunbridge
    twin 454's
    MV Mar-Y-Sol
    1979 Bayliner Conquest 3150 hardtop ocean express.
    Twin chevy 350's inboard
    Ben- Jamin
    spokane Washington


      Bring engine up to normal operating temperature. If you cannot remove the spark plugs after that without getting burned, let it cool enough to be safe. Remove all of the spark plugs.

      Disable fuel flow to cylinders. This may be by plugging a hose, removing the fuel pump relay, or disconnecting the fuel hose from the tank and letting the engine run out of fuel prior to removing the plugs. Completely remove the wire to coil to prevent any spark. Block the throttle plate(s) wide open. Screw the compression gauge into #1 spark plug hole hand tight.

      Crank the engine over to get maximum reading on the gauge. Some will say to only crank engine over a specific number of times, to each their own, I am going from memory of the instructions that came with my compression gauge. Write down the reading on the gauge, # 1 - xxx psi. Record each cylinder's results. Release the pressure on the gauge, and move it to the next cylinder's spark plug hole. Screw it in hand tight, crank engine, etc. Do the same with all cylinders. Hopefully they will come up reading within upper 10% of the highest reading. Low readings on two adjacent cylinders usually indicates a head gasket issue. One low isolated from other cylinders indicates valve, or ring issues. If more than 10% differential you may want to consider leak down tester to find out why. Kawasaki tested several brands of leak down testers. Their conclusion was the NAPA leak down tester is the best. I know this because I just completed some factory training as a commercial engine servicer for Kawasaki. This is the procedure Kawasaki teaches their mechanics. Some may be overkill but better safe than sorry.

      Newport, Oregon
      South Beach Marina
      1986 3270 with twin 110 HP Hino diesels. Name of boat "Mr. Darcy"
      Past work history: Prototyping, tooling, and repair for Reinell,. General fiberglass boat repair starting in 1976.
      Also worked as heavy equipment mechanic, and machinery mechanic for over 30 years.