Kohler CH25 blown head gasket????

Discussion in 'Mechanic and Repair' started by rallen, Apr 25, 2007.

  1. rallen

    rallen LawnSite Member
    Messages: 10

    Bought a '94 Bunton with a Kohler Command 25hp that misfires from a neighbor who was moving. My neighbor was ignoring the backfiring since it didn't seem to happen under load, and tried to compensate by choking it a little. I don't know how long it has been doing this. This year, when I went to start it up I pulled both plugs. One plug was dry with a a little black deposit, but white on edges, like it had been running a little rich (probably from choking?). The other, however, was completely fouled, wet black hard deposits. The hour meter shows 930 hours.

    I cleaned the plugs and re-installed, and just re-inspected them after about 2 hours of use. The "good one" seems to be dry and burning cleanly, with a touch of white on the edges; the "bad one" is a little wet and black, and my guess on its way to being fouled again. The mower appears to consume some oil, but I can't see any smoke from it when it is running.

    When I purchased the mower I thought maybe it was a carb issue, but now I wonder if the misfiring isn't cylinder related. I posted these symptoms on another board and someone suggested that I may have a blown head gasket on one side. I don't have a leakdown tester, though that sounds like the right tool to diagnose what is going on. Any advice? Could a blown head gasket cause the misfiring?

  2. Restrorob

    Restrorob LawnSite Fanatic
    Messages: 11,029

    If only the one plug is oil fouling it surly sounds like a blown gasket, But with this many hours the leak-down test would tell all.
  3. rallen

    rallen LawnSite Member
    Messages: 10

    OK...purchased a leakdown tester at Harbor Freight... says to check with the piston of the cylinder being tested at top dead center of the compression stroke. Is there any way to know if it is at tdc of the compression vs. exhaust stroke other than to take the valve cover off? Or do I need to remove the valve cover, turn the engine manually until the intake valve closes and the piston is comes up to the top, which I can verify by sticking a straw in the spark plug hole?

    Sorry about this being a stupid question, but I wondered if there was another way. The engine is not dissassembled whatsoever, and in its location on the mower I can't access the flywheel side; only the clutch side.
  4. khouse

    khouse LawnSite Bronze Member
    Messages: 1,465

    One easy way is to remove the plug. Close the hole off with your finger and bumb the engine over. Not a full crank. Just off and on real quick until your finger gets pushed off with the compression. Then stop and rotate the piston to TDC by hand. Then make sure you lock the crank in place before you apply the pressure test.
  5. Restrorob

    Restrorob LawnSite Fanatic
    Messages: 11,029

    Pull the plug on the good cylinder then put a wrench on the bolt holding the PTO clutch on and turn the bolt counterclockwise (standing in front of the clutch) until you feel it start to get hard. Thats the beginning of compression stroke, Then take that plug out and stick your straw down the hole and rotate the piston up to TDC.

    Now, In the instructions it did say to pin the engine at TDC so when air is introduced into the cylinder it won't push the piston down ?
  6. rallen

    rallen LawnSite Member
    Messages: 10

    Thanks! Seems kind of obvious once you explain it.... With regard to pinning the engine, I thought I would clamp the wrench holding that bolt so it wouldn't move in either direction.

    If i understand correctly, doing this test I should "see" a leak in the fouling cylinder, and hear air escaping somewhere. If I don't, then I have another problem. I am looking forward to hopefully confirming that this is the head gasket that is causing my issues.

    Someone else wrote in another forum that I might want to check the engine timing; doesn't this engine have an ignition module with fixed timing?
  7. Restrorob

    Restrorob LawnSite Fanatic
    Messages: 11,029

    If it doesn't have the spark advance module (SAM) it is fixed timing other than the flywheel key.
  8. rallen

    rallen LawnSite Member
    Messages: 10

    OK...didn't get to do the leakdown test yet BUT....did a compression test.

    The side that fouls: 165psi
    The side that doesn't: 135 psi

    Is the compression higher on the fouling side because of the oil that gets in the cylinder? If I have a blown head gasket on the bad side, wouldn't it have lower compression?

    Also, how can you tell if your Kohler has a Spark Advance Module? If it was a timing issue, wouldn't both cylinders have fouling problems?

  9. thecrankshaft

    thecrankshaft LawnSite Member
    Messages: 172

    A CH25 engine will have the SAM module and variable ignition timing.

    To diagnose a head gasket failure you really need to do a leak down test, not just a compression test.

    What is the serial number of this engine?
  10. EMTDUCK1984

    EMTDUCK1984 LawnSite Member
    Messages: 180

    Could Be A Head Gasket B/c You Should Have No More Than A 15% Difference In Compression Between Cylinder 1 And 2

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