Mercruiser 3.0L zero compression

Discussion in 'Gas Engines' started by CDK, Aug 27, 2013.

  1. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    One of my tourist guests put his Bayliner in the water by crane in a nearby city because he wanted to keep the trailer dry. After approx. 2 miles the engine started to run rough and he noticed the temp gauge.
    Someone towed him to Merc. service where they replaced a blocked thermostat. No further issues during the 10 miles at 3500 rpm to our bay.

    Then he took his family and friends for a sightseeing trip, but the engine lacked power, so he returned within a few minutes, the engine stopped and they had to row back to the jetty.

    He asked for my help so I checked compression: zero on all cylinders! He called the tech who repeated the test with the same result. Removed the valve cover to check for valve train damage: everything looked OK.

    Does somebody have an explanation?
     
  2. latestarter
    Joined: Jul 2010
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    latestarter Senior Member

    I should warn you I have no practical experience of diesel engines and I am working from first principles here.

    The reasons for lack of compression that come to mind are hole in piston, piston rings, hole in head, hole in block or blown head gasket. The chances of this happening to all cylinders at once should be vanishingly small.

    Is the engine fitted with a decompression system which could malfunction?

    The fuel injection system is connected to all the cylinders, would it be possible the loss of compression is going back through it?
     
  3. CDK
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    CDK retired engineer

    No Latestarter, this is a GM gasoline engine designed around 1940 and - slightly modified - still used by Mercruiser, especially on the European market.
    It has the camshaft mounted low in the block, driven by gears. No belt or chain that can break.

    Because the guy came happily planing to our bay the defect occurred within minutes when he took off again with 4 adults and 2 children. Hardly time for a meltdown of all four pistons, valves or the gasket around each cylinder.
     
  4. gonzo
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    gonzo Senior Member

    I would think that only valves stuck open on all cylinders would do that. Even if the timing had been overadvance, it usually makes a hole in one or two pistons. Did the engine overheat?
     
  5. CDK
    Joined: Aug 2007
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    CDK retired engineer

    It did overheat on the 1th part of the trip, after installing a new thermostat everything seems O/K.

    Eventually I will hear from the guy what was wrong after he gets home and delivers the boat at Merc service in Germany, but it annoys me I cannot find a satisfying explanation.
    During my boating years I've had 5 of these engines and encountered any issue you can think of, but never something as serious as this.
     
  6. Ike
    Joined: Apr 2006
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    Ike Senior Member

    Had a similar thing happen with a Mercedes diesel. I went away on a business trip for several weeks. While away my son drove the MBZ, but despite me telling him to check the oil (it had abut 200,000 miles on it) at every fill up, he didn't. When I got back it wouldn't start no matter what. Towed to dealer. They said zero compression. Apparently the last time he drove it the oil got low enough it was no longer lubricating the rings. Result, no rings, scored cylinders, etc. New engine, $6500.00
     
  7. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    A good starting point would be to obtain an endoscope you can get cheaply from an electronics store, not sure what they call them exactly, but you stick it through the spark plug hole and see what the cylinder/piston looks like. Cheaper than taking the head off, if there is damage to the bores it should be evident in those cylinders that are near the bottom of the stroke.
     
  8. jehardiman
    Joined: Aug 2004
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    jehardiman Senior Member

    Generally when you weld the rings to the cylinders through overheating, the engine stops and cannot be spun. Did you check the oil? Was it too thin? Too rich a fuel mixture / stuck floats / debris in the needle valve will score the cylinders without overheating, but it will show up in the oil. Could be cracked valves / valve seats. Only a tear down will make it know for sure.
     
  9. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Yesterday evening I towed the Bayliner to the slip where the guy put it back on the trailer and pulled a tarp over it. I also advised him to install an oil pressure/temperature buzzer, but I guess after one spoiled holiday he doesn't need that because from now on he'll watch the temp gauge like a hawk.
    The buzzer should of course be standard equipment, there even is a wire reserved for it in the Merc instruments cable.

    I appreciate Mr Efficiency's idea and I do have an endoscope but never thought about using it for this purpose. The cheap ones probably don't pass through the spark plug hole but one with fiber optics does.
     
  10. Dave T
    Joined: Sep 2011
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    Dave T Senior Member

    Did you find the reason that it didn't have compression on all cylinders? What I would do is first check to make sure the distributor is turning, possible broken camshaft. If ok then I would put compressed air in through the spark plug holes. If it's rings the air will come out through the crankcase vent system, exhaust valves out through the exhaust, intake back through the carb. Don't think I've ever seen an engine lose all compression on all cylinders at one time. The only other thing I can think of would be a blown head gasket between cylinders that lets the compression go from one cylinder to the next. This would show with an air test as air coming out of the spark plug hole of an adjacent cylinder.

    Dave T :)
     
  11. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    I've never seen a compression lose, on all holes at once either. All the valves open at once isn't possible, even with a broken cam, since the valve train looked normal with the cover off, who knows, but the air in the spark plug hole trick will work.
     
  12. CDK
    Joined: Aug 2007
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    CDK retired engineer

    If it were my boat I wouldn't have any rest until I found out what happened, but luckily it isn't mine.
    I wrote the owner a mail today to remind him about his promise to send pictures.
     
  13. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    He'll think of it again next summer, I guess.
     
  14. PAR
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    PAR Yacht Designer/Builder

    Agreed, this would drive me nuts and I'd have the head off that puppy in a heartbeat, trying to find what was up.
     

  15. powerabout
    Joined: Nov 2007
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    powerabout Senior Member

    i saw a severely overheaded engine lose comp ( not zero though)and it was the rings that lost their tension
     
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