200 Merc overheating, erratic water psi, prop & poppet questions

Discussion in 'Outboards' started by sbklf, Mar 15, 2014.

  1. sbklf
    Joined: Feb 2006
    Posts: 45
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    Location: Houston,TX

    sbklf Junior Member

    While running close to WOT, the RPM increased and shortly afterwards (20 seconds) it overheated. The motor was jacked and trimmed up a little higher than normal so thought that was the problem. Now it seems to maintain good water pressure & run normal temp at mid throttle but when close to WOT, at times we get the RPM increase again and then notice erratic water pressures (literally bouncing between 0 and 20psi) and overheating. Running in a tank, the water pressure at idle is good (2-5 psi) and the head temps are good (150F). Water pump, poppet and t-stats were replaced last year and we ran with no problems till now.

    -My prop guy had to put some cup in the prop and advised it is pretty worn. Can a prop issue create a cavitation condition and cause the water pump to lose prime?
    -What are the symptoms of poppet issues?
    -Can poppet issues cause erratic water pressure?
  2. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    In a tank near WOT the drive sits in a dry hole with water all around it, so there is extensive cavitation and lack of cooling water!
  3. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    Isn't he saying it overheats at speed on the water ? The motor may be too high, the cooling water intakes are intermittently exposed. That would explain the erratic water pressure, and overheating, and runaway revs. It is surprising just how much prop can be exposed before they "let go", in some instances. A combination of being jacked too high, and trimmed out a lot, perhaps is causing the problems. Drop the engine down and see what happens. An added clue is the prop being worn, sounds like it has lost a bit of blade area and diameter, and with the extra cupping, means the whole shebang can rise further out of the water without over-rev. On second thoughts, I think you need a new prop.
  4. sbklf
    Joined: Feb 2006
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    Location: Houston,TX

    sbklf Junior Member

    Yes, WOT only in the bay, idle speed in the tank. My tank is actually a big feed bucket and I have to remove the prop to get it in there.

    I will start with water pump replacement and have the prop rebuilt.

    I like learning these terms that tell the story. When they "let go", do they typically speed up? It seems they would slow down.

  5. powerabout
    Joined: Nov 2007
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    Location: Melbourne/Singapore/Italy

    powerabout Senior Member

    run a clear hose from where the water pressure take off is ( top middle of the exhaust in the block) and into the boat, have 10 or so coils wound up then lead the end overboard.
    Now go for your run and see if you get air bubbles in that hose at WOT
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