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Just a gentle reminder of why it is important to keep those engines maintained. Especially in our area where we have a lot of floating grass. This was pretty typical of all four of my thermostats.

Contender 27 Cuddy


  • troutman57troutman57 Posts: 3,691 Captain
    That's great info AP. Thanks
    This place Rocks if yer a crabber
  • Fishin RodFishin Rod Posts: 2,620 Captain
    Making sure your thermostats are working is important, I just replaced mine on my Yamaha 115 and my water pressure at the top of the head went from 5psi to 30psi....... If they are stuck open you will not get the heat transfer necessary to properly cool your engine. Another thing to check is the valve at the bottom of the head where the hose comes out. There is a plastic shaft with a spring and that needs to be clean and working properly, I have heard it called the "Pop off" valve but not sure what it's really called. I'm no mechanic but simple maintenance saves dollars. Especially when the going rate for outboard mechanics is around $90.00 and hour now.
    "Be what you is"....... Isaiah Minter
  • Doc StressorDoc Stressor Posts: 2,784 Captain
    If they are stuck open you will not get the heat transfer necessary to properly cool your engine.

    My Yamaha mechanic told me the same thing. I just can't get my head around it though. Outboard thermostats let water flow out of the block when they open. You would think that it would let the engine run cooler if they get stuck open. They should only run hot if the thermostat fails to open properly.

    My F250 runs hot intermittently when I run offshore. If I slow down for a while and then get up on plane again the temp goes back to normal for a while. I used to think I was sucking in floating weeds, but I just don't know. It's going into the shop today still it's still under warranty.

    The valves you are referring to are pressure relief valves, which are often called poppet valves. They are supposed to open up when you run at high rpm and close when you are going slow. Since the impeller pumps more water as the rpm increase, there has to be a relieve that pressure. They are supposed to open when the pressure gets over 25-30 psi. It is pretty common for them to stick in the open position when grass and junk gets in there. When that happens the engine will overheat at low speed since the cooling water will drop out of the bottom of the jacket too fast. At high speed the engine runs at normal temperature since the poppet valves would be open anyway.
  • Almost PerfectAlmost Perfect Posts: 602 Officer
    Fishin Rod wrote: »
    my Yamaha 115
    My Yamaha

    There's ya'lls problem right there :wink
    Contender 27 Cuddy
  • Fish HaidFish Haid Posts: 8,417 Admiral
    Out of focus thermostats are bad news.
  • HighPlainsDrifterHighPlainsDrifter Posts: 270 Deckhand
    There's ya'lls problem right there :wink

    "I live in a free country. I've got all the licenses, registrations, and permits to prove it!" :huh
  • Fishin RodFishin Rod Posts: 2,620 Captain
    There's ya'lls problem right there :wink

    Touche' or "Tooshy" , if your from South Georgia....
    "Be what you is"....... Isaiah Minter
  • CyclistCyclist Posts: 23,340 AG
    I use a water pressure gauge with my 60 ETEC. I would assume I would see some changes in pressure if my thermostat is clogged up?????
  • lakemanlakeman Posts: 821 Officer
    I have to agree, thermostats are often over looked for issues. On one of my last motors in for a water pump, I ask for the thermostats to be checked, the repair place said why, we just put in new ones a year or so ago, any way, they did check them, and they were trashed, evidently some one had put in the wrong thermostats, not in size, but in material, any way I got free new ones.
    They are easy to check so just do it.
  • screamin seamanscreamin seaman Posts: 1,492 Officer
    Water flow can never be "too fast to pull heat out!" ! ! !
    The old wives tale comes from car mechanics thinking hot water flowing through a radiator couldn't dissapate heat to the Al/brass and then to the air fast enough to cool the engine.

    So this is especially false with outbaords which have open cooling systems. The cooling of metal is non-linear, as in if you take a glowing piece of hot metal and dip it on water for a half a second it will drop a few hundred degrees VERY fast... yet for it to drop another couple hundred degrees it would have to be held in water, for say, 3 seconds... and for a couple hundred more of a drop another 10 seconds and so on.

    The thermostat and poppet valve both regulate water but in different ways,sort of. At idle the poppet valve stays closed, blocking water (or at least most water from passing) and the thermostat opens and closes to regulate water temp in the block. The main reason is to keep a minumum temp for proper combustion. Once you advance the throttle,the pressure from the water pump starts to over power the spring holding the poppet valve closed and begins flowing 'extra' water through the block. The more RPMs you gain the more pressure you build in the cooling system and the more water flows past the poppet valve. At a curtain RPM the poppet valve is wide open and you are completely free flowing water through the block with the thermostat doing nearly nothing.

    Dpending on which is bad you can have an over heat issue at idle but not WFO or cool fine at idle but over heat at WFO.
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