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150TXRZ Overheats at idle

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  • 150TXRZ Overheats at idle

    Sorry for the long opening, but I've done quite a bit already, and I'm really confused.

    Ran motor all last season with no issues. The motor has never been in salt water. The motor was not used for about 2 months before I winterized it. When I winterized in the fall, just as I was ready to shut it down, the overheat siren came on. Hoping it was a fluke, I put it away anyway. I started it again on earmuffs yesterday. The overheat problem wasn't a fluke.

    Here's what I've checked so far:

    1) Motor always pees good.

    2) Checked thermostats in pan of water. They both open at approx 122 degrees like they are supposed to.

    3) Pulled lower unit and checked impeller. Everything looks fine.

    4) I drilled a hole through one of my kids bouncy balls and stuck that on the end of my air hose. Stuck the ball/air hose on one thermostat opening, covered the other with the palm of my hand, and had my son make sure there was good air flow coming out the tube in the lower unit. He said yes, so it seems there was no obstruction there. Repeated the test by holding air hose on other thermostat opening and still good air flow.

    5) Started motor on earmuffs for about a minute with no thermostats and no thermostat covers, just to see if water was filling the block. No water made it to the top of the motor.

    6) Pulled Poppet/PRV pressure relief valve. It seemed like it may have possibly been stuck in the closed position. However, from what I've been reading, it should be closed at low RPM anyway. Cleaned it up a little and don't see any reason it wouldn't be working properly now.

    7) Hooked a hose up to the hose connector on the port side of the motor and tried flushing the motor. I had the poppet valve out, and both thermostats out. Water did come into the opening where the poppet valve goes, but didn't seem to have much pressure even though the hose was on full force. I stuck my finger in the PRV hole to plug it, still no water to the top of the motor on either side.

    I'm really confused.

    I think the water flow is supposed to:
    1) Come in water inlets in lower unit to feed water pump
    2) Water pump pushes water up the tube to the bottom of the motor where the poppet valve is.
    3) If there's too much pressure, some is released from the poppet valve.
    4) Water is pushed into water jackets surrounding the cylinders to the top of the motor.
    5) Thermostats hold water and keep it from flowing out of the water jackets into the heads until proper temp is reached.
    6) Thermostats open and allow water to flow through the heads and back down through the bottom of the motor into the lower unit.

    I'm pretty sure water is supposed to get mixed in with the exhaust, but there seems to be very little coming out the center of the prop and none coming out of the ports in the upper lower unit.

    Based on what I think I know, I would think the water pump isn't generating enough pressure or there is a blockage between the water pump and the pressure relief valve location. However, if that were the case, I'd think I'd get water to the top of the motor when I hooked it up to the hose connection on the port side of the motor.

    Please help!

  • #2
    Have you removed the cover on the top of the engine between both cylinder heads. I recommend changing the water pump anyway. If there is no corrosion, then it is most likely the water pump. With the lower unit off, but thermostat covers off, take you hose (on full blast) and hook up to the end of the water tube. This will send water through the proper cooling route.
    Regards
    Boats.net
    Yamaha Outboard Parts

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    • #3
      Thank you. Actually that's basically what I did this morning and you are right, it does seem to be the water pump. Even though the water pump visually looked OK, it must be bad.

      Here's what I did:

      1) Pulled off lower unit.
      2) Plugged the tube going from the water pump up to power head (electrical tape across the end).
      3) Put PRV back in. Left both thermostats out and caps off.
      4) Hooked up to hose connection turned on water (low pressure) and plugged the pee hole with my finger.

      This left the water no place to go but into the water jackets and out the holes where the thermostats go. Sure enough, that's what happened. The port side thermostat hole is slightly lower than the starboard side. After the water started out the port side, I put the stat back in and made sure it came out the starboard side stat hole.

      When I removed the tape, water that had collected in the tube came rushing out.

      Just to double check, I used that rubber ball on the end of the hose and at the bottom of the tube. Plugged pee hole and water eventually came out the stat hole again.

      Great news for me! No engine teardown trying to find a restriction that isn't there.

      Water pump must be creating just enough pressure to get water up the tube to the bottom of the motor where the PRV and pee indicators are, but not enough to fill the block.

      I'm off to buy and impeller.

      Comment


      • #4
        That was the problem. Replaced impeller and it runs fine now. Really can't believe it. It was only a year old. Had a little bend in the blades, but still seemed to have plenty of flex in them and absolutely no visible wear.

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