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Over heating at full throttle

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is the seacock open? 

hoses not kinked?

have you cleaned debris out of the transmission cooler?  

is your strainer sucking air?

inspect impeller: all the fins healthy and the splined shaft does not spin wrt molded rubber?  cover gasket healthy (cover not sucking air)?  DON'T over torque those cover screws.  torque spec is in inch-pounds NOT ft-pounds.

next i'd drop the boat into the pond, remove the output side hose from the engine water pump and run the engine at idle for 15 seconds to insure it was pumping A LOT of water.

assuming all is jake to this point, next step is acquiring a gasket for reassembly then pulling the thermostat cover and thermostat. 

is that thermostat in good shape?  does it open when dropped into a pan of hot water?

get back w feedback and we'll talk more.


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It's way too soon for you to go to this extreme but I did exhaust ALL of the above options on a 1986 Supra.  The culprit was the actual raw water pump housing.  Over time the brass on the intake side wears until it is no longer able to effectively "suck."  There are rebuild kits available.  https://www.amazon.com/Water-Pump-Rebuild-Jabsco-2570/dp/B07LCW8HZC

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Here are some candidates below;  not sure if last bullet point applies to you or not, but I'd definitely look at circulation pump.

  • Verify the raw intake valve is completely open; or if utilizing a flush or fresh water intake that the water input is routed from an open port/hose/valve as appropriate.
  • If on land, verify good water pressure to water intake (avoid using a fake-a-lake!)  Note that when using any type of external system it could mask a cooling issue.
  • If on water, bump the throttle to “prime” impeller in case of air in system.  If you have a “run dry” impeller (such as the Globe 100J) they are known to be prone to priming issues at times; if so, a short 1500-2000 rpm engine rev should prime it.
  • Verify the impeller is good; inspect or replace with new if ANY overheating.
  • Verify the transmission cooler screen (and raw water intake screen) is clean.
  • Verify the thermostat is good.  They are inexpensive, replace if in doubt.
  • Verify the temp sensor is good. They are inexpensive, replace if in doubt.  There are up to 3 temp sensors, depending on engine: 1 sensor for dash, 1 sensor for ECM, and another for light/buzzer.  See picture below; applicable to many Indmar 5.7 or Chevy small block 350 based engines.
  • Verify all hose clamps are tight and hoses are not kinked. Trace from the raw water intake through the entire system.  Check for loose hose clamps along the way and any kinking, especially around the tiller arm area upstream of impeller.  Any small openings can allow air to be sucked from impeller resulting in no prime, especially on impeller face plate.
  • Verify circulation pump is working.  If overheating at speed vs idle, it may be that pump.  Remove the drive belt and open the top port on the circulation pump and then turn the circulation pump by hand, with water in system you should see movement.  If a broken pump is suspected, remove entire housing and shake: if you hear noises, it is a broken circulation pump.
  • If you have a heater verify the heater circulation pump is running. You will get heat even if it isn’t running so don’t make any assumptions based solely on if you have heat. If it isn’t running first check the fuse for it.


  • Verify the vent line to transom is clean. On the transom you’ll see a little metal port about the size of a button. After running the engine and shutting down it will continue to vent water like its peeing for a few minutes. If it isn’t venting water it’s plugged up. There’s a hose that will run to the top of the engine. Take that hose off and blow it out with compressed air.
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