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87 Johnson 150 Blowing Steam out the Telltale

R

rage

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"We get a good steady stream w

"We get a good steady stream when we start the engine but when we travel at headway speed the cooling water starts to heat up then steam blows out the telltale. Shut it down for a few seconds and restart and the steady stream of cool water returns then heats up and starts to steam. The high temp alarm isn't sounding but we didn't run it with the steam for very long. The initial steady stream seems to indicate the water pump is okay. If figured it was the thermostats but the manual says they usually fail open. Any ideas? Shouldn't that temp alarm be sounding if we've got steam? How do you know if the engine is really overheating? Thanks,"
 
I would start with the waterpu

I would start with the waterpump. It takes far less pressure to make that stream then it does to circulate enough water to cool the engine properly.
 
"Thermostats fail either way.

"Thermostats fail either way. It sounds like you need to replace thermostats and also check out the warning circuit. I have a 1987 110 Johnson and last month both thermostats were bad. Pee stream was always good but alarm would sound after 5 minutes on the hose in the driveway. Pulled the thermostats out, put them in water on the stove with a thermometer to monitor what temp they opened at. 1 opened at 160 degrees and the other at 175 degrees. They should open somewhere around 145 degrees if memory serves me. On a 150 those are in the top of the heads and they don't look too hard to change."
 
"The warning horn should beep

"The warning horn should beep momentarily when the key is turned to the ON position. Does it?

With the engine NOT RUNNING, have the key in the ON position, then ground out the TAN wire that is protruding from the heat sensor at either cylinder head. The horn should sound steady. Does it?"
 
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