OMC 3.0 Liter 4cyl engine 1979 OVERHEATS when slow or stopped in water

by James

I have a 87 bayliner, I had to replace the engine with a new block..(OMC 3.0 liter 4cyl, drive unit is a OMC cobra)I found 1979 block and head. The engine will run fine when crusing, but when slowing for docking and stopping for skiers the unit will over heat. ENgine is very hard to start when shut off at this point. I have replaced the water pump (engine) and I have installed a new impeller and cover. When the engine is running with a hose, the unit runs fine and stays cool. I have no thermo.. When in the lake, the main water hose to the waterpump when squeezed doesnt seem to have must water going thur it.. Recap. Engine runs fine with water hose attached, runs fine at the lake when running over 1500 rpms, but below that in the lake it over heats and then does want to start. New waterpump, new impeller, NO thermo installed.

If anyone has this problem or has an idea please email @


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Jul 22, 2010
by: Anonymous

And I thought I was the only one with issue. I also have a 87 Bayliner that does the same thing. I have replaced the impellar as well, and it still over heats. I have not had the boat long, and it is my first boat...If you get your issue fixed, please let me know...

Jun 13, 2010
overheating problem
by: Anonymous

i will try to keep this short but there are a few problems it could be
1. your thermostat it would be the cheapest to fix

2.the radiator could need to be flushed out

3. water pump alot of times with the water pump it will have a drip under the water pump but i have seen them not drip at all.

4. i have also seen the radiator baffles fall out and that would cause the air to blow right threw and not give the air enough time to cool the water and the higher speeds are pushing more air threw and cooling it

there are only really 5 tings that can make an engine overheat let me know what fixes it

Aug 18, 2009
overheat on mercruiser.
by: oscar

put the thermostat in it.

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