Answer
Apr 22, 2025 - 09:20 PM
Alright, so the cooling system on the Mercury 9.9HP 4-stroke is a pretty simple setup, but if water’s not coming out of the telltale (the little pee hole), something’s up.
Basically, the water pump (down in the lower unit) draws in water through the intake screens, pushes it up through a tube into the powerhead, circulates it around the cylinder block to keep things cool, and then spits it out through the telltale port. If you just replaced the pump but there’s no flow, a few things could be going on:
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The pump impeller might not be seated right — sometimes if the drive key slips or the impeller vanes are bent the wrong way during install, it won’t pump correctly.
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The water tube might not be lined up with the pump housing. If it's even a little off, water can’t get up to the powerhead.
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The telltale port itself could be clogged — mud, sand, or old salt buildup loves to block that little hole. Try poking a thin wire or even compressed air up there.
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Worst case, you’ve got a blockage in the cooling passages, but that’s less common unless the motor’s been run in dirty or salty water without regular flushes.
Luckily, if you really want to know what's wrong with your particular unit, the manual has diagrams and a full cooling system flow path so you can double-check everything’s lined up right. Wish you the best of luck 👍🏻
