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Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 10590
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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I got seaweed in the intake of a 2-stroke, 8hp Nissan. I

Customer Question

I got seaweed in the intake of a 2-stroke, 8hp Nissan. I first noticed a problem with steam coming from the pee hole. I cleared the moss and it started up and ran "fine." After one more incident with the steam, I replaced the impeller and gaskets, etc. (I was afraid to pull the ss cup from the plastic housing, as I don't have a replacement. There was a fair amount of rubber residue in the cup, but it scraped out OK.
I blew out the water tube and pee hole from above. (I didn't think it prudent to blow air UP the tube in case there was debris up there from the worn impeller.)
Now, after running for a while in gear, in the test barrel, after I drop to neutral, and resume forward, the pee hole reverts to pulsing steam and only hot air.
After letting it sit for a few minutes, it starts up right away and jets cold water from the pee hole. I suspect "foul play" from the thermostat (is there one?). But logic says--to me--that flow should increase with heat, not the opposite. And, it just continues to get hotter if I don't shut down. The block gets as hot as the exhaust area in the lower unit/leg. Soooooooo, my question.
Is there any correlation between the thermostat and the pee hole? Or is there some other bypass/restricted passage that might be partially blocked?
(It's the 4th weekend, and I have volunteered to lead the boat parade around our lake tomorrow. This is gonna' be a fiasco if I don't make it underway. We are also not anyplace I can get Nissan parts in less than a week.)
Submitted: 3 months ago.
Category: Boat
Expert:  Tan replied 3 months ago.

Hello. My name is Tan.

It is possible you have a blown head gasket. Now you could have seaweed blocking the thermostat.

The thermostat is in the top of the cylinder head. So you have to remove the cylinder head to inspect the thermostat. If it were me I would remove the head replace head gasket and thermostat and be done with the problem.

Now it the water pump plastic housing was melted this could be an issue also. This won't allow the pump to deliver sufficient water to the engine.

How hot did the engine get?

Customer: replied 3 months ago.

Hot to touch, but I didn't let it run for more than half a minute with steam.

I'm not in a position to be pulling the head where I am. But, what I need to know, is if the pee hole is downstream from the thermostat. And, how it can go from running lots of water, to hot air, and back.

Also, I suspect that if the head gasket was blown, it would steam all the time, just not when returning to idle from full ahead. (??)

Expert:  Tan replied 3 months ago.

Ok there is a way to test to see if the water pump is the problem. Remove the lower unit. Now attach the garden hose to the water tube going up to the engine. Now run engine with city water on about half volume. Does engine run ok and not produce steam? You will have to make up a fitting to connect the hose to as the water tube is real small in diameter.

Customer: replied 3 months ago.

OK. I can't do that where I am, but will try it.

But, the biggest question is why the flow goes from strong to non-existent when I change speeds up to fwd-ahead, to idle-neutral, to reverse, and back ahead. Through these changes the pee hole shuts off water and goes to hot air. Otherwise, with a steady-ahead there is no change.

I can shut it down and get it back online after a minute pause..

Expert:  Tan replied 3 months ago.

This will tell us if you have a water pump issue or an engine issue.

If engine runs good with no steam then we have a water pump issue. Most likely the plastic housing and cup. Possibly the base of the pump is distorted .

Keep me posted.

Customer: replied 3 months ago.

OK. I've got to go launch and do a longer test.


Expert:  Tan replied 3 months ago.

Ok keep me posted.

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