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Old 13-01-2020, 10:00   #1
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Nissan/Tohatsu M3.5A cooling water routing

I had replaced the water pump impellar. I had the motor all together. It would start and run. So I took it out to the lake. I put it in the water and broke the shear pin. Which led me to past my last note.



I noticed when it was running no water was spurting out the engine as I can see on various videos of the same engine on youtube and on russian video sites.


I took the lower unit off and I hooked up a water hose to the pipe that leads up from the water pump. I can see water running down the inside of the lower unit but none spouting out in any manner that would be visible if the engine is all together.



I also don't see how the water gets through the engine in this drawing from tohatsu : https://www.tohatsu.us/index.php?mai...1005_1036_1038


Is the water tell tale a new feature that this engine is just too old to have?
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Old 17-01-2020, 16:12   #2
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Re: Nissan/Tohatsu M3.5A cooling water routing

So I figured out my problem.


First here is how the water flows inside a Tohatsu M3.5A:



My problem is that the water pump doesn't put out enough water pressure to push the water up the tube from the pump to the power head.


I found this out by dismantling the motor hence the picture shown above. I blew air through the passages and observed air flow through the tell tale hose.


Then I pushed water through the tell tale hose and observed water flowing around the cylinder.


Then I put drill on the drive shaft of the lower unit. While running the drill I saw that no water flowed out of the pipe. I could blow through the pipe. I could push water through the pipe. When I pulled the pipe out of the pump and ran the drill, I could see water flowing out of the pump, but with out much pressure. It is possible the drill doesn't run fast enough, but I think the pump is the problem. So apparently replacing the impeller wasn't sufficient to solve this problem.


I'll will continue working on it.
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Old 18-01-2020, 20:46   #3
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Re: Nissan/Tohatsu M3.5A cooling water routing

The clearance of flexible impeller pumps is critical. You used a factory gasket or o-ring I assume. The reason I ask is I made my own on a raw water pump & the pump didn't develop much pressure at all. ( had an impeller but no gasket )
I ended up using Loctite 518 flange sealant with no gasket to get it to work.
Nice pics & explanation BTW.
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Old 18-01-2020, 21:58   #4
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Re: Nissan/Tohatsu M3.5A cooling water routing

Clearances as mentioned above are critical. Was the pump housing scored at all? Might have to buy a whole new housing. Sometimes using a scored housing even with a new impeller won’t allow it to build pressure.
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