Nissan/Tohatsu M3.5A cooling water routing

[tbodine88]

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.

https://www.youtube.com/watch?v=-OEgiMTnn54

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?

[tbodine88]

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.

[Compass790]

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.

[rbk]

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.

[Rumbero]

Hi, I am trying to bring up to date and clean up the refrigeration system of my Tohatsu M3.5A, since little water flows out. I am also changing the impeller, o-ring in the pump body, and even the oil seal.


It seems that tbodine88 posted long ago a diagram on how the water flows inside a Tohatsu M3.5A, however, it seems the diagram is not visible anymore. If tbodine88 or someone else could post the diagram again it would be of great help.
Thank you very much!


Rumbero

[tbodine88]

Here it is.



I'll probably need this soon, since I've acquired a 2.5 for my Dinghy.

[ggray]

Random thoughts on this old post since the OP is still around:

You mentioned a broken shear pin. This happened to me a couple of times until I changed my operating process (I have a neutral shift which I'll bet you do as well). It seemed to me that it was the sudden shift into gear with the motor running is what sheared the pin, so I began starting and running the motor in neutral until it warmed up a little. Then shut down and restart in gear. Never sheared a pin again. Perhaps a stronger pin is needed?

The water pump impeller, IIRC, is in an unusual place, different than most motors.

I still have a problem forcing lower unit oil into the L.U. Haven't resolved as this motor is no used much now. But if it happens to your motor search CF for a long
thread on this.

[tbodine88]

The motor that sheared the pin had no neutral since it was just a small 2hp motor.

I broke the shear pin since I ran it in a lake with a rocky bottom. The prop hit a rock at high speed which sheared the pin.

My larger Nissan 5hp has a clutch between the drive and propeller that keeps the shear pin from breaking most of the time. In 20 years the shear pin hasn't broken

That engine has a neutral and can not be started in gear.

If I was wise I would have cut the engine before I was in shallow water.

Thanks

[tbodine88]

Originally Posted by tbodine88

I'll probably need this soon, since I've acquired a 2.5 for my Dinghy.

But I forgot where the pump is


Here is a photo for anyone's viewing pleasure





Since the pump is sound, but there is no water flow I will try blowing out the water pipe with compressed air.