Tohatsu outboard not pumping water

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peter2407

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Decided to fire up an old Tohatsu 3.5 Hp 2 stroke outboard in a wheelie bin full of water, but the cooling water flow is very weak. I have checked the water vent thingies behind the prop and there was a little bit of blockage but not much. I have also checked the Tohatsu manual expecting to see an impeller location but there isnt one. Any ideas on what this might be and any guidance on how to do a proper service? TIA
 
peter2407 said:
Decided to fire up an old Tohatsu 3.5 Hp 2 stroke outboard in a wheelie bin full of water, but the cooling water flow is very weak. I have checked the water vent thingies behind the prop and there was a little bit of blockage but not much. I have also checked the Tohatsu manual expecting to see an impeller location but there isnt one. Any ideas on what this might be and any guidance on how to do a proper service? TIA
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Depending on which model of 3.5 it is, the impeller is either a doddle, or a bit laborious, to change. The 3.5A simply requires removal of the prop and the plate behind it; the more common 3.5B requires the leg to be dropped. VicS will probably be along shortly to tell you how to get a proper manual on-line. It's not a difficult job, but doing it without is a recipe for tears.

It might simply be that the tell-tale channels are partially blocked. Some people have had success adding vinegar to their wheelie-bin water to clear this. A long run helps, as the water heats up.
 
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Yes there is no guidance on the 3.5 impeller location in the owner booklet-it just says "take to dealer"!
I did mine recently and assumed it was behind the propellor backplate. It is in the leg.
You just have to take out the rubber bung at the leg bottom and the clamp screw for the gear lever is there 10mm head. loosen that but not completely and when you undo two bolts under the cav plate the leg can be lowered and the water pump is visible. Be very careful not to lose the little pin that drives the impeller when you remove the pump. It is only a half hour job but can be fiddly getting the water pipe end located and the drive shaft into the spline. There is a plastic grommet that the water pipe fits into and this can lift off when you split the leg and it need re fitting in place so that the water pipe can then slide into it, with a bit of grease.

I replaced my impeller as I had never done it and the engine has given 10 years of frequent annual use, estimated as in excess of 300 hours. The impeller was perfect without any trace of cracks at the root of the vanes or other perishing deterioration.
 
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Thanks both. How do I tell if it is the A or B model? Take off the prop and backplate and if the impeller isnt there its a case of drop the leg?
 
C08 said:
I replaced my impeller as I had never done it and the engine has given 10 years of frequent annual use, estimated as in excess of 300 hours. The impeller was perfect without any trace of cracks at the root of the vanes or other perishing deterioration.
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Yes, I've never yet needed to replace an impeller and I run some very old outboards; lack of water flow is always just the waterways in the powerhead furring-up.
 
First of allc heck the tell tale outlet for blockages

Models with a fixed drive,ie no clutch, have the water pump located behind the prop and changing the impeller is as stated simple.

Models with a clutch will have the water pump located on top of the gear case and therefore will require separation of the gear case from the exhaust housing. This will entail releasing the clamp between the upper and lower sections of the shift rod. It would be wise to mark the position of the clamp to aid correct reassembly.
Slide the top of the water pump up the shaft without pulling the shaft from the gear case ( to avoid damaging the seal below the pump) watch out for the impeller drive key when you remove the impeller
Lightly grease the splines at the top of the drive shaft when reassembling, and take care that the water tube locates in the pump correctly

I do not know of any on line workshop manuals but the parts diagrams at https://www.tohatsuoutboardparts.com/Parts-Books.html can be useful
 
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Thanks everyone. So its a B model as it had F/N gears. Whats the easiest way of checking and removing a blockage? I would rather do that first to avoid the f@ck up fairy paying a visit when I try to drop the leg....
 
peter2407 said:
Thanks everyone. So its a B model as it had F/N gears. Whats the easiest way of checking and removing a blockage? I would rather do that first to avoid the f@ck up fairy paying a visit when I try to drop the leg....
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I believe the B models have the provision to connect a hose to the power head for flushing ( see your owner's manual)

Do this and you should get a full bore flow from the telltale and a good flow from the cooling water and exhaust outlet as well as some flow back through the water pump and out of the water intake slots


If you get little flow from the tell tale check that for blockages.

If you don't get a good flow from the outlet then take the cylinder head off and remove the power head from the leg and clean out the water passages

Then try it

If you still don't get a good water flow then replace the water pump impeller, and housing if worn, and also check that you can get a good flow up the water tube from a hose ( Or down it from the above flushing connection.)


Personally I'd do the water pump impeller first esp if if has not been changed recently
 
My Tohatsu tell tale blocks up all the time-I just use a 6" length of stiff wire and it always unblocks ok. By the nature of the use of a small outboard it does not normally cause a big problem on a tender so better to wait until the impellor goes and then fix it. I have previously used a 20year old outboard the owner said had never had a new impellor and was used heavily each year and I never changed it in 5 years!
 
Soon after I got my Tohatsu 3.5 the tell tale stopped spouting water, but the engine was not getting hot. I did open the engine, clean the cooling channels I think and replaced the impeller.

I would try having a gentle poke with something the right size and blunt up the tell tale pipe but it is probably a good idea to replace the impelleras well.
 
Most times a wire pushed up the rubber piddle hose will clear any blockage, but occasionally the salt crystals will be in the metal pipe to which the hose is attached, which is at 90 deg to the hose. If you take off the gear lever and remove the cowling, the hose can be pulled off so that you can poke around in the metal pipe - that should do it.

Rob.
 
The Tohatsu impellers on the small stuff are almost bullet proof. As others have said likely to be blocked passages, poke around up the pee hose with a bit of bendy wire.

You can see if you have water getting through the powerhead by tilting the engine slightly with it running, some water should be spraying out of exhaust slits at the bottom of the leg.

NB Reassembling the leg to the powerhead is a very fiddly job, in fact a total PITA.
 
You won't get a proper flow of water coming out if testing in a bin. Cavitation will cause too much air to be sucked up hence the poor flow. Better try it off a dinghy afloat.
 
chrishelen said:
You won't get a proper flow of water coming out if testing in a bin. Cavitation will cause too much air to be sucked up hence the poor flow. Better try it off a dinghy afloat.
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You can test in in a bin Ok if you run it in neutral, or in the case of fixed drive models with the prop off.
 
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