Tohatsu 25HP 4 stroke

I,m having a problem with my Tohatsu 25HP 4 stroke. I was running it last week at just under full power and things were fine, I increased to maximum revs and after 5 min it died. I turned it off and did not run it for an hour or so. It started ok but when going to higher revs it died again. I ran back to shore on half revs. The water pump works fine - a good stong water jet is being discharged. A mechanic has told me to check several things - change the impellor, check the thermostat etc. Does anybody know what the temp should be when the thermostat opens?
I don,t have a workshop manual- I have ordered one but it will be a few days before I receive it and I want to get on with sorting it out.

Re: Tohatsu 25HP 4 stroke

There's an easy and quick way to check that your thermostat is atleast working. After about 5 minutes of your engine running at low to mid idle you will be able to increase your rpms to a high idle (don't go over) and put your hand in the water jet and you will actually be able to feel the water going from luke warm to very warm and back again as it bypasses the cylinder housing. I do it all the time with my 20hp Tohatsu. This isn't gonna tell you that your thermo is opening at the correct temp but will atleast tell you that it is opening and closing to begin with.

Re: Tohatsu 25HP 4 stroke

You didn't mention whether you have an older MFS25A (with 3 carbs) or a 25B (EFI). Completely different fuel systems. Regardless, a slight overheat would not normally cause the motor to stall from WOT. FWIW, the stat opens at 60C. Check your owner's manual, available at http://www.tohatsu.com/tech_info/manuals.html and you will see that a low oil pressure or overheat condition will limit RPM to 2800.

Is the gas tank vent open? Is the crankcase oil at the correct level? Do you have a water separator filter? What do the plugs look like?

If you have a 25B, your dealer can connect it to a laptop and review the engine's history.

Re: Tohatsu 25HP 4 stroke

The engine is a MFS25B purchased new in 2008 by the previous boat owner. I bought the boat and everything with it July last year. He claimed he had only run for about 50 hours. I used it last year for about 20 hours. I don,t know if he had any servicing done on it if he had there was no paperwork to support it. I did an oil change before using it this year and have done about 12 hours this year before the problem occured having said that it wasn,t run at maximum revs untill last week when I took out to sea. In the main I use the boat on our local river and there is a max 6kts speed limit.
The fuel vent on the tank is fine. I intend to do a full service on it when the parts are delivered and change the water pump - even if that is not the cause of the problem it could probably do with it in anycase. Thanks for your input

Re: Tohatsu 25HP 4 stroke

Have you ever drained the vapor separator? That is one spot where some fuel junk could accumulate. If the FFP (fuel feed pump) has an issue, that might cause your problem. A poor connection at the ECU could also cause odd issues. Again, on a 25B, it can be connected to a laptop to review the ECU memories (including temperatures, RPM speeds, etc.) and any faults. If you were to overheat, it would just drop back to "limp-in" (max 2800 rpm) mode, and would alarm. Is this a tiller motor, or a remote? Is the oil level too high? If it's equipped with a water separator (it should be), have you inspected/changed that filter?

If you are going to turn wrenches yourself, be absolutely sure to get the Factory service manual from any Tohatsu/Nissan dealer, including me.

Re: Tohatsu 25HP 4 stroke

pvanv said:

You didn't mention whether you have an older MFS25A (with 3 carbs) or a 25B (EFI). Completely different fuel systems. Regardless, a slight overheat would not normally cause the motor to stall from WOT. FWIW, the stat opens at 60C. Check your owner's manual, available at Tohatsu Outboards: Authorized North American Distributor for Tohatsu Outboard Motors, Parts, & Accessories. and you will see that a low oil pressure or overheat condition will limit RPM to 2800.

Is the gas tank vent open? Is the crankcase oil at the correct level? Do you have a water separator filter? What do the plugs look like?

If you have a 25B, your dealer can connect it to a laptop and review the engine's history.

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Captain I have a similar problem for my 02 tohatsu 25A It runs fine with no load on it then you put into gear it starts to rev up then it boggs down wont getout of its on way I have changed all filters OIL and Gas and Water seperater fule tank was empty when I first took inflatable out. Had it cheched out by mechanic says its fixed last week well went out yesterday went up river about a mile everything was working fine waited there half hour and then headed back started out okay then bogged down and I limpted back to the big boat where do i begin to find out what my problem is ? Is it electronic or fuel ? I don't have a clue