# 25hp tohatsu 4stroke FI issues



## mpancari (Jul 21, 2014)

I've been chasing an idling issue plus other issues for a minute. Here's what I got,
Fuel getting in oil
Spark plugs fouling
Rough idle 
Random stalling (last time after running an hour and was putting back to ramp. Did fire up again) 
(2011 tohatsu 4 stroke fuel injected MFS25B)

Here's what's been replaced
-thermostat
-fuel pump 
-low/pressure fuel filters
-fuel injectors cleaned and tested
-checked vst (float, needle, and filter all look good) 
-spark plugs 
- idle air control valve 
-new fuel tank, water seperator and hoses 
- new piston rings 

Probably done other things I don't remember at the moment. 

If anyone has expirienced these issues or thinks they know what could be causing it I'm happy to listen. Thanks


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## jonny (May 1, 2007)

Jesus man you replaced the rings already? I was gonna say possibly blow by. But new rings would have fixed that. And you got a good look inside. Don’t have the schematics in front of me. Shouldn’t be a fuel solenoid being injected. You would think with the injectors tested. Leaking into the cylinder would be out of the equation. Did you pull the head gasket when replacing the rings? Or just do it from the bottom? A blown head gasket is about the only thing left. How is the compression? I would check it hot and cold. To see if there’s a huge change. Are all the plugs fouling?


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## jonny (May 1, 2007)

Just had a thought. Could it be possible if you are fouling plugs. The unused fuel is being forced by the rings? Due to the increase in compression. And it could be a spark issue you are chasing. Did you change plugs recently? 
Did you use different oil weight? Are your plugs fouled with fuel or oil/both?
Another option is a stuck thermostat. Not allowing it to get up to temp. For proper metal expansion and allowing blowby. How much fuel are you getting into the oil. Are you getting a raise in the oil level?


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## JC Designs (Apr 5, 2020)

My guess is rings failed to seat or the lift pump has a bad diaphragm. Where in Central Fl are you located? I’d be happy to do a cylinder leak down test for you and maybe more diagnostics if needed, James


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## mpancari (Jul 21, 2014)

Hey guys, thanks for all the input! 
Got the boat out to water test after thermostat replacement and cleaning corrosion off the water temp sensor. Boat no longer runs rich. Pulled spark plugs afterwards and they checked out perfect. No more gas fouled plugs.


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## mpancari (Jul 21, 2014)

jonny said:


> Just had a thought. Could it be possible if you are fouling plugs. The unused fuel is being forced by the rings? Due to the increase in compression. And it could be a spark issue you are chasing. Did you change plugs recently?
> Did you use different oil weight? Are your plugs fouled with fuel or oil/both?
> Another option is a stuck thermostat. Not allowing it to get up to temp. For proper metal expansion and allowing blowby. How much fuel are you getting into the oil. Are you getting a raise in the oil level?


I believe you were right on the money with this. I purchased this outboard knowing it was not taken care of. Therefore my reasons behind the full tune up and over 200 hour service. Also my friend wanted to rebuild it so I didn't object. I did all the external engine work. Reason for me taking so long to check the thermostat out of all things.


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## mpancari (Jul 21, 2014)

jonny said:


> Jesus man you replaced the rings already? I was gonna say possibly blow by. But new rings would have fixed that. And you got a good look inside. Don’t have the schematics in front of me. Shouldn’t be a fuel solenoid being injected. You would think with the injectors tested. Leaking into the cylinder would be out of the equation. Did you pull the head gasket when replacing the rings? Or just do it from the bottom? A blown head gasket is about the only thing left. How is the compression? I would check it hot and cold. To see if there’s a huge change. Are all the plugs fouling?



Yeah ended up being the thermostat and coolant temp sensor not allowing motor to get to correct temperature causing flooding and blow-by. My buddy offered to rebuild it for me for free that's the whole reason behind having it rebuilt. I did the external parts replacement and trouble shooting. Should of known the check the thermostat sooner but you live and learn.


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## jonny (May 1, 2007)

Glad you got it figured out. Should be good to go for a long time hopefully👍


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