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Old 23-03-2020, 08:13   #1
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Can I Repair This?

I have a tohstsu Outboard (8hp 4 stroke, bought new 3 years ago) that overheated even though the telltale was squirting water And continues to squirt water.

It’s seized up but you can still use the pull starter to turn the engine over. Just not very easily. It'll start but dies after it warms up. I didn't realize I was seizing the engine because it was still spraying cooling water and ran it like that several times. Oof.

I only figured it out when it became harder to pull the pull start. It's now nearly impossible.

Since I am on leave during coronavirus, I figure this is a reasonable project to look at. Is this Outboard salvageable? What are my steps? Do I need to just immediately replace the cylinder? And rings? Do I need to replace the piston? Do I need specialized tools like a micrometer to measure for any warpage? Or are there some tips and tricks to doing this?
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Old 23-03-2020, 08:14   #2
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Re: Can I Repair This?

What are some of the things I couldíve damaged by overheating it?
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Old 23-03-2020, 09:17   #3
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Re: Can I Repair This?

Interesting that the pistons are still moving freely. Could start by doing a compression test and see what you have going on. If low compression, squirt some oil in and test again. If more compression, then the rings are suspect, if no difference may have a warped head or bad valves.
The compression test is just a diagnostic and most likely you'll need to take the head off to see what's going on.
Do you know why it over heated? Do you have oil pressure?
This could be a fairly extensive repair if you have more than a warped head/bad top end, even then it could be expensive.
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Old 23-03-2020, 09:30   #4
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Re: Can I Repair This?

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Interesting that the pistons are still moving freely. Could start by doing a compression test and see what you have going on. If low compression, squirt some oil in and test again. If more compression, then the rings are suspect, if no difference may have a warped head or bad valves.
The compression test is just a diagnostic and most likely you'll need to take the head off to see what's going on.
Do you know why it over heated? Do you have oil pressure?
This could be a fairly extensive repair if you have more than a warped head/bad top end, even then it could be expensive.
A lot of things about this were interesting. LOL. A little too interesting for me. Ha ha

But yeah. I assumed I would have to take the head apart and replace things. Thank you for the thoughts on the diagnostic compression test. To at least narrow some things down.

I can only guess that it overheated actually. Because it started getting more difficult to pull the starter rope. And I can feel the piston moving in there. I can feel it drag as I pull the starter rope. It actually starts up and runs, I can get zooming across the water at full throttle, and then it slowly fades away and seizes. To the point you can’t pull the pull start anymore. The piston jams in there. Then later, when it cools down, I can pull it a little bit. And the more I pull it the looser it gets. I think it puts more oil on the cylinder wall or something when I do that.

I do have oil pressure. The little green light on the tohatsu that stays on for properly operating oil pressure is lit during the entire thing. Until the end when it cuts out.

And yeah. It’s an extensive repair for sure. I was planning to just buy a new one, but with more time than money kicking around right now, maybe it makes sense to fix this one.
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Old 23-03-2020, 09:30   #5
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Re: Can I Repair This?

Partial engine seizure is not all that uncommon, to be able to tell anything you’ll need to take it apart, and yes to really tell anything you will need an inside micrometer which most often amounts to something that looks like a T that is adjusted to fit the bore, then removed and measured with a regular micrometer.
Most often the pistons are toast and of course the rings too. I’ve never put old rings on new pistons, but I guess you could.

It’s very common on high performance two strokes, I had a racing jet ski many years ago, and every time it got to really running hard, it would melt a piston, that would sieze the piston or sometimes just seize the ring into the piston and lose compression in that cyl.

It happened often enough that I learned a trick, you could use muratic acid to eat out the melted aluminum smeared into the cylinder wall, it was steel so the acid had little to no effect.

But your going to have to disassemble to determine extent of the damage, unlikely but it could even be a spun crank bearing, very unlikely but possible.
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Old 23-03-2020, 09:36   #6
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Re: Can I Repair This?

Probably 30 years ago I was sitting in the waiting room at Bruno’s Porsche in Jax, I was talking to an older guy who also had a VW bug, ( I had my Brothers 911 in to have it lowered to European specs), he said he was running down the road and it kept slowing down, finally it just shut off and he pulled over, went to check the oil and it was too hot to touch, engine finally cooled down enough so that he could pull the stick out, no oil.
Took I think he said three quarts to bring it up to full, restarted it and drove home.

Well I told him he had seized the engine of course and advised him that he had best get it overhauled now, don’t drive it again as it was sure to seize again and create even more damage, the whole time he was looking at me funny.
Then he told me that was three years ago, it’s been running fine since

So you never know.
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Old 23-03-2020, 09:53   #7
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Re: Can I Repair This?

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A lot of things about this were interesting. LOL. A little too interesting for me. Ha ha

But yeah. I assumed I would have to take the head apart and replace things. Thank you for the thoughts on the diagnostic compression test. To at least narrow some things down.

I can only guess that it overheated actually. Because it started getting more difficult to pull the starter rope. And I can feel the piston moving in there. I can feel it drag as I pull the starter rope. It actually starts up and runs, I can get zooming across the water at full throttle, and then it slowly fades away and seizes. To the point you canít pull the pull start anymore. The piston jams in there. Then later, when it cools down, I can pull it a little bit. And the more I pull it the looser it gets. I think it puts more oil on the cylinder wall or something when I do that.

I do have oil pressure. The little green light on the tohatsu that stays on for properly operating oil pressure is lit during the entire thing. Until the end when it cuts out.

And yeah. Itís an extensive repair for sure. I was planning to just buy a new one, but with more time than money kicking around right now, maybe it makes sense to fix this one.

Based on this description, you probably scored the cylinder wall(s) and the top end is ok (so far). Typical repair would be to hone the cylinders then put oversized rings on the pistons. Off the top of my head, wouldn't even know if those were available or not for your motor. If not available kinda of a moot point to attempt a repair.
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Old 23-03-2020, 11:33   #8
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Re: Can I Repair This?

You certainly have nothing to lose (except time - which you now have in abundance) by disassembling the beast and finding out. Repair manuals are online with a bit of goggling.
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Old 23-03-2020, 11:47   #9
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Re: Can I Repair This?

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I do have oil pressure. The little green light on the tohatsu that stays on for properly operating oil pressure is lit during the entire thing. Until the end when it cuts out.

Sorry to say but that light is supposed to light ONLY for a few seconds when the engine is first started to confirm that it is working and then turn OFF and remain off while the engine is running. If that light is ON it indicates low oil pressure and the manual states never run the engine if the light is on or blinking.
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Old 23-03-2020, 11:51   #10
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Re: Can I Repair This?

Does Tohstsu offer any type of engine kit? I'd start there. The cylinders may be warped in different areas depending on the amount of heating. They may no longer be round. If they're not round they need to be bored and you'd need oversize pistons. If Tohstsu doesn't offer oversize, any place making racing pistons will usually make pistons for anything.
It might be you could buy a block assembly.
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Old 23-03-2020, 12:40   #11
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Re: Can I Repair This?

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Sorry to say but that light is supposed to light ONLY for a few seconds when the engine is first started to confirm that it is working and then turn OFF and remain off while the engine is running. If that light is ON it indicates low oil pressure and the manual states never run the engine if the light is on or blinking.
Uh oh. I donít remember what the case was. This was last year. Maybe it did turn off. It was operating as it normally does. Definitely definitely something to check into. When I return. The motor is 1000 miles away from here right now. I was trying to get it started back up.
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Old 24-03-2020, 08:58   #12
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Re: Can I Repair This?

I have a 15 hp 2 stroke yamaha,,, kept seizing when hot. Figured out that even when the P stream was coming out heavy, the head was plugged with salt in the water passages. So the indicator was not exactly indicating what I was thinking. Cleaned out the passages and never had the problem again.
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Old 24-03-2020, 09:09   #13
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Re: Can I Repair This?

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I have a 15 hp 2 stroke yamaha,,, kept seizing when hot. Figured out that even when the P stream was coming out heavy, the head was plugged with salt in the water passages. So the indicator was not exactly indicating what I was thinking. Cleaned out the passages and never had the problem again.
Seems like a good place to start. That was my disappointment. I was annoyed that the little stream was still working at full capacity, but I’m still overheating. I never leave the thing in the water. I always pull it up.

And you know, in posting this, makes me think that there might be one of the vanes from the impeller that broke off and stuck up there. A thorough check of the cooling system is probably the first to do.
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Old 24-03-2020, 09:11   #14
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Re: Can I Repair This?

Not to quibble but all the Tohatsu engines I've seen have a red light. So this may lead to less certainty wrt what you recall was happening. And as previously noted, it goes out when there is sufficient oil pressure. What was the oil level when you started to notice the problem? (Sorry if you already answered this)



Check the condition of the oil pump rotor when you start to disassemble. It might help at least in your understanding of where the problem began. But otherwise getting inside the power unit to see what the condition of the cylinders does sound like the next step.
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Old 24-03-2020, 09:13   #15
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Re: Can I Repair This?

if oil pressure was the issue, more likely the bearings are toast, maybe no damage to piston if the crank was splashing oil around.
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