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Jason, Marine Mechanic
Category: Boat
Satisfied Customers: 15682
Experience:  Degree in Marine Technology. Gas and diesel marine mechanic.
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Have a 2006 8hp Tohatsu 4-stroke with an excessive amount of

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Have a 2006 8hp Tohatsu 4-stroke with an excessive amount of steam/white smoke coming from the exhaust relief. Oil is fine (it's not "making" any), just changed the thermostat. Runs great.
It makes this steam at idle after warm up, all the way through open throttle when running. Mind you, I don't run it at full throttle all the time: It is on a sailboat and has a 4blade thrust type pitch for slow running.

Hello, my name is XXXXX XXXXX I will be helping you today.

When was the last time the impeller and thermostat was replaced?

Customer: replied 3 years ago.

Just changed the thermostat before last outing (for the same reason). Impeller not changed because I have a really good flow out of the telltale. Another fellow with the 9.8 (same engine) had same problem and drilled a 3/32 hole in the thermostat and it relieved the problem.

I was seeing if this is a common problem with the Tohatsu/Nissan/Mercury 4-stroke 8/9.8hp.

It's pretty common for them to smoke when they need an impeller, tstat, or they are a little big clogged. The engine has different parts to it that cool differently. For example, the block cools differently than the cylinder head than the exhaust. And the little tell tale stream can be fine, but the exhaust is still running to hot. I've seen this alot on the little tohatsus and Mercury (its the same engine). All the steam means is that it needs the cooling system service, which means a new impeller, thermostat, and a block flush. This needs to be done every other year regardless of hours, as the impellers are just rubber and do dry rot and get stiff on their own just from sitting.

Does that make sense?

Customer: replied 3 years ago.

Block flush: use something like "salt-away"? It has been in salt water.... Any mechanical scraping out to do, like wire in relief port, etc. I assume the exhaust is down through the leg and either out the prop or bottom of leg.

No it's nothing like that. There is a water tube in the mid section that connects to the water pump assembly. When servicing all we do is remove the lower unit, remove the thermostat, and hold a garden hose against that water tube and just let it run. This will push any sand that might be in the block, out of it.

Customer: replied 3 years ago.

Got it. Good info. Any harm running without the thermostat? How about the other guy's fix with drilling a hole to allow more flow?


(I know I need to rate you to get done, just trying to get my money's worth more than "change the water pump and thermostat". I already have changed the thermostat.)

Yes, you never ever run any engine without a thermostat. You can overheat an engine internally that way. If water flows through a block to quickly, it doesn't sit around to pick the heat up. Because of that, what a thermostat is, is a restrictor. It slows down the velocity of water running throught the engine so the water can take the heat with it. This is a car thing. And I know that in some cars you can get away with removing a thermostat, but they have closed cooling systems were the water/coolant always recirculates. But on top of that, car engines don't run at high rpms. If you popped a thermostat out of a car, started it up, and then ran laps around a track at 4k plus rpms the car engine would overheat internally as well. So you never ever run any engine without a thermostat.

As far as this other guy, you would have to ask him. If it was supposed to have a hole in the thermostat I would think Tohatsu would put one there, don't you? Sounds like he was trying to mask the problem, not actually correct it.

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