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Outboard Motor Powerhead Cooling

In this video I look at the path the cooling water takes through an outboard motor power head.

Dangar Marine

About Dangar Marine

MarineEngine.com proudly sponsors the Dangar Marine YouTube Channel. Our friend Dangar Stu draws on his experience as a commercial boat skipper and mechanic to make some great how-to videos. Dangar Marine videos cover a range of relevant topics in outboard repair, boat repair/upgrades and seamanship. Subscribe on YouTube to be notified when new videos are released.

Outboard Motor Powerhead Cooling – Video Transcript

Hey there Dangar Stu here. Today's video is about the path that the cooling water takes through an outboard power head, and is proudly sponsored by MarineEngine.com.

I think most people are pretty familiar with the path the cooling water takes from the leg down those side grills up into the water pump, through the impeller then up the water pipe to the power head. But I think it gets a little bit more mysterious after that so that's what we're going to be focusing on today. What I've got here is the power head from a Yamaha two-stroke 2 cylinder, probably about a 15 horsepower or something like that, I can't quite remember where it's from, and we're just going to sort of take it apart and have a look at the actual path the water follows.

So once the cooling water comes up from the pump it enters this particular outboard block down here, so I'll show you this. The water comes up to this little opening here, and then it comes in under this exhaust cover, so we'll be taking that off as well. I also just remembered where this outboard came from. It was a Yamaha 20 HP that had thrown a bearing and put the rods through the side of the block. This is the exhaust cover here and this little barb here, is where the hose for the telltale comes from. So we've got this cover, nothing particularly special about it, and the outlet here. This outlet for the tell-tale isn't actually from this cavity here. This telltale outlet from this side comes through actually to this flange surface and picks up from below here as well. So, we'll actually work a little back. We'll go inside here and then I'll show you the part that takes before it comes back out that hole. These have always got gaskets and things on them obviously they're missing in this case because it's been pulled apart. But, we've got this cover, then underneath here we've got the exhaust ports from the outboard. So, these shapes here are just designed to deflect the gases. to give it a sort of a smooth curve to flow so you don't get that turbulence.

Quite a bit goes into designing this side of a two-stroke motor because in order to get those exhaust gasses out it needs quite a specific shape to the exhaust. Whereas with a four-stroke, it's simply that pistons sort of pushing the exhaust gasses out. Before we move on though, I will show you one other thing to do with this exhaust. Which is, there's two little openings here and here, and with this on, you can see that inside this other cover that opening, here, goes through this little gallery. When this cover's on, this port, here, is connected to this port here, through this little sort of convoluted gallery. And this port here, we take this head off, inside the cylinders here you've got your intake and your exhaust port. Your exhaust ports the one on this side, here, heading over to here. This one's actually missing a huge chunk, you can see the cylinder wall's broken. But then you can see this little port, there we go, a little round on there in front of a torch. So if we come back to these two ports, the main exhaust port and this little hole here, you'll see this little hole, if I shine the torch through it, comes out through that little hole. So this little gallery here is a path that connects this little exhaust port with the main exhaust port there.

And I'm sure somebody who's really into two strokes will be able to comment, and tell us exactly why that is because I've got no idea. Anyway, I'll stop telling you I don't know and tell you I do know, which is about the cooling water. So, when the water comes through here, one of the first places that it can go is come out through this hole here. So I'll show you where that then leads. So you can, hopefully, see that that's directly connected, light shines straight through there, then they can come up into the top here. Once cooling waters come through here, up here, and onto the top here, they can go down all these little holes we just saw. So one of them, is this one here. If I come round this way and show you, and then we shine torch down there, hopefully you can see, that this one lights up. So that's actually connected by nothing more complicated than being a single space here. So once water's gone down here, the pressure forces it out here, and that's the outlet for the tell-tale. So if you're telltale is running, what it's telling you is that water's coming up from the block, it's coming through here, it's coming down here, back out here, and out the tell-tale. That's not enough to cool the motor. So that's why having your telltale working tells you your pump's pumping water, but it doesn't tell you that your motor's actually being cooled by that water.

So I think that's really the first take-home message from this. It's that a telltale is simply an indication that your water pump's working, it's not indication that your motor is actually being cooled. So you can have a really good telltale and still have a motor overheating. The real job of cooling the outboard comes when the water goes down these other holes here, because they lead to the water jackets around the Pistons. So I'll show you, I'll sort of shine the light and show you how that works. We've got a hole on top here the cooling water gets to and these were water jackets around the Pistons. So if I shine the torch in that hole up there, you can see, light shining here down, here goes the other one shining down, inside the water jackets. I'll then take this little inspection camera and put it inside the water jackets of this bottom cylinder so you can see here. That's the cylinder, then there's the cylinder wall, and the water jacket here. Then if we go inside this water jacket, upper top, there's this little passage. We can go into there and we'll see that light shining.

You take that light away and that light is to here. So we know that's directly connected too. Right, so where does that get us? What it means is if we have all our cover plates on, like this, we've got a good working pump. Water's going to be coming in here, it's going to be coming on to the top here, it's going to be going directly into this bottom jacket, it's going to be going down these passages into the jackets as well. It's going to be coming down here and out the tell-tale. So we'll have water sitting around the water jackets, so there will be water there, and we'll have a flying telltale. What we'll do now is move around to the actual cylinder head, and we'll see what happens next. All our waters come in here from this side, it's free to flow and fill all these water jackets around the Pistons here, and then it kind of stops. And what happens, if I pop the cylinder head on? So once the water comes up through the bottom here, it can come up into here, it can flow through all these spaces, it can flow around here, can flow right up the sides.

When all this water eventually gets to the end of the road this is where it stops. There's a little aperture here that also just goes down in the water jackets so water can flow up through there as well, and then it comes up and goes into here. What's in here is your thermostat. So thermostat cover, gasket goes around that, and then inside here is a pretty main-key looking thermostat. But the point being the hot water comes up to this side of thermostat, this is the side with a little wax plug that expands with temperature. So this here is a thermostat in its closed position, and if we heated that up, this would open and allow the water to flow. But in this case it's currently blocked off. I'll go boil some water and I'll throw this in and show you what happens. It will be interesting to see if it still works. It's pretty monkey but it may still work. Just a glass of hot water, boiling water, we'll pop the thermostat in and we'll see what it does. I think we can pretty much call this thermostat dead. As you can see there, it hasn't opened at all. What I will do though is just open a new one and show you what they're supposed to do. So that's what they look like closed, and that's what they look like open. And eventually as this cools down that little plunger will recede back in. So it's a pretty simple test you can do to make sure your thermostats working.

What it means for the flow of the water, if your thermostat is stuck closed like this, is that water will come up, it'll come out the tell-tale, all these water jackets will fill, presuming they're not blocked with anything, then the water will get to the top here where the thermostat is and it'll stop. It'll keep flowing out the tell-tale because it needs to come out somewhere and you'll have this sort of pressurized water sitting inside the jackets and that water will just get hotter and hotter and hotter. Eventually it'll turn to steam and you might have a bit of steam coming out the telltale but the engine will gradually overheat even though it is actually surrounded by water. The purpose of the thermostat is to get a motor up to temperature quickly, so it lets that water sit there until it's hot enough, and then when it gets to the right temperature, it opens and allows the water to flow. If the engine actually starts to get too cool it'll just close again. It just regulates the temperature so it gets it to a temperature quick, and then it keeps it at that correct temperature, the whole time.

Thermostats will often have the temperature printed on them, the temperature they're designed to open at. So if we look at this one, hopefully you can see there, 50-degrees Celsius, is when this one opens I'll bring you in close again and just sort of show you the final piece of the puzzle now. So, cooling water was coming up here to the thermostat, the thermostat opens, this little cover just provides a path for the water to loop back over, so the work comes out the thermostat, and then back down here. That passage simply comes into here it runs down the other side. When the water gets to the bottom here, it goes down this hole, and if I shine a light through here, we can see that light coming out in two places, inside the casing, and then down here, where that goes inside this exhaust manifold. And this is where the exhaust comes out here. This is the exhaust for this outboard, and if we imagine this bolted on here, this outlet here, comes into here, which actually just goes straight to here, and then it flows down the outside.

So it looks like it's designed to run down the outside of the exhaust to keep it cool. It doesn't actually go through the center. So, I hope this video helps you understand the path the water takes, and I hope that knowledge of the path the water takes will help you diagnose overheating problems if you're having them. A good tell-tale just means your pump's working. And a working pump means nothing, unless you've got a working thermostat. Taking the thermostat out means the motor is going to be running cool all the time. It's better than the motor overheating. You know this failure may have actually been due to that thermostat being stuck, so if the thermostat wasn't there, chances are this motor would still be working. But if you take the thermostat out, it's going to be running cool, your efficiency is not going to be good, your fuel efficiency. You might find your plugs foul up and your emissions won't be as good. It's not ideal to have no thermostat in a boat. I wish I'd filmed it, but I actually had a boat in the shop at the moment that it was overheating.

It had a good telltale and the thermostat was working, and if I took the thermostat out, ran the motor, lots of water was coming out the thermostat hole. So in that situation I'm normally pretty confident that cooling is working properly. If you can take the thermostat cover off and water's coming out the thermostat, and you know it's good thermostat, it's actually brand new, something else is going on. So I took the head off that motor and this is what I found. There was a big chunk here missing from the cylinder wall. Now oddly this actually had perfectly even compression on both cylinders, so it goes to show, compression tests are a little bit hit and miss. But there was obviously a problem here.

Now another big clue to it was the nature of the telltale. The telltale was actually quite forceful and was a little bit sporadic in its kind of pulses. And what that is, is exhaust gases coming past this cylinder wall through a blown head gasket and displacing all that water. The water was being pumped in by the pump but then the exhaust gasses were much stronger, and they were just pushing exhaust gasses into the water jackets, flushing that water out with such force it was actually flowing all the way backwards and pushing these gases out the tell-tale. So that's a really good sign of a blown head gasket. So although the cooling system on this motor was working perfectly, good pump, good thermostat, no blockages, it was actually the blown head gasket that was causing this motor to overheat.

Well I hope you enjoyed this video. I sort of hope it solved the mystery of where all this cooling water goes if you weren't familiar with it, and I hope that information does help you if you're having a problem with your outboard. Because although you don't always need to understand it, it certainly can help with your thinking when you're trying to figure out what's going on with your outboard. Alright, we'll take care and I'll see you next week.