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Why Outboards OVERHEAT!
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Aug 16, 2024
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View Video Transcript
0:00
Over the years, I've seen some pretty serious overheats, from smoked impellers to sand-packed heads and thermostats
0:08
but when it's happening to you, it's a different story, and you might not be as familiar with the systems as you would like to be
0:15
and know exactly why an engine might be overheating. So let's go through the reasons why, and start with how an outboard cooling system works
0:23
We will focus on outboards today, but the same principles will apply to your inboards and stern drive
0:30
engines as well. They just have two different options between being raw water
0:34
cooled and operating almost exactly as an outboard, just in a different style
0:38
because of the engine design, or being closed loop cooled by using a heat exchanger
0:44
which is basically the same as what the radiator in your car does. The closed
0:49
loop system saves you from running raw salt water through the engine and worrying
0:54
about salt buildup like you would down here in the islands where you won't find
0:58
many inboard boats. simply because of how damaging the salt water is to the engines and the drives
1:04
Now there aren't any outboards that have closed-loop cooling, but all of these cooling systems will have a heart
1:10
which is going to be this little thing here that is your water pump impeller
1:17
The inlets on the endboards can either be scoops on the bottom of the boat
1:21
that will go to a water pump that is usually driven by a belt or inlets on the drives themselves similar to your outboard
1:29
They will be located on top of the lower unit and are driven by the spinning of the drive shaft on the gear case
1:37
Water enters into the system by these pickups here on the lower unit
1:41
they can either be here on the sides or there are also some lower water pickups that help provide water when the engines are trimmed up and you're running
1:49
which is a technique that some of you know about because you've watched the video on how you can improve your fuel economy and save some gas money
1:57
I'll put a link to that video below and you can see that these pickups run the water straight up to the bottom of the water pump housing
2:05
With the water sitting under the housing, the impeller then moves the water
2:13
and pushes it up the water tube and up through the midsection of the outboard
2:18
Then this is where things will vary depending on brand and model of the engine
2:23
Some models will go to a screened area that then routes the water throughout the water
2:26
throughout the engine others flow the water straight up into the block and around the engine But all have the same basic functionality next They route the water around the cylinders throughout the powerhead and the cylinder heads
2:41
This water flowing around the cylinder cools the engine down by absorbing the heat into the water
2:47
which brings us to the thermostats, because each engine will have specific thermostat temperatures
2:53
that allow the thermostats to open up when the water gets to a specific degree
2:57
specific degree once they are opened up they allow the water to dump out
3:06
where it is routed to the exhaust system putting water into the exhaust
3:11
cools the exhaust and then is sent out of the engine through the propeller
3:15
this is the basic functionality of the cooling system in a boat engine but there
3:20
are many other things that can be involved in the cooling system depending on the
3:24
brand and the model a lot of engines incorporate coolers into the fuel systems to cool down the fuel as well as air
3:32
coolers, oil coolers, supercharger cools, even Evanrudes they water cool the engine
3:38
computer that they call an engine management module or EMM. There are many engines that run
3:44
cooling water through the rectifier regulator as well, which is just the component that
3:49
converts the AC electricity produced by the engine into DC electricity that is then
3:54
used by the boat and stored into the boat's batteries. There is one last component to talk about though before we go into the actual overheating of the engine
4:03
and that is the poppet valve, which can get kind of confusing, but I'll try my best to describe it in a way that will be understandable to everyone
4:12
So when we are just sitting in the boat idling, the water pressure being produced by the impeller spinning at only 600 to 800 RPM is only producing a few pounds of pressure
4:23
And this isn't moving a lot of water through the block very quickly, which is why the third
4:27
thermostats are used to open up at a certain temperature to allow this hot water
4:32
out of the block and let it flow easier regulating the temperature of the engine
4:36
now at high r perems when the impeller is producing much higher water pressure
4:41
there needs to be a way to get that large amount of water out of the engine
4:45
quicker and this is done by the use of that poppet valve that is located in
4:50
different areas on the engine depending on what brand and model you have most of
4:54
the valves are located just before the exhaust or close to it, whether that be in the midsection
5:00
the bottom of the engine block or even externally like on some Evanrudes The point of the poppet valve is to make sure the engine is full of water at all RPMs by allowing water to dump into the exhaust at high RPMs by the use of spring pressure
5:16
When a certain water pressure is hit, the valve opens and allows cooling water to be dumped
5:20
into the exhaust. But at low RPMs, it stays closed and helps to keep water in all of the cooling system during
5:28
these idle or low water pressure times by closing. When it comes to overheating, it's all about this water flow process
5:38
If the water flow is clogged anywhere in this system, then the engine could overheat
5:43
The most normal reasons for an overheat condition usually deals with the water pickups, the impeller, or the thermostats
5:54
Starting with the water pickups, you could get seaweed stuck in the pickups, run over a bag that wraps around the intake
6:01
or get into some shallow ground and pick up a bunch of grass or mud. If you do get an overheat, slow the boat down and quickly check to see if water is coming out of the tail tail
6:12
If you don't see any water, turn off the engine and check the pickups for any obstructions
6:17
After checking the pickups, if you don't find anything, and if you have the tools and you can't get the boat out of the water
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you can pull out the thermostats and see if water flows through the head and up into the thermostat housings
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If you don't see any water, then either the impeller is not pumping water or there is a clog in the system between there and the water pump
6:46
Now if the engine is ran for too long without water going to the impeller, the impeller will burn up, break blades off, or even melt the housing and clog it up
6:56
So addressing an overheat immediately when it happens is a bubble. big deal and you should have the habit to quickly slow the boat down to an idle and check the
7:04
tail-tail to see if the engine is peeing. Now there are other models that have certain issues when it comes to overheating like where corrosion can eat away at the exhaust and allow the engine to lose water pressure but these are model specific and only affect those that have these engines another cause for overheats can be related to the thermostats being clogged but you will notice
7:48
something like that whenever you pull the thermostat housings then there could
7:52
also be an issue with the poppet valve if the poppet valve is stuck open and the
7:56
water pressure from the impeller is already low then all the cooling water can be
8:00
escaping out the poppet valve and not allowing enough water to flow through
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the engine at low speeds, causing a low speed overheat issue. That could swing the other way
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as well, though. In cold climates or with cold water, if the water pressure is normal and the
8:15
poppet valve is stuck open, then the engine could be running cold, causing different issues
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like fouling out spark plugs quicker than normal because of the cold running temps. It's important
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to remember that all models are different and each situation is different as well, because
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of water temperature, water flow, RPMs, impeller, state. thermostats functioning, and other factors that affect the cooling system, there could be a number of different reasons to experience an overheating outboard
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If you have a shallow c and are always running through mud or sand, then the impeller could get eaten up or you could have sand buildup in the engine
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The thermostats or poppets could get packed up or even the water passages and the cylinder heads could get clogged up as well
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Understanding how your engine's cooling system functions and the different components that make it up is going to help you
9:04
you spend more time on the water by avoiding hours and hours of troubleshooting and trial
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and error tests. Since there are so many reasons for an engine to overheat, we'd like to hear
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if you've ever had to deal with one and what your problem was in the comment section below
9:19
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