Without a doubt outboard motors are the most ingenious means of propulsion for boats; they’re a fully self contained power plant, they come in a range of different sizes, and they’re convenient to maintain and transport.
However in some ways their supreme utility can be a curse as much as it is a blessing. People are often prone to thrashing outboards around while forgetting that they aren’t indestructible, which inevitably leads to issues.
One of the most common ways an outboard can fail is when it overheats during use. There are a number of different reasons for why this happens; from a blockage or damaged component within the cooling system, to low engine oil, or a faulty thermostat.
Whilst most manufacturers recommend that outboards are serviced every 100 hours or every year, you can’t always second guess when a part will fail, so it’s important to be aware of what can go wrong, and how to remedy the issue.
An overheated engine shouldn’t be ignored because the damage (eg. cracked cylinder head, or warped cylinders) and subsequent cost to repair will be far greater than repairing the issues that cause the overheating in the first place.
Water Pump Failure
Taking full advantage of being partially submerged, outboard motors are most commonly water cooled, using an ‘open system’ design that uses the water the boat is travelling on to cool the engine; hence the familiar ‘pee stream’ that is continually pumped out whilst the engine is running.
If the pump that draws in sea or fresh water fails you’re immediately left without cooling water. Indeed it might be the lack of water in the first place that causes the pump to fail as it relies on water to lubricate it, and therefore keep it running without fault.
You’ll know if the water pump is faulty if the outlet cooling water (the pee stream) is hotter than usual. This could indicate that the pump isn’t pulling water through the system and across the heat exchanger/exhaust manifold, and thus not dissipating the heat fast enough.
Impeller Failure
If a water pump is allowed to run dry for even a short amount of time it can prove catastrophic to one very important part of the pump assembly; the impeller.
The impeller is basically a small rotating propeller or fan like component that draws cooling water into the engine. Impellers are almost exclusively made of rubber, both because it’s a cost effective, and resilient against the impact of sand and other foreign material that might accidentally be sucked in along with the cooling water.
Unfortunately the downside of a rubber impeller is that if it runs ‘dry’ (ie. not submerged in water) it will not have any cooling or lubrication, and will quickly overheat, disintegrate, or even melt.
No impeller means no cooling, which means the engine will overheat. The good news is that replacing an impeller isn’t a particularly difficult job as long as you’ve got a decent set of spanners and/or sockets.
Removing the lower unit (ie the lower section of the motor with the propeller attached exposes the water pump housing. This can then be unbolted and the impeller can be replaced.
It’s good practice to replace the associated plate and gasket, which normally all come as part of an impeller replacement kit.
Being so notoriously prone to failing as it is, it’s worth checking the state of the impeller on your outboard at least at the start of each season, as you’ll almost certainly notice wear and tear after a season’s use, at which point you’ll need to make a judgement call over whether or not it should be replaced.
Exhaust Manifold Blockage
A fault with engine exhaust manifold can be catastrophic, especially if the pipework corrodes and exhaust cooling water gets in to the manifold. This can find its way back to the cylinders potentially resulting in a total engine failure when the water (which can’t be compressed) causes the pistons and cylinders to warp.
From an overheating perspective, an exhaust manifold doesn’t necessarily need to be corroded or faulty to cause problems; a blockage in the cooling water supply can cause overheating issues.
Flushing through the cooling system by connecting a hose to the water inlet on the bottom of the motor unit is usually enough to flush out any debris, but it’s important to try and avoid taking on said debris in the first place by avoiding using your boat in shallow water, or other places where silt and sand could accidentally be sucked in by the cooling water intake.
Low Engine Oil
Oil is the lifeblood of any engine, it keeps all the moving parts lubricated, reduces wear and tear, and extends the life of all the critical components.
Without the correct amount of oil not only will the engine wear out faster, but the increased friction will heat up the cylinders and pistons, leading to all kinds of potential damage.
Bottom line: Check the engine oil regularly, top up as required, and fully replace the oil after as many hours of use as the manufacturer stipulates you should.
Thermostat Failure
An engine thermostat is really the ‘brain’ that regulates the temperature that the engine runs at. When the engine is cold the thermostat valve stays shut, allowing the engine to warm up, as the engine heats up, the thermostat valve opens and allows coolant to flow freely.
All engines are factory fitted with a thermostat that is designed to function within the temperature range the engine has been designed for, and yet for various ill judged reasons (usually because the thermostat isn’t operating correctly, causing the engine to overheat) many people have developed a bad habit of removing the thermostat altogether.
I would strongly advise against doing this, for the simple reason that the engine will then have the potential to operate outside of its safe working temperature range, which could result in overheating and engine damage.
If you suspect the thermostat is faulty, it’s much better to replace it (like for like) with a thermostat that matches its specifications precisely. If you swap it for one that operates within a different range than the original then the same risks around not having one at all exist.
Faulty or Incorrect Spark Plugs
In gasoline (petrol) engines, faulty spark plugs can cause a whole range of issues from poor fuel economy to sluggish performance. They can also cause engine misfire, which is essentially when one or more cylinder fails to fire as it should due to a faulty spark plug.
There are many symptoms to look out for with a misfiring outboard, from unusual popping and sputtering sounds, to a strong smell of gas/petrol, to excessive vibration as a result of the uneven movement of components in the engine. This is particular can cause the engine to get noticeably hot.
Most boats don’t see the mileage that cars do, so as such there’s an argument not to change them with the same frequency as you might on a car, during an annual service for example. However spark plugs are pretty cheap in the grand scheme of things, so replacing them every year is worth it in my opinion, especially if it keeps more serious problems at bay.
Air Cooled Outboard Overheating
If your outboard is air cooled then you would be forgiven for thinking that cooling water related problems wouldn’t be an issue you would need to deal with.
In many cases this would be true, but bear in mind that just because the engine itself isn’t water cooled, doesn’t mean that other components such as the exhaust manifold or clutch are not water cooled. This means that many of the same issues around blockages/scale deposits can be apparent on such systems as they are on fully water cooled systems.
So whilst test running an air cooled engine out of the water might be ok for a short time, I’d advise checking the engine specs/manufacturers instructions before you make too many assumptions!
Cleaning a Cooling System With Vinegar (Salt and scale deposits)
A outboard’s cooling system is prone to developing a build up of salt and limescale deposits. This varies depending on where you use your boat, but it’s always going to be a problem to some extent. Once again this can cause the engine to heat up as the deposits in the cooling system could be restricting the flow of cooling water.
Blockages can occur anywhere in the cooling system, from the thermostat to the exhaust manifold, and you might be wondering how to unclog these components without having to strip down the engine entirely.
One of the easiest ways is to clean out limescale from a cooling system is to pump a vinegar/water solution through the engine to react with and break down said limescale.
To do this fill a plastic bin or barrel with roughly 2 parts water and 1 part white vinegar, position the engine in the the container so that the water intake is submerged, and the exit stream returns back into the container (it would be wise to remove the prop too!) and run the engine for about 20 minutes at a time (the water will get too hot otherwise). Allow the water to cool and repeat the process over the course of a few hours, and eventually you should find much, if not all, the scale deposits have cleaned off any visible components, and more importantly the pressure and flow of cooling water should be as they should be.
Overheating at a high RPM
Sometimes you’ll find that your outboard performs exactly as expected at low speeds, only for it to overheat when you open the throttle right up.
There are a number of possible reasons for this, in truth it can be any of the problems discussed above, but in my experience I’ve found that a worn water pump or impeller might shift enough water through the system at low rpm, but struggle to keep up with the demands of the engine when it turns over faster. So this would be the first thing to check, and go from there.
