Here is How Yamaha Outboard Cooling Systems Work
On the surface, outboard engines seem identical to those you typically find in any roadgoing vehicle — and that’s mostly true for the fundamental design and working principles. However, the cooling system, lubrication process, and electrics are widely different. Hence, even if you are an experienced mechanic, a Yamaha outboard manual and the illustrations it contains would really help here. Still, we’ll try our best to explain how they work using only words here.
The main difference between a motorcycle and an outboard engine is that the latter has an open cooling system, picking up raw water through inlets in the lower section. A water pump is then responsible for moving the water through the engine before it exits through either the propeller or a separate pilot hole. So, as you’re driving your boat, the water you are traveling on is flowing through your outboard motor, cooling it in the process.
Now, as you can imagine, there is more to this system; it’s also cooling the fuel and oil, among other things. Let’s see how it all works.
Yamaha Outboard Cooling System
Cooling your Yamaha outboard starts in the lower section where the water inlet is, facing forward. Once the raw water enters the system, the first thing it meets is the water pump, which ensures a constant flow regardless of whether you are moving or stationary.
Since the engine sits at the top, the water first passes through the midsection, a critical phase since this is the part where exhaust gasses go through Indeed, if the cooling system is faulty, it’s not uncommon for the midsection to crack because of the temperature difference between the exterior and interior. Moving further upward, we reach the oil pan, our last stop before the engine.
It’s important to note that the oil pan is only in this location on smaller 4-stroke engines. If you aren’t sure if it applies to your specific model, have a look at your Yamaha shop manual first. I would highly suggest having a look at eManualOnline here — you’ll thank me later.
After the oil pan, the water will pass through the engine block, cylinder head, and the thermostat, just like it would in any other roadgoing engine. Once through the thermostat, if it’s open, the water will exit through the propeller boss, cooling the main bearing or a pilot hole.
Waterflow for Yamaha four-stroke outboards
Water inlet> Water pump> Exhaust-guide/Mid-section> Oil pan (if it applies)> Engine block> Engine head> Thermostat> Exhaust-guide/Mid-section> Propeller boss/Pilot hole.
Yamaha Outboard Fuel, Oil, and Regulator/Rectifier Cooling System
When it comes to outboards, cooling only the engine won’t get you far, and that’s why outboard cooling systems get a little more complicated. Indeed, Yamaha outboards have three secondary cooling systems: fuel, oil, and regulator/rectifier. Of course, only some models have them all. As a result, understanding the system before starting any repairs is crucial. And since finding illustrations online is often impossible, your safest bet here is a Yamaha OEM service manual. Sure, it will cost some money, but that’s nothing compared to how much you can save using one.
Cooling the fuel might be peculiar, but there is a good reason why Yamaha employs it in the engine. For starters, cooler fuel is denser and more stable, leading to better combustion and higher performance. Considering the return lines can reach 130°F, the fuel can get really hot, resulting in a significant loss of power. Unlike cars, the fuel doesn’t have a long way to travel back to the tank giving it time to cool down, so the entire outboard fuel tank can get up to temperature quite fast.
We often see oil coolers on performance cars, motorcycles, and heavy-duty equipment. Oil is crucial in keeping the engine operating temperatures down, which might not be enough in high-stress applications. Since outboards will often run at high speeds, the oil cooler helps keep the engine healthy and always performing at its best.
A regulator/rectifier is present on all outboards and regulates the voltage in the electronic system. More precisely, it converts AC power to DC and regulates how much current flows to the battery; when the engine is idling or running at slow speeds, all the extra current converts into heat.
While smaller regulator/rectifiers use integrated cooling fins to get rid of the heat, bigger ones need additional water cooling. Lower temperatures will let the regulator/rectifier work more efficiently and last longer, so keeping them cool is essential.
How Yamaha Outboard Secondary Cooling Systems Work
The secondary cooling system has all the same steps until the mid-section, where two smaller hoses separate; one goes first through the fuel cooler and into the oil cooler, while the second one passes through the regulator/rectifier.
All the hoses will then meet back in the mid-section, where they will escape either through the pilot hole, the propeller boss, or the poppet valve. A poppet valve regulates the cooling system pressure when the thermostat is closed or in case of an issue.
Waterflow for Yamaha outboard secondary cooling system:
Water inlet> Water pump> Exhaust-guide/Mid-section> Fuel cooler/Oil cooler/ Regulator/rectifier cooler> Pilot hole/Propeller boss/Poppet valve.
Two-Stroke Yamaha Outboard Cooling System
Two-stroke outboards are much simpler and have fewer components than their four-stroke counterparts — and the same applies to the cooling systems, eliminating many of the four-stroke systems mentioned above.
For starters, the water intake is sometimes located on the same side as the propeller instead of facing forward. Water will pass through the water pump and the midsection; if the thermostat is closed, it will exit through the pilot hole, or the propeller boss, depending on the model. Smaller two-strokes that don’t have an exhaust guide will have an exhaust cooling chamber next to the cylinder instead.
In this case, water will go through the cooling chamber, a bypass valve around the cylinder, and out the pilot hole. Also, on small engines that don’t have a propeller exit, you can use the pilot hole to determine if there is sufficient water flow through the engine.
Waterflow for Yamaha two-stroke outboards:
Water inlet> Water pump> Exhaust guide/exhaust cooling chamber> Cylinder> Thermostat> Pilot hole/Propeller boss
Cooling systems in marine outboard engines can get quite complicated rather quickly. If you are just starting to work on them, there is no simple explanation to help you become an expert.
Hence, considering how many different systems and engines are out there, your best bet to make sure you understand how the specific system you are working on works is through factory illustrations, flowcharts, and exploded-view diagrams, all of which you can find inside a Yamaha service manual.
Sure, you can always try winging it and learn as you go, but investing in a manual is a much smarter option and will spare you from wasting time and money on simple mistakes.