The purpose of your motorcycle’s cooling system is shed excess heat caused by the combustion process and keep the engine operating at an optimal level for long life and efficient power production. All internal combustion engines need a cooling system of some kind, either air cooled or liquid cooled. Most modern bikes are now liquid cooled, and they will be the focus of the article.

Importance of your cooling system

If your engine is allowed to run too hot, engine parts start to expand from the heat, decreasing clearances, increasing friction and robbing your engine of power. Additionally, oil thins to the point of failing to project your engine adequately, potentially damaging or ruining your engine. Things can get so overheated in your engine that they actually weld together from the intense heat, “seizing” the engine.

On the other hand, if an engine runs at too low a temperature, it is inefficient, oil gets excessively contaminated by the byproducts of incomplete combustion (adding wear and subtracting horsepower), deposits form, and fuel mileage decreases.

So yeah, I’ve dropped a big bomb on you about the importance of your cooling system. A part of your motorcycle you’ve taken for granted, or possibly never even put a thought to before! It’s called tough love, and you will be better for it! 

Let’s take a look at the components of your cooling system, their roles in the overall system, and common maintenance issues:


Coolant is the fluid that absorbs heat from the engine and then dissipates it through the radiator. Coolant, which is commonly called antifreeze, is a mixture of ethylene or propylene glycol and water, and contains rust inhibitors to help prevent corrosion. Think of coolant as the “blood” of your engine’s cardiovascular system.

Because your coolant is operating in a hot, hostile environment, coolant will slowly break down over time. Additionally, your coolant's rust inhibitors get used up, leaving the small cooling passages in your engine and radiator vulnerable to corrosion. Even with these rust inhibitors, some corrosion will inevitably take place, contaminating the coolant with debris. We need to periodically flush they system to remove the debris, and replace with fresh coolant. 

Most manufactures agree that your engine’s coolant should be changed at least every 2 years.
It's basically a "drain & Fill" opertaion, but we do have some tips & tricks to pass on... and we'll be covering that full procedure in a future post.

Water Jacket & Radiator

Liquid cooled engines have passages for the coolant, throughout the cylinder block and cylinder head (item #2 in the diagram below). Through these passages, your coolant gets close contact with the hottest parts of your engine, the combustion chamber, cylinder walls, and valve components. Coolant flows through these passages, absorbing heat from the combustion process and then flows out to the radiator for cooling. The radiators job is to shed the heat absorbed by the coolant in the engine. As the motorcycle is in motion, air passes through the radiator absorbing the heat from the cooling fins. After getting "cool" again, the coolant cycles back through the engine. This cycle continues as long as the engine is running, with the coolant absorbing and removing the engine's heat, and the radiator cooling the coolant. 


Thermostat and Fan Switch

A thermostat (#3 in the diagram) is an electrical component which monitors the temperature of your coolant and re-directs the flow of coolant as needed. When the engine in below its optimal operating temperature (i.e. just warming up), the thermostat cuts the radiator out of the loop and keeps coolant flowing only inside your engine (providing minimal cooling). This allows your engine to get up to operating temperature quickly.

As your engine temperature rises, the thermostat re-routs the coolant through the radiator as needed for maximum cooling. In this way, the thermostat regulates the amount of cooling needed to keep the engine in the optimal temperature range.

The fan switch is a temperature sensitive switch mounted on your radiator. This switch is used to monitor the temperature of the coolant in the radiator, and switch on the radiator fan (#7) when additional cooling is needed.

You should be able to audibly hear your radiator fan kick ON an OFF as you are riding (much more in hot weather or stop and go traffic), but if you have any doubt, you can always do a test.

Fan Test

Set your bike on its kickstand in open area (your driveway will do) and start her up and let her idle. Keep an eye on your temperature bars as the engine heats up. You should hear your fan kick on two to three increments before it reaches full bar. If it reached\s full bar without the fan kicking on, you either have a problem with your fan switch or the fan itself. Switch off the motorcycle and let it cool back down and investigate!

I also recommend conducting this test after every major service, as it’s very easy for you to forget to re-connect the fan’s connector when buttoning back up after a valve adjust or other major service. Ask me how I know. ☺

Radiator Cap and Overflow Reservoir
(Items #5 & #6)

Your radiator cap is designed to hold a certain pressure in the coolant system. Most caps hold 8 - 12 PSI. This pressure raises the point in which the coolant will boil and helps maintain a stable system. Additionally, your radiator cap has a built in relief valve to handle the expansion and contraction of your coolant’s volume as it is heated up and cooled back down. Overflow coolant released by the radiator cap flows through a rubber hose to the coolant reservoir, As as the radiator cools down, coolant is sucked back into the radiator to maintain the correct level in your radiator. 

Your coolant overflow reservoir has an “min” and “max” mark, and should always have a coolant level at least to the “min” mark. Get into the habit of checking its level when you check your oil. It’s a quick visual check, and will help you keeps tabs on the health of your cooling system.


Water Pump and Hoses

Think of the water pump and associated rubber coolant hoses as the heart and arteries of your cooling system. 

The water pump is simply an impeller driven off the crankshaft of your motorcycle, making sure that when your engine is running, coolant is flowing. Coolant hoses take care of transporting coolant outside the engine, usually to and from the radiator.

Because of their simplicity, water pumps rarely entirely fail, although they are prone to leaking (both internally and externally). Both the water pump cover and associated hoses should be regularly inspected for leaks, weeping and tears. Engine coolant has a particularly “sweet” smell to it…and you’ll know it when you smell it… trust your nose! Even small leaks can cause big trouble overtime, so get them addressed asap.


If you’d like to get some real world experience on this procedure, Moto Republic offers private one-on-one instruction on this procedure or any maintenance task you want to learn more about!  Additionally, Moto Republic handles the waste disposal for you (oil, coolant, batteries & tires), making this every maintenance task that much more pleasant to tackle. We encourage you to come on in and get your service on!