06/10/2026
With the coming onset of the Super El Nino which will bring abnormally higher temperatures in the coming months to the South East Asian regions, I think its timely for owners of liquid cooled motorcycles to check on the condition of their bike's cooling system.
The higher the ambient temperatures...the higher will be the operating temperature of the engine. Heat is the greatest enemy of the engine oil. High temperatures will thin the engine oil making it lose its lubricity causing friction to occur between engine moving parts which in turn create even more heat. It is a vicious cycle.
Although the following write up is meant for my DRZ customers... it is also applicable to any liquid cooled engines. Generally, on most liquid cooled engines,, the cooling system will be made up of the following components.
1. Coolant
The purpose of the coolant is to absorb the heat from the engine cylinder walls. There are coolant passages around the cylinder block...which basically wraps the cylinder with a coolant jacket. The coolant is the medium for transporting the heat from the engine to the radiator.
All commercially available coolant like Engine Ice, will contain a mix of deionised water and ethylene glycol. Ethylene glycol is listed as a toxic chemical requiring care in handling and disposal. Ethylene glycol has desirable thermal properties including a higher than normal boiling point, lower than normal freezing point, stability over a wide range of temperatures and high heat and thermal conductivity. Most coolant today also contain additives to lubricate the water pump and the seals within the system.
Tap water should never be added to a radiator system unless as a last resort. Radiator coolant contains corrosion inhibitors... adding water which is not deionised... will promote not only corrosion within the radiator passages but also scale buildup. The other negative effect of using water in the radiator system is that water has lower boiling point than coolant. To effectively transfer heat to a liquid.. there must be full fluid contact.. when a liquid is boiling.. its air bubbles that contact the metal surface not the liquid.. the heat transfer is poor.
When we race at Sepang... FIM rules do not allow the use coolant in the radiators of the race bikes... reason is that its for the safety of the other riders as coolant is very slippery. So what we use on the Team Raptor MotorSports race bike radiators is deionised water. with Redline Waterwetter additive.
2. Radiator.
Size and area matters.
The function of the radiator other than as a reservoir... is to allow airflow to remove the heat from the coolant. Normally made of aluminium for ease of construction and weight saving. The greater the surface of the radiator exposed to the airflow, the greater is the cooling capability.
Sadly.. adding a fine wire mesh radiator screen guards in the name of 'protection' and aesthetic looks actually reduces the amount of airflow available to and reduces the efficiency of the radiator.
On the radiator, there will be a coolant inlet and outlet port and a provision for a radiator cap. At the receptacle where the radiator cap sits there is a little pipe that will convey the excess coolant to the reservoir overflow tank.
For a standard single piece radiator...Hot coolant from the engine will enter the radiator from the upper inlet port. The lower outlet port goes to the water pump to be circulated to the engine again.
For the DRZ and some other bikes with a two piece radiator set up... the hot coolant will enter the primary (RH side) radiator through the bottom and then flow over to the secondary (LH side) radiator through the top.
The coolant hose going to the water pump will always be coming from the lower (cool) part of the radiator.. simply because cold liquids sink. Also it makes more sense to send cool coolant to the water pump instead hot coolant just out of the engine or the heat will destroy the plastic impeller of the water pump.
3. Radiator Cap.
The radiator cap is the removable lid that seals the radiator system to allow it to build up pressure. When the temperature of liquid in a confined space is raised.. its pressure is raised.. .. the temperature at which the liquid boils is also raised. This is the same principle that is used by your mum or KFC when they use the pressure cookers to cook the food faster. Most radiator caps come with a 1.1 Bar rating on the cap. 1 Bar (Barometric Pressure) is equivalent to 14.7 PSI at sea level. There are some race radiator caps that come with a 1.6 Bar rating. And if you want put a higher bar rated radiator cap... you better make sure your radiator hoses can take it.
A radiator cap is made up of 3 parts. The seal, the spring loaded pressure relief valve and the vacumn relief valve. When you close a radiator cap.. the resistance you feel when pressing down on it to lock it is from the spring loaded pressure relief valve.. which is set at whatever Bar rating printed on the cap.
During engine operations... heat from the engine will expand the coolant and increase the internal pressure within the radiator system. Excess pressure (as dictated by the cap pressure relief spring) will unseat the cap seal allowing excess hot coolant to flow out and into the little pipe at the base of the radiator cap receptacle which diverts the excess coolant into the reservoir overflow tank. When the engine is cooling down.. the coolant in the radiator system will "contract" and create a vacumn in the radiator, the vacumn relief valve on the radiator cap will open and literally allow the radiator to 'suck'' back the coolant from the reservoir overflow tank and top up the radiator back to full condition.
4. Coolant Overflow Reservoir.
The coolant overflow reservoir is made of plastic, for lightweight and visibility of the liquids contained. A coolant overflow reservoir tank should NEVER be empty.. it should be half full all the time.. and its level should be checked when the engine is COLD. On some of these coolant overflow reservoirs.. there are high and low level marks. As a quick reference for a normally functioning cooling system, if the overflow reservoir has coolant in it... chances are the radiator will be completely filled. When the engine is hot... the coolant level at the reservoir should be higher than when the engine is cold.
During engine operations, hot excess coolant from the radiator will flow into this plastic container. Heat and plastic just don't get along and so over time the container will age harden... become whitish due to crazing and crack. These coolant reservoirs seem to have a life span of 4 to 5 years only.
5. Radiator Hoses.
The original hoses on the bike that come from factory will be braided rubber hoses. Radiator hoses are the transportation lines for the hot coolant to travel to the radiator for cooling and back to the engine. Over time and use... the rubber hoses will weaken due to the heat. The sidewalls of the hoses will weaken and will feel soft and spongy when pressed. An advanced stage of weakening will be the hoses look 'bloated'. As per the workshop manuals... the stock radiator hoses are recommended to be replaced every 3 years.
Aftermarket radiator hoses are made of high heat resistance rubber silicon reinforced with durable mesh fiber... Other than being colourful.. they are also of a much better quality then the stock OEM hoses and provide better flow. Having said that.. they should also be replaced every 3 years or when they start getting soft.
6. Thermostat
The thermostat is a temperature activated flow gate that controls the access of engine heated coolant to the radiator, A wax like material is in the thermostat and when heated by hot coolant.. will melt and expand and open the thermostat allowing the hot coolant to flow to the radiator for cooling. When the engine is cold and just started... the coolant is prevented from flowing to the radiator to expedite the engine warm up process.
Only when the engine temperature has hit about 95 C does the thermostat start to open an allow hot coolant to flow to the radiator for cooling. Engines are designed to work efficiently at certain temperatures.. the same applies to the engine oil that is recommended for the engine... its efficiency is also affected by engine temperature.
You do not see this kind of temperature on your meter display simply because the temperature indication system takes its temperature reading of the coolant from the area of the radiator that is going to the water pump and not at the outlet of the thermostat.
There are some who think removing the thermostat will help the engine run cooler.... WRONG!!!
There are 2 functions that the thermostat performs... the first is to restrict the coolant flow to the radiator initially to help the engine reach its operating temperature... the other is to regulate the flow of coolant to the radiator... it does not allow the coolant to flow freely to the radiator. By regulating the amount of coolant flow to the radiator.. the coolant has time to be cooled by the airflow because of its longer 'dwell' time in the radiator. If the coolant is rushed through the radiator... it will not have enough time (dwell time) to cool... and hot coolant will be pumped back into the engine.
When a bike has no thermostat or if the thermostat is stuck open... the engine temperature tends to go up slowly... on the other side.... it also takes it a long time to come down even when there is airflow to the radiator.
Removing the thermostat on race bikes makes sense because there are no traffic lights on the race track.
7. Water Pump
There are two sides to a water pump that is connected by a common shaft. The coolant side is a plastic vane impeller and the mechanical side is a gear driven by the engine. Dynamic seals are used on the common shaft to seal against leakage. On most water pump housings.. there is a hole at the bottom for leaks to flow out. If coolant comes leaks out.. the coolant side dynamic has failed, if oil leaks out.. the mechanical side has failed.
Since the mechanical side of the water pump is geared directly to the engine crankshaft.. the output of the water pump is then directly dependent on the engine rpm.
The plastic vane impeller is molded onto the shaft.. and it can fail, I have come across a case before where the impeller has lost its grip and would not spin with the shaft.
8. Radiator Fan
The radiator fan will only come on when the temperature of the coolant in the radiator has reached 100 C as detected by the fan temperature switch that is on the radiator.
9. Fan thermal switch
Located on the radiator... turns on the radiator fan when the coolant has hit 100 C.
10. Temperature warning thermal switch
Located on the radiator - turns on the engine overheat light on the dashboard when the coolant temperatures have reached 100 C and above.
How to keep the cooling system running efficiently?? Flush the system and replace the coolant every 2 years... change the hoses every 3 years.
Raptor MotorSports - Always Keeping It Real.
