What Causes High Engine Temperature in Motorcycles?
2 Answers
Introduction to the causes of high engine temperature in motorcycles: Insufficient or leaking coolant: When the engine is cold, open the radiator filler cap and check if the coolant level is adequate (for CH125 models, the front fairing must be removed). Coolant should be replenished at idle speed by filling the radiator to the brim, while the reservoir should only be filled to about two-thirds of its total capacity. Check if the engine oil has emulsified or deteriorated. If the oil appears milky white, it indicates internal coolant leakage, necessitating engine disassembly to identify and rectify the cause. Typically, internal leaks occur at the junction between the cylinder head and cylinder block, which can be resolved by replacing the cylinder head gasket. The ratio for coolant mixture varies depending on the region and the concentration of the original fluid. The following table uses Honda Super Coolant as an example: Additionally, carefully inspect all hose connections for leaks, check for hose damage, and examine the water pump leakage hole for any signs of coolant seepage. Blocked circulation system: Inspect the circulation system for any blockages. The radiator should be cleaned with a radiator cleaner every 5,000 km, with special attention paid to whether the small circulation hose is twisted or obstructed. If the small circulation is blocked, the coolant temperature in the cylinder block and head water jackets will continuously rise after engine startup but cannot circulate. The thermostat will not open if the water temperature at its location does not increase. When the water temperature in the jackets exceeds the boiling point, the increased molecular motion gradually raises the temperature at the thermostat, causing it to open. The high-temperature, high-pressure coolant then rushes out through the filler cap, resulting in a "boiling over" phenomenon. Valve clearance too small: To ensure engine performance, valve clearances are set to specific requirements and are not necessarily better when smaller. For instance, the exhaust valve clearance for Honda CH125 is specified at 0.10 mm. However, due to dimensional discrepancies in domestic engine components or user intolerance to valve noise, many manufacturers (or users) adjust the valve clearance to very small values during production, leading to incomplete valve closure. This can prolong the post-combustion period of the air-fuel mixture, with much of the heat generated during this phase contributing to temperature rise rather than useful work, thus causing engine overheating. In reality, as long as the valve clearance is adjusted according to specifications, slight valve noise does not affect usability. Excessively lean air-fuel mixture: Typically, the carburetor's air-fuel mixture ratio is pre-adjusted by professionals using specialized equipment before leaving the factory, and riders need not adjust it. Only if overheating is confirmed to be caused by an excessively lean mixture should the carburetor adjustment screw be appropriately tuned. Faulty thermostat: The thermostat's role is to reduce coolant circulation when the engine is cold, allowing it to reach optimal operating temperature quickly. A genuine wax-type thermostat should begin to open at around 70 degrees Celsius and fully open at approximately 80 degrees Celsius. If the thermostat fails to open properly, it will inevitably lead to poor circulation and engine overheating.
I've encountered the issue of high motorcycle engine temperature several times and fixed it myself. There are quite a few possible causes. The most common one is insufficient coolant - either forgetting to top it up or not changing it for too long. Another reason could be a faulty water pump that prevents proper coolant circulation, leading to poor heat dissipation. During summer rides, if the cooling fan isn't working, the temperature will spike quickly. A stuck thermostat is also a frequent culprit as it disrupts normal coolant circulation. Riding steep slopes or prolonged low-speed operation can also cause overheating due to excessive engine load. Additionally, external dust clogging the radiator or internal rust buildup can significantly reduce cooling efficiency. Motorcycles have more compact cooling systems than cars, making them more prone to issues, so regular maintenance is crucial - change coolant at least every six months. Once during a long ride, my temperature gauge maxed out; inspection revealed a failing water pump that nearly caused engine damage. Since then, I've made it a habit to perform regular checks.
