DIY! How to Understand Engine Failures and its Maintenance!

How to

The thermostat, which is built into the cooling system of a liquid-cooled engine, is an important component that opens and closes according to the water temperature to maintain an optimal cooling environment. The pattern of thermostat failure is that the valve may not open or close, and if it does not open, it will cause overheating and if it does not close, it will cause overcooling.

Signs of trouble are hard to understand, but knowing how they work can help you predict the reasons for sudden changes.

Thermostat that is the key to cooling the liquid-cooled engine

The KAWASAKI GPZ900R's thermostat is located on the underside of the main frame and the top cover is held in place by 3 bolts. After removing the hose leading to the cover, remove the bolts and open the cover to reveal the top of the thermostat.

The basic cooling principle of a water-cooled engine is that the heat generated by the cylinder head and cylinders during engine operation is conduction to the cooling water, which is then dissipated by the radiator, and the temperature is more stable than air-cooled engines, which rely only on the wind hitting the engine when traveling.

One of the indispensable parts of the circulation system is the thermostat, which opens and closes according to the temperature of the cooling water and adjusts the water temperature. The engine is in a cooled state immediately after the cold start, and raising the temperature in the cylinders and around the combustion chamber as soon as possible will lead to a stable combustion state.

At this time, the thermostat valve is closed and the cooling water inside the engine does not flow to the radiator, so the atomization of the mixture due to the rise in water temperature is promoted and stability is improved (however, the valve has a small hole and some of the cooling water pumped from the water pump is circulating). When the heat generated from the engine eventually rises and the cooling water around the engine reaches a certain temperature, the thermostat valve opens and flows to the radiator to promote heat dissipation and prevent overheating.

Thus, the thermostat is an important part of cooling control for a water-cooled engine.


  1. Liquid-cooled engine not only for cooling but also for warming up
  2. The thermostat is the key component for water temperature control.

The effects of a thermostat failure vary.

The valve opening temperature is imprinted on the main body of the thermostat, and in the case of this thermostat, it becomes fully open at 82℃, and the cooling water inside the engine flows to the radiator. It can be cooled by a running breeze or radiator fan, or it will automatically close if the outside temperature is low and the water temperature drops to prevent overcooling.

A typical thermostat opens and closes automatically as the wax sealed inside expands and contracts in response to changes in water temperature, and the opening and closing temperatures are often indicated on the thermostat itself. So what are the problems that occur when the display does not open and close at the indicated temperature?

The most common trouble with thermostats is overheating due to the valve not opening. It is correct that it is closed when it is cold for the purpose of promoting warm air, but if the thermostat is not opened even when the temperature is set, the coolant does not circulate to the radiator, so only the water temperature in the combustion chamber and around the cylinder rises, leading to overheating.

For motorcycles equipped with a water thermometer, if the radiator core does not heat up even though the needle on the water thermometer points to a high temperature, the suspicion arises that the thermostat is not open. On the contrary, the cooling water will be circulated to the radiator even when cold if the opened valve remains stuck, and it will be over cool where the engine does not warm up easily even though you want it to warm up quickly.

If anything, overcooling is often perceived as less serious than overheating, but overall performance is achieved when the temperature of each part of the engine rises properly during operation.

For example, engine oil, which rises in proportion to the temperature of the engine, is generally designed to perform at temperatures in 60°C or higher. Therefore, if the water temperature is too low and the oil temperature remains too low, the viscosity may be too high and the shift touch and lubrication performance may deteriorate. In addition, if the cylinder is kept at a low temperature, the pistons will not expand evenly, and the pistons can dance and load the piston rings while the clearance with the cylinder is wide.

In other words, overheating with the thermostat closed is a problem, but conversely, overcooling with the thermostat open is also a problem. The negative effects of overcooling are especially pronounced in winter when the outside temperature is low, so it is important to check if the water temperature gauge needle continues to point to a low temperature or the low temperature indicator does not turn off.


  1. The valve is opened and closed by the expansion and contraction of the encapsulated wax.
  2. Overheating with open valves and overcooling with closed valves are both problems

Measuring the actual opening temperature by simmering in a pot

When restoring the thermostat cover, make sure that the O-ring or packing on the mating surface with the housing does not engage. Especially when the cover is displaced from an angle such as GPZ900R, be careful not to shift the O-ring.

If air accumulates in the cooling path, it may expand as the engine temperature rises, obstructing the flow of cooling water, so connect an appropriate hose to the air release bolt on the thermostat cover and start the engine to expel the air. When air bubbles are no longer mixed in the hose, the air release is complete.

The thermostat is often set in the middle of the hose connected from the engine body to the top of the radiator, and the specific mounting position varies depending on the model, but in the case of KAWASAKI GPZ900R, the main frame is located above the cylinder head. It is right below. When you open the case that divides the top and bottom and take out the thermostat, the temperature at which the valve opens is indicated on the body.

If you have overcooling, you can see it because the valve remains open even at room temperature. In case of overheating, to know if it really does open at the indicated opening temperature, heat a thermostat and thermometer in a pot of water to see if there is a link between the water temperature rise and the movement of the valve. If the valve does not open when the set temperature is reached, its thermostat is abnormal, so replace it with a new one.

High pressure is applied to the cooling system as the water temperature rises, so when you open the thermostat case, clean the mating surfaces of any foreign objects and restore them with a new packing or gasket (depending on the model).

Compared to cooling water and radiator caps, thermostats that are in places where there is little trouble are often neglected, but it is necessary to understand that they are the coordinators of the cooling system in general, and that they should be settled when a problem occurs and respond to it.


  1. The mounting position of the thermostat varies depending on the model.
  2. Check valve opening by heating in the pot.

See KAWASAKI Moto Index Page
See Accessories for KAWASAKI GPZ900R Ninja

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