16-07-13, 10:13 PM
As others have said it is an expansion tank. When the engine heats up the coolant expands and has to go somewhere, hence the expansion tank. When things cool down some coolant is sucked back into the engine. The coolant is topped up at the cap on the thermostat housing, which is under the fuel tank, but the expansion tank should always have some coolant in it.
Also, when changing the coolant there will often be a bit of air in the system and in use this will find its way to the top of the thermostat housing. On heating up this air gets pushed into the expansion tank and is replaced by coolant when the engine cools down. After that the level should stay the same other than being higher when hot.
Unlike the engine and radiator system the expansion tank does not become pressurized when hot, hence only has a flimsy plastic cap.
Also, when changing the coolant there will often be a bit of air in the system and in use this will find its way to the top of the thermostat housing. On heating up this air gets pushed into the expansion tank and is replaced by coolant when the engine cools down. After that the level should stay the same other than being higher when hot.
Unlike the engine and radiator system the expansion tank does not become pressurized when hot, hence only has a flimsy plastic cap.