Originally Posted by Pleiades

Firstly, there is no coolant flow in the expansion tank on the XT�s cooling system. It is not really a header tank at all (as it sits below the coolant level) and it is not within the pressurised part of the cooling system (and why it only has a rubber bung). It is literally there just to collect the overflow of coolant forced out when the radiator cap�s valve opens under pressure due to the volume of coolant expanding when the engine gets up to normal operating temperature. When the engine cools a vacuum is created in the pressurised part of the system sucking back the same amount of coolant that was expelled when the bike reached operating temperature. The level in the expansion tank will rise on a warm engine and fall back when cold. It should always return to the same level on a cold engine if there are no leaks in the system and the radiator cap is working properly.

This is why you should really check the coolant level in the radiator; don�t rely on the level in the expansion tank. Theoretically the expansion tank could appear full, but there may be a big air gap at the top of the radiator. Remember it�s not a header tank; what you put in the tank isn�t going to fill the radiator. Unscrew the rad cap and check the level is up to the neck. It�s quite tricky to do on a Z as the tank is very close but I find a small mirror helps to get a look inside the radiator filler neck.

Your problem could well be a radiator cap that has a weak spring, bad seal or is opening too early and not holding pressure. It should maintain a system pressure of 1.2bar (17psi). If it doesn�t, then the coolant temperature will rise much more quickly than normal and the coolant will boil at close to 100C, rather than nearer 115C when under pressure. This may explain why the temperature warning lamp is coming on early. There have been a lot of documented problems with weak or failed radiator caps on XTs, so I�d have a look here next (after having checked the coolant level in the rad.) Yamaha changed the cap design in 2010, so if you have an earlier one � be a little suspicious.

Also check the radiator fins are free from crud and that the fan is operating correctly � it should be cutting in before the temperature warning light does.

Of course all the above suggestions are based on the fact you have no obvious physical coolant leaks (as you didn�t mention any) and there is no coolant dripping out of the tell-tale on the base of the water pump housing?