Hi people,

My bike is loosing coolant. 0,5L every 250km and it's boiling/leaking from the overflow of the reservoir. After some time, the red light of the engine overheating turns on, Luckily. Then Incan easily add 0,75-1L of coolant to the radiator and/or reservoir.

What could it be?

- already hard-wired the fan to the battery so always on.
- the waterpump is pumping
- tested the thermostat, within specs, opens and fully open at 85C.
- no leakings
- ambient temp variation of 0-30C doesnt seem to do much.
- clean radiator
- drained any air by warming engine and topping radiator up.
- radiator cap seems ok
- all hoses in correct condition.
- temp sensor (of no influence) checked and ok.
- the water in the reservoir is hand-warm, maybe 30C.
- when opening a hose a little with running engine: coolant is pumped out so there is flow.

The fact it's boiling and leaking from the reservoir, which not part of the pressurised part of the system, would strongly suggest the the cooling system isn't retaining pressure. With no system pressure, the coolant will boil at a much lower temperature and also a no pressure will allow the expansion tank to fill as soon as the motor fires up and the coolant starts expanding and eventually overflow and the coolant will evaporate - be lost. This lost coolant (in the expansion tank/reservoir) will never get sucked back into the cooling system if there is no system pressure because that pressure is required to produce a vacuum as it coolant drops back to ambient temperature when the motor is stopped. Effectively what's happening is every time your engine warms up coolant is pushed out by expansion, but each time the motor cools it doesn't return, the net result being a lower and lower coolant level (and premature boiling).

"Seems" - You can't tell whether it is retaining the correct pressure visually. Radiator cap issues are relatively common on the XT. As everything else is OK, I would suspect this first as a good radiator keep is crucial for maintaining the correct system pressure. Swap out for a known good one.

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Doean't that contradict with the fact that the coolant in the reservoir is cold? Or it it being squeezed out by the pressure of the overflow of the radiator?
Great hint. As I'm nowhere near a Yamaha dealer, does a radiator cap of a car also works? Or do I need an XT specific one?

No. As the engine warms up, the thermostat is shut, so the water in the radiator is going to be basically cold (there will be a little creep of temperature, but not much as the only access to the rad is from the bottom hose - there is no flow), hence the 30C or so you are feeling in the tank. As the coolant circulating around the barrel and head gets hotter it expands, this expansion is relieved through the bottom hose to the radiator forcing out the still cool water from the top of the radiator into the expansion tank in your case. Once the engine is up to temperature and the thermostat opens and the radiator becomes hot the coolant will essentially be at fully expanded (engine at normal operating temperature set by the thermostat and operating pressure set by the radiator cap). Now if the radiator cap fails the system will never reach operating pressure, the temperature will rise quicker than normal and the thermostat will open earlier allowing coolant into the radiator and straight out into the expansion tank for longer, and expansion will be greater, and it will do it all the time because operating pressure has not been reached and the cap is leaving the opening to the tank partially open.

You will need the correct cap, not only because it needs to release at the right pressure (16-20 psi), but the sealing surface needs to be in the right place to open and close the opening to the expansion tank. You can get them online fairly quickly from various places. Do you know anyone else who has an XT who can "lend" you their cap to try it out?

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Yes! A new cap on the radiator did it! All problems are gone. Didnt loose a drop in 1500km.
Found a similar car radiator cap here in Kyrgistan, removed the spring and replaced that in my XT cap.
Ready to make the 8000km back home. Thanks for the good advice! 😃