My ZX12 runs good for a few kilometers but then the radiator pusshes the water back into the water bottle and then boils out the breather pipe.

Look for a stuck closed thermostat first. Then make sure there is enough water in the radiator. Next look at the exhaust pipe. Is it black or a shade of tan? Is there steam coming from it when the engine is running? Compression gases getting into the water can mean a bad head gasket or worse a crack in the head or block.

air lock or thermostat stuck try flushing with hose pipe

When sitting still, yes both fans on a car equipped with two fans will run. The temperature ranges you posted seem within acceptable range. That said, the water (coolant) should not boil at those high temperatures, but some overflow is expected/normal. Ensure that you are not adding pure water to the radiator and instead are premixing it with antifreeze coolant or adding premixed 50/50 that you can buy at auto stores as this will keep the water from boiling and freezing.

As an added point of information, the hose leading into the overflow jug is two-way, meaning that when the coolant in the radiator cools down enough, it creates a vacuum and pulls coolant back into the radiator.

Zach

Stuck shut thermostat, blocked radiator.

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Hi and welcome to FixYa!

You may have a faulty radiator blower motor. If the blower blows weak it will surely be not enough to make the engine cooled down. One good reason the water boils on the bottle. Another issue can be a clogged radiator and a faulty water pump. These can make the engine hot too when faulty. Please give enough attention on the possible causes to prevent severe damage to the truck.

Hope this helps and thank you for using FixYa! Have a good one!

1. When setting the temperature to MAX, blower level to FULL and outside air Vent CLOSED, no hot air comes out after 2-5 minutes?
If YES, when did you last flushed your radiator and is it full?
2. if more than 5 years ago, flush radiator system, instructions are usually in Owners manual. Use a bottle of Radiator cleaner, run egine hot for about 10-15 minutes, carefully open petcock in bottom of radiator to flush out radiator, WARNING, careful as flushed coolant would be boiling hot. As sson petcock is open and water start to flow out, carefully open radiator cap to enhance flushing flow. Use garden hose to fill radiator while flushing until water coming out from petcock is cold and clear.
Once clear, start engine, turn heter full and blower full blast and keep hose in radiator flowing full, leave petcock open until clear water flows out again. Once clear waters comes out, turn off engine,turn off garden hose and let water drain empty radiator. Once it stops flowing out, close petcock snug and put radiator cap on, pull off radiator coolant catch bottle, clean bottle for any debris and dirt. Drain any water, leave bottle empty and put it back onto its place.
3. Fill radiator with 1 gallon (4L) of concentrated coolant, top up radiator until full, fill catch bottle with concentrated coolant, start engine and keep it overflowing until no bubbles comes out and the radiator is fully hot. Once it is flowing good, close and connect everything. Drive with it and keep catch bottle filled as necessarry.
Good luck. Give as vote and good comment if this advise solve youor problem. Thank you.

FIRST CHECK FOR LEAKS ON BLOCK THEN HOSES OR RADIATOR. THEN IF NOTHING FOUND RUN VEHICLE TO OPERATING TENP WATCH FOR EXCESS WATER OR MOISTER OUT TAIL PIPE.IF NOTHING FOUND CHECK THERMOSTAT,CHECK FOR FLOW IN RADIATOR

if the water in your radiator or reservoir is boiling means your cars engine auxilliary cooling fan is not perfectly working enough to dissipate heat inside the radiator( heat exchanger) check cooling fan if its silicone oil filled if its rotating free and no resistance then remove fan and refill silicone oil/ if its equiped with electric fan then fan motor is busted

make sure there is no coolant leak. is the radiator fan running? it should run whenever the A/C is on. if not, that will be the problem. Engine running, A/C on, see if the fan is running, if not get a long rod, broom handle ect and tap on the motor, if it starts to spin or starts running, replace the fan assy.