Boiling coolant?

Hi, the radiator coolant in my Monster 1200 starts boiling as the temperature reaches 98-100 degrees C. Radiator is full of coolant liquid (pink), the radiator cap gets very hot.
Fan comes on at 103 degrees as standard. The coolant keeps boiling once bike is turned off until the temperature hasn't gone down.
I read another post ( http://www.ukmonster.co.uk/monster/s...oiling+coolant ) that the issue was with a broken radiator.
The only difference before the appearing of the problem is the installation of Rizoma crash protectors which slightly push over the hose (left side of the bike) which feeds coolant to the engine part. However that's how the crash protector is meant to go and the hose seems to have enough bend around the protector.
The mechanic has no idea what the issue may be.
Would someone have further thoughts? Thanks

PS: there seems to be no leaks of air around the radiator cap or other areas of the hose

[Mr Gazza]

Water boils at 100C at normal atmospheric pressure. If you keep it under pressure, ie sealed in a coolant system. it will boil at a much higher temperature.
(incidentally, it boils at lower temps at lower pressure. hence why they cannot make a decent cup of tea, high up mountains.)

It sounds like you have a leak to atmosphere? Maybe a faulty rad cap?

[AndyC_772]

Agreed, it must be a leak somewhere, preventing the system from pressurising and allowing the coolant to boil. It's possible that when you've installed crash protectors, you've applied some force to the joint between hose and radiator which has opened up a small gap.

Try applying leak detector spray (soapy water may work at a pinch) to the area and see if you spot any bubbles. It may be something as simple as a hose clip which needs tighening.

Hi Andy and Mr Gazza, thanks for your responses. We checked the cap leaks but none. Will recheck everything this weekend and hopefully it's a clip requiring some tightening as you suggest. Will update. Thanks, G

How do you know that it is boiling?

[Mr Gazza]

The cap will not be seen to leak.
It is spring loaded and designed to release excess pressure via a tube, either to the road or more likely into an expansion tank.
If the spring gets damaged or weak it will not hold the pressure. Foreign matter could also get under the cap seat or the seat become distorted.

All cured with a new cap.
I'm sure that different pressure value caps were available years ago, from my Mini days!