I have a coolant issue that's making my brain hurt. When riding the valve on top of radiator bubbles a little yet the fan comes on and off as I think it should so no problem there. When cool the rad needs about 50ml of water to bring it back over the fins !!. Have just run it with the cap off and it boils up quite quickly,and boils over. I can only think that coolant is not being circulated hence a boil at some point in the system not in the rad.On the 2015 gasser the water pump is a sealed unit I think so how would I check this. I'm thinking that i should take the pipes off and kick the bike over. Any other ideas chaps?

GG Water pump impellers have a habit of failing, might be worth checking (new ones £10 on ebay)

 The water will boil with the cap removed, that’s why it’s pressurised to stop it boiling

Its not a sealed pump, 3 cap screws hold it together 

Edited September 11, 20178 yr by suzuki250

I had a similar situation with a previous bike.  This may help you diagnose your bike.  After doing a water pump change and some other maintenance I went riding and after a while and noticed steam coming from the radiator overflow tube.  

Rechecked my work – all looked OK.  Topped up radiator and next time riding, same steam.

 

To learn how much was escaping, I zip-tied a small plastic bottle to the frame and ran the radiator overflow tube into the bottle.  Rode the bike and it spit 5 – 10 cc’s into the bottle once hot.  If I topped up the radiator, it would spit the same amount.  If I didn’t top up it would not spit any. 

 

Conclusion:  The cooling system needs a little air space in the radiator to allow for normal heat expansion of the coolant.  Bike never overheated but if I overfilled, the bike would spit the excess. 

Removed the bottle and relaxed, knowing there was no real problem. 

 

I hope this helps you.

On a car/truck/tractor/etc you have a small overflow bottle for that same reason.  Catches that overflow coolant and allows for a reserve.  Our machines are too limited on space for the bottle so we need to just check ours and pay a little more attention to the cooling system.