Coolant Boiling

The last couple of days my ZX7 has been seriously overheating to the point where the coolant in the overflow bottle has been boiling (bottle is still cracked and leaking too, but don't think that would cause boiling).
What would be the cause of this?
Just happened suddenly. First time on the way home from work Friday, stuck in traffic and on a hot afternoon. Then again this morning, bugger all traffic and a lot cooler.
What should I be looking at as the cause of the problem?

The crack and leak is your problem.

When the system is pressurised, it raises the boiling point of the water (from 100deg to about 120). The liquid expands as it boils, and under pressure there is no room to expand. As you have a crack it allows the expansion at the normal temp (100 deg) and so the liquid boils. Fix the crack, fix the boiling problem, as most engines are designed to run around the 100+ mark.

Well, that sounds a lot better than the possible water pump failure I was expecting. Thanks mate

check your fans .fuse maybe?

Hey Punisher
I have to agree with Mike that is exactly what was happening to mine as the coolant expanded it sent the coolant into the overflow bottle which leaked it out and left me with little or no coolant in the engine.

Do yourself a favour and get a new coolant overflow bottle ASAP. It will save you a lot of grief, trust me I know from experience.

Also after I put the new bottle in with new coolant, after about a week the coolant turned brown, you will need to do a coolant flush because as you overheat the bike it will loosen all the scale and crap in your engine. After I did this the bike now runs happily in all kinds of weather.

As well check to make sure your fan is working.

Coolant bottle has been ordered, will be here tomorrow. Have to hope this one doesn't crack now as well.

Fan works too, thanks Paul

Have to hope this one doesn't crack now as well.

Do not use all the fixing points on the new bottle!

If you stress the plastic out of shape it is fine while new but as it ages the plastic goes brittle leading to cracking ......was a common problem back in blighty, but mainly due to the cold weather stiffening the plastic here in Aus it's the UV rays

Yeah I wondered about doing that, just leaving the bottom mount loose.
Just rode home today, filled radiator up with water before leaving work and it didn't get overly hot. So you guys are probably right on the money

Yeah I wondered about doing that, just leaving the bottom mount loose.

Double sided sticky pads

Thought about putting the bolt in, minus the washers.

i was gunna suggest a flush and new coolant while ur doing it, just to make sure its clean and fresh...but since ur already onto it, i wont!

lol, thanks mate. Your input is appreciated

all well with the thermostat?

also id check the radiator cap too to make sure it wasnt letting ater vent prematurely.

all bottles ive had just vent to atmosphere via the top just under the cap, the pressure is maintained/lost via the cap itself, none of this pressurised overflow malarkey afaik

Thought about putting the bolt in, minus the washers.

talk to Stace, the one and only member of Team Bolt, think thisa is what he did too but think there are nuts involved, Neka could confirm that for me.