water/steam from overflow bottle

[big_tedman]

Just noticed that my overflow bottle started boiling and steam coming out of it. 2000 mk4 golf gti 1.8t
what could cause it, car has never overheated.
top radiator hose is hot going to check lower.
what else?

[leighaus]

air in the system possibly.

[Tim]

a faulty cap can cause cooling system issues too. Make sure the o-ring on it isnt partially dislodged. Ive seen that before

[jords]

+1 for the cap. Our Mk3 use to spew water out all the time. Took it to get the air con regassed, and they noticed it and said the cap was no longer holding the pressure in. Bought a new cap and never had an issue since.

[grtuned]

Water pump and thermostat!!!!! If they have never been changed. Never seen a faulty cap cause a motor to overheat. But who knows?? Vee dubs can do weird things sometimes!!!

[big_tedman]

will check the cap, shouldnt the waterpump leak if it breaks?
new to non jap cars lol
motor hasnt overheated yet, car was fine then all of a sudden steam n water came out. no sign of anything before.

[VW Convert]

Never seen a faulty cap cause a motor to overheat. But who knows?? Vee dubs can do weird things sometimes!!!

Faulty cap can definitely cause overheating (not that I'm saying that a faulty cap is the cause in this case). Cap is designed to keep the cooling system pressurised, increased pressure raises the boiling point of the coolant thereby increasing cooling capacity. No pressure = lower boiling point = potential for overheating.

Definitely worth getting system pressure tested.

Cheers

George

[big_tedman]

will get the cap checked tomorrow.
if the thermostat failed wouldnt it just hold water in the block as apposed to letting water to the overflow?
is it possible for air to get in if the water has never ran low?
things certainly seem different to jap cars lol finding it hard to find some particular info.
cheers for all your help guys