A4 2.4 V6 Coolant boiling over

[sined]

Mate of mine owns this car and replaced the temp gauge sender about a month ago and all seemed fine last weekend while stuck in traffic he noticed steam from under the bonnet and found the header tank split.Replaced the tank yesterday but now cant get the coolant to stop boiling out of the tank after refilling.Is this just an air lock or something else? The temp gauge stays half way but the fan does not kick in at all.