Problem with mini engine please help

Jasonbbo

hi all , iv a problem with a mini which I can’t seem to figure out I’ll give some details of the car
Mini one
2011
1.6

Ok there’s pressure building in the coolant system and bubbling so my first guess was the head gasket so tested for  exhaust gas to my surprise there was none picked up (using that color changing liquid) so my next guess was the thermostat stuck closed and overheating making the coolant bubble, so changed that  didn’t fix it so this is were it gets tricky only happens when engine is revs on Tick over its fine , now   If you rev the engine and watch the coolant rise in the expansion as soon as you put the fan on hot or cold the coolant will return the original level and no more gas bubbles or pressure so can’t figure out what the fan has to do with pressure in the system please help

k17chy

Could be the water pump.

un1eash

I'd try a new expansion cap. Sounds similar to what happened to my Lotus. Water at pressure has a higher boiling point, if the cap seal has failed the system boils over.

VFR

Yes agreed fit a new cap first before you go any further.