I have a 2017 1.6 Tdi Octavia estate . My problem is this the low coolant warning light came on so I pulled over lifted the bonnet and sure enough the expansion tank was nearly empty but here’s the strange thing when I took the cap off the coolant came rushing back through. Any ideas

Hello, welcome to the forum.

Removing the cap releases the pressure in the cooling system - which reduces the boiling point of the coolant and can cause a hazardous, explosive release of steam as the coolant flashes-over. Unless you topped it up, what did the coolant level stabilise at when the system finally went cold?    

Edited 24 February, 20242 yr by Warrior193
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The adoption of the expansion vessels was in part mandated because of todays compensation culture that rewards those who do foolish things and punishes the innocent manufacturers who can write all the warnings in the manuals and on stickers in the world and still be liable.

The main function of an expansion vessel in any closed system, is (as it states on the tin) to allow for the expansion of the fluid as it heats up - fluids are normally incompressible, without an expansion vessel, fluid pressure would either increase to an unacceptable level through expansion as it heats up (causing failures at joint) - or be lost via any pressure relief valve - that's the reason there's a maximum fill level marked on the EV - to allow space for the heated fluid to expand.  

Edited 24 February, 20242 yr by Warrior193
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