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[Ewan]

Hi,
Suspecting a leaky gasket, I checked the flow chart for this and found the symptoms seem to tick all the boxes. The system stays pressurised when cold and on undoing the cap on the (empty) expansion tank coolant gurgles up into it. It smells of exhaust fumes inside. There is no mayo in the oil filler cap, oil and water are clean, and no external water leaks. What I don't understand is, presumably the water is getting sucked into one of the cylinders and exiting the tailpipe as vapour, so why does the system stay pressurised long after the engine has been switched off? Why does water not trickle down past the piston and into the sump? Can any of you miggie boffins please enlighten me?

[TheBoy]

Could be a straightforward coolant leak somewhere. HBV is often a favourite.

Compression test before condemming....

[Ewan]

I replaced the HBV a few months ago. Any external leaks must surely depressurise the system soon after switching off?

[TheBoy]

I replaced the HBV a few months ago. Any external leaks must surely depressurise the system soon after switching off?

Leaks may not necessarily cause depressurisation...  ...take a minor hose split for example, it may not 'open' until until significant pressure, resealling once pressure has dropped slightly.

[Ewan]

Aha, clever. Never thought of that.

[Abiton]

Hi,
Suspecting a leaky gasket, I checked the flow chart for this and found the symptoms seem to tick all the boxes. The system stays pressurised when cold and on undoing the cap on the (empty) expansion tank coolant gurgles up into it. It smells of exhaust fumes inside. There is no mayo in the oil filler cap, oil and water are clean, and no external water leaks. What I don't understand is, presumably the water is getting sucked into one of the cylinders and exiting the tailpipe as vapour, so why does the system stay pressurised long after the engine has been switched off? Why does water not trickle down past the piston and into the sump? Can any of you miggie boffins please enlighten me?

You have to take into account that the piston-ring to cylinder wall seal is designed to be virtually gas-tight, so if the rings are in good nick, coolant can happily sit for some time on top of the piston if/as it leaks past the fire-ring after shutdown.
That's how I confirmed our HG problem, by looking down the sparkplug holes and seeing coolant.   
Grab a little bright torch, whip out the plugs and have a look.  Could be very reassuring if there's nothing to see. 

[Ewan]

Things have moved on a bit since my first car then. A 1960 Hillman minx. It had a leaky head that went through a gallon of water every 40 miles. I swear it literally went like a train due to the water injection. The oil looked like thin salad cream. But it went well.
Thanks all.