Cooling

[jimh]

Did it spew the water on its own with the cap on, or as you took the cap off?
After shutdown, the water in the engine will go over 100 degrees and only the pressure in the system is stopping it boiling. Removing the cap will drop the pressure and allow the water in the engine to boil.

To flush the system, you're obviously going to have to drain the coolant.
I think if you remove the thermostat, you should be able to reach into the hole and check the water pump impeller from the back to check it hasn't come free on the shaft. Drop the thermostat in hot water on the stove.

Of course, it could well be that the rad is all blocked up with scale, it's just the thermostat & water pump are easier to check and get a definitive answer on - might as well verify when the coolant is drained.
How is the condition of the outside of the rad? Fins in place / reasonably straight / not filled with dirt etc?

[darreno'd']

Yes cap on Jim, water pump is new as is the thermostat, the rad is original as far as I can tell, but in externally good condition and not blocked or folded over on the fins. I think a good flush is in order first, to try and remove limescale. I had been considering going to waterless coolant, but no point if I have an underlying problem.

[Isimmo]

If it’s the original radiator Darren, with the system drained you might as well send it off for a re-core during the winter downtime. Ours was re-cored by Guildford Radiators about 10-years ago. Came back like new and not had any issues. Think they might have been bought out since though.

[BackintheFold]

Is the expansion tank cap new? If they leak they allow the pressure to drop and the coolant boils over.

[darreno'd']

Yep new cap too, I’d already considered that, no cracks in the tank either, I’m going to go down the radiator refurb or replacement route now, and also try for a good engine flush beforehand.