Non-OEM replcement parts.

Rover 75 1.8 Coolant boiled furiously if left to idle when warm and heater not getting warm. Engine is an open deck drop in wet cylinder liner type and has a known reputation cylinder head gasket failure.

Customer reported this particular engine is known to have one cylinder liner dropped low below spec flush with the deck blow rather than slightly proud. History of a new head gasket fitted by another shop in June with no coolant loss between then until it started boiling again.
Loosing coolant at alarming rate and any attempt to bleed the cooling system didn't produce any change.

So far sounds like a no-brainer head gasket failure BUT I tried to remove the coolant header tank cap when cold (same cap as BMW e46) and it was very stiff to unscrew -- strange and wouldn't lift off when fully unscrewed hmm. To pulled at it with pliers and levered with an screw driver as a result the top came off the cap leaving the business end in the header. The centre part eventually out after opening a heater hose to vent vacuum in the header tank.

Ask custome new non-OEM radiator cap had been fitted in April to minor coolant loss, it did until the head gasket was replaced.

Fitted a new genuine pressure cap and refilled with plain water, bled the air out, pressure tested and checked for coolant circulation, and fan operation and it has now been running for a week without coolant tank level dropping?. It will be back in next week for fresh coolant