E34 Overheating - Coolant loss, but expansion bottle at max!

[John S Busby]

2 months back noticed a faint smell of coolant when driving my 1992 95K miler M50 engined 525i. Level in the expansion bottle remained OK and no obvious leaks. Recently during a fast drive coolant temperature warning message came on and temperature guage needle headed for the red. Backed off speed, turned on heater and temperature guage dropped but made occasional and brief excursions into the red. Got home and let the engine cool overnight. In the morning started to top up the expansion bottle and bleed the system; bottle filled to top but no coolant or air from the bleed valve. Left expansion bottle cap and bleed valve off and detached radiator bottom hose. About 1 litre of water came out but level in expansion bottle stayed as was! What's going on? Clearly I've been losing coolant and it's not been replaced from the expansion tank; very nasty.

[Tiger]

Plastic water pump impeller separation... or bad thermostat