also heater is not working properly
what are the reasons for the cooling system to boil or fluctuating cooling temp on 1998 BMW 540i
Thermostat problem, most likely.
1 more answer
Sounds like the thermostat is bad. Need to replace soon but make sure coolant is at proper level and not leaking. Other cause is heater core obstruction not allowing fluid to flow through properly. Hopefully, not the water pump.