If the valve or valve spring in the radiator cap becomes defective the pressure in the cooling system
will be reduced, causing boiling over.
If the radiator cap is suspect, have it tested by an authorised BMW dealer. This job requires
specialist equipment and cannot be done at home. The only alternative is to try a new cap since the cap has a screw thread
fixing which means that most normal testing equipment cannot be applied to it unless an adapter can be devised.
The BMW tool is number 17 0 500 and is connected using a suitable connector/adapter. A specified
pressure is applied, which must be held for at least 6 seconds; above this the cap valve will open. If it sticks or opens
too early the complete cap must be renewed. Note that the cap gaskets are available separately and must be renewed if
damaged or worn.
It should be noted that when tracing an elusive leak the entire cooling system can be pressurised to
its normal operating pressure by connecting the test equipment described above to the radiator filler orifice. Remove the
radiator cap, check the coolant level, topping it up if necessary, and apply a pressure of no more than 17 psi (1.15 bar)
by means of the hand-operated plunger. Any leaks should soon become apparent. Most leaks will, however, be readily apparent
due to the tell-tale traces of antifreeze left on the components in the immediate area of the leak.
6.1 Radiator filler cap can be tested only using special equipment