If your BMW 2002 suffers from a continual loss of coolant with no explanation, try checking the heater control valve — a leak here is virtually unnoticeable and coolant quietly drains away without you finding out where it all went.
This valve, located near the firewall high up, will leak either due to the hose deteriorating or to the hose clamps being too tight and cutting into the hose. The O-rings within the valve can also distort and then no longer seal properly, also causing leakage.
Removal of this valve requires much patience and effort because of its confined location — many universal joints and extensions are needed for your trusty socket to be able to remove the two bolts holding the valve to a bracket on the heater top and the third bolt used to adjust the cable connected to the heater slide switch inside the car. Once you get the valve out, carefully remove the two screws and one bolt — they are usually very tight. The square O-rings can be removed, cleaned off and inverted, then reinstalled if you cannot find something to replace them. During re-installment make sure to correctly adjust the control cable from the heater slide switch — this will take some fiddling around but it is worth it. This entire operation will take a couple of hours but will be worth it in coolant saved and lack of future problems with the valve.
Author: Jack Christensen.