Volvo: 1983 Volvo 240, head gasket, water leaks

Question
My 1983 Volvo 240 is running fine. However I have to top up as much as 20 FL OZ of coolant (I use water only) every 2 or 3 weeks. The mechanic could not find the leak.  
What should I do?
Thanks.

Answer
Few things to think about.  First, do you see a puddle of fluid underneath the car after having it parked for any amount of time?  If so, see if you can account for most of the fluid you have to add every month.  If there is no puddle, then the fluid must be going somewhere else.  A lot of times this means either a gasket leak, where the fluid is mixing in with the oil and burning off, or the engine runs unusually hot and boils off some of its fluid.  Because you say the car runs fine, and you didn't mention steam escaping after being driven, I would start leaning toward water getting into the oil system through the head gasket.  You can check a number of ways, first off by checking the oil for water through the dipstick.  Also, check to see if the car starts harder then normal, meaning overly lengthly crank time, or rough low idles while cold.  The reason why you can have this problem with little or no symptoms is because one the engine is warm, the water leaks into the oil, then burns off immediately, leaving little evidence other than a slight loss of power and having to add fluid.  Let me know if you have more detailed information.