You should start with having the cooling system pressure tested as this will locate any external coolant leaks, also have the pressure cap tested, start here and report your findings.
November, 18, 2008 AT 5:36 PM
IT HAS BEEN PRESSURE TESTED AS GOOD.
November, 18, 2008 AT 5:42 PM
OK, the coolant can only go two places, that is an external leak, or internally, so if the pressure test has been done correctly and there is no external leaks, it must be an internal problem, you can have a head check done, this is a chemical test done in the cooling system, that detects exhaust gasses in the coolant, this is a sure sign of a combustion leak, get this done next.