If you are topping off the coolant and it is not staying at normal level than you have a leak or it is burning. Common leak points are the lower intake manifold gasket, which you will see coolant pooling up on the transmission bell housing and a wet trail where the manifold meets the cylinder head. The coolant elbows are also prone to leaking. Aside from these 2 points, all of the normal cooling system parts need to be checked... radiator, hoses, water pump, coolant overflow bottle, heater hoses, heater core, thermostat, etc. Take your oil cap off and see if you have a brownish milky mess on it. This indicates coolant mixing with the oil. Best thing to do is top off the coolant and then pressure test the system. If there is a leak, it will do it. If you can see coolant bubbling while the car is running and it smells like exhaust, you may have a blown head gasket or crack in the block or head where exhaust is leaking thru. Dense white smoke that smells like coolant comming from the exhaust is usually a blown head gasket or crack on the intake side, allowing coolant to get into the combustion chamber and burn.
