Sometimes, the cooling system in your car leaks, but you can't see the leak. The best way to find the leak, which could be in the radiator, water pump, radiator hoses, heater core or any of the heater hoses, is to pressure test the system. The coolant system is pressurized, and the pressure increases as the engine temperature increases. The average pressure for most cars is between 8 and 12 pounds.
Allow the engine to cool, if it is hot. Remove the radiator cap.
Fit the pressure tester and the appropriate fitting onto the neck of the radiator.
Pump the pressure tester up to 16 pounds. Watch the gauge. If it drops, there is a leak in the system.
Pump the pressure tester up to 16 pounds again, then look at the radiator, water pump and hoses for leaks. If the heater core is leaking, it will leak inside on the passenger side of the vehicle, unless the heater core is in the engine compartment, which is not common--it's usually under the dash.
