External coolant leaks from your car can be easy to determine because the green or brownish liquid -- depending on the type of coolant -- is visibly leaking from hoses or other engine components onto the ground. However, coolant leaking internally can be more difficult to diagnose and usually means a blown head gasket. If your car is overheating and the coolant levels are decreasing, there is a radiator cap test, or what is sometimes referred to as a champagne test, you can administer. This test can determine if coolant is leaking in the head gasket and if it is blown. This will take just a few minutes of your time and no tools.
Turn off the car and let it cool overnight. Open the hood.
Locate the cap toward the front of the engine bay with a warning message imprinted on it, "Do not open when hot." This is the radiator cap. Twist it counterclockwise to open it, and set the cap in a safe place.
Climb in the driver's seat and start the car once the cap is off. If the car has head gasket leaks or a blown gasket, you will see bubbles forming in the coolant when you look into the radiator filler neck. If your vehicle tests positive for a head gasket leak with this test, you should take it to be inspected by a licensed mechanic.
