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 bonnet.
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.
Other signs of head gasket leaks or a blown head gasket include a sweet musty smell, lots of white smoke coming from exhaust, coolant disappearing but no leaks on ground, the car overheating and the engine light coming on (a trouble code check should generate a misfire code). Another way to test for a leaking head gasket is with a radiator pressure test kit. This is a tool that has a hand pump with a combination pressure gauge. However, these kits can cost between £81- £162. If your car tests positive for the above test described in the steps and you take it in to a mechanic for a suspected head gasket leak, they will test the cooling system with a radiator pressure test kit and this may be unnecessary to do yourself unless you already have the kit. If you do have the radiator pressure test kit, attach the fitting on the kit to the radiator filler neck. Check underneath the radiator cap for the pressure rating for your particular vehicle. Squeeze the hand pump on the kit to the desired pressure. For example, if the cap says 6.8kg., increase the pressure on the pump to this number. Wait to see if the pressure holds for 10-15 minutes. If the pressure does not hold, the head gasket is leaking and may be blown or the car could have a cracked block.
If you open the radiator cap while the vehicle is hot, coolant could spew out and burn you.