A Honda Civic's cooling system is a sealed system, meaning that it uses pressure to raise the boiling point of the fluid within the radiator. As the engine operates, the fluid within the radiator circulates throughout the engine. Most overheating problems with these vehicles can be traced to gasket failure, leaks or to worn components.
Twist the radiator cap off of the radiator after first waiting for the engine to cool, then look into the radiator and observe the fluid level. The fluid level should be approximately 1 inch below the top of the radiator. Add radiator fluid if necessary.
Visually inspect the cooling system for signs of leaks. Even a small leak will gradually lower the fluid within the radiator and result in overheating. Inspect the upper and lower radiator hoses, the seams surrounding the water pump, the thermostat elbow and the radiator itself.
Pressurise the cooling system with a radiator pressure gauge and check for radiator fluid leaks. The pressure gauge fastens to the radiator in place of the radiator cap. The gauge consists of a pump and a gauge. As the pump is operated, air will fill the radiator and the gauge will display a pressure reading. Pump approximately 6.8 Kilogram-per-square-inch (psi) of air into the radiator. If the radiator rapidly looses pressure, inspect the cooling system for leaks.
Check for signs of antifreeze in the crankcase. Withdraw the engine's oil dipstick and observe the fluid at the tip of the dipstick. Radiator fluid will appear as bubbles within the oil. If water is in the crankcase, the cylinder head should be removed and its gasket replaced. If the problem persists, the block and cylinder head should be inspected for cracks.
Check for combustion leaks into the cooling system if there are no signs of radiator fluid in the crankcase. Pressurise the cooling system with the radiator pressure gauge to approximately 15 psi, then start the engine and observe the needle on the gauge. If the gauge needle moves erratically, pull each spark plug wire off of its spark plugs, one wire at a time, and observe whether the needle steadies. If the needle does steady, that particular cylinder is leaking into the cooling system and the head gasket must be replaced.
Check the release point of the radiator cap with the radiator pressure gauge. As a safety device, the radiator cap will release the pressure in the radiator if the pressure exceeds a certain amount. If the cap releases the pressure too early, overheating will result. The pressure gauge's pump attaches to the underside of the cap. Quickly pump pressure into the cap and note the reading on the dial when the cap releases the pressure, then compare that reading with the cap's specified rating. A radiator cap's rating varies on the size of the radiator. As Honda does not use the same radiator in all of its Civics, its caps differ. The pressuring rating will be stamped into the cap. If the cap releases the pressure early, replace the radiator cap.