Coolant is the fluid that absorbs heat from the engine and then dissipates it through the radiator. It is also dissipated through the heat exchanger in the passenger compartment when you crank your heat in the winter.
Most cars have a plastic coolant recovery tank connected to the radiator by a hose, so there’s no need to open the radiator cap itself. The coolant level should be at the tank’s MAX or HOT line when the engine is hot, lower when it’s cold.
Servicing the coolant involves draining and/or flushing your car’s cooling system and then replacing the old coolant with fresh coolant. Draining out the coolant and refilling the system removes dirt and rust particles that can clog up the cooling system and cause problems in winter and summer.
Because It is operating in a hot, hostile environment, coolant will brake down over time. Most importantly, the coolant’s rust inhibitors get used up, leaving the small cooling passage in your engine and radiator vulnerable to corrosion. Even with these rust inhibitors, some corrosion will inevitably take place, contaminating the coolant with debris.
If the rust inhibitors stop working, the cooling system will rust from the inside out. The biggest source of rust in a car’s cooling system is the engine block. In time these bits of rust will also clog the tiny passages within the radiator and heater, causing your engine to overheat. When an engine overheats, the result is a bonus yacht payment to your mechanic that someone else should be making.
Disappearance of coolant could be caused by either an external or internal leak, the latter being more expensive. Either on should be addressed quickly, since running low or out of coolant lead to catastrophic engine failure.