Checking the Coolant

If pressure in the cooling system is too low, or if the coolant does not contain ethylene glycol antifreeze, the coolant will boil. Pure water could boil at 119°C (247°F) even with a 124 kPa (18 psi) in the cooling system. Pure water lacks the necessary rust inhibitors and water pump lubricant. The engine should never be operated without the pressure cap, or with the cap loose, unless performing work on the cooling system.