Checking and renewing a radiator cap

The spring loading of the cap keeps the cooling system under pressure, and raises the boiling point of water so that it does not boil at the working temperature of the engine.
When the coolant heats and expands, air or water escapes through an outlet valve into an expansion tank or overflow pipe. When the coolant cools and contracts; air or water is drawn in through an inlet valve.

Guide

1 To check the cap, run the engine until it is warm, but not too hot. Stop the engine. Put a cloth over the cap to protect your hand from scalding steam. Unscrew the cap a quarter turn. There should be a slight hiss noise as you undo the cap. Note If there is not, fit a new cap as soon as possible.

2 The new cap must have the same dimensions as the old one take the old one with you when you go to buy it. Check the pressure stamped on top of the cap. The cap must have the same Pressure marking as the old, or the engine will run at the wrong temperature. This could lead to overheating or uneconomic fuel consumption and damage to the engine.