Also known as Pressure Caps, these small bits of the cooling system are often ignored for the entire life of the vehicle. They are an important part of the modern cooling system that is designed to run under much higher pressures that those of yesteryear.
Unexplained water loss or temperature running too hot can sometimes be attributed to a faulty cap. We recommend that caps be replaced at least every 2 years when the cooling system is drained and flushed.
With some vehicles now running pressure caps of 200kPa (about 30 lbs), that translates to approx. 137°C that the water will boil. Most caps will carry a warning to “Never open when Hot”. This makes sense looking at both the temperature and pressure. This vehicles’ running temperature will probably be somewhere in the region of 100 – 110°C.