The coolant in your engine plays a vital role in giving longevity to the life of your car, and the pressure cap plays a vital role in keeping coolant levels constant. When the coolant reaches extremely high temperatures, it rises and forces itself through the pressure cap. This overflow is subsequently gathered in a reservoir, and when everything cools back down the coolant is sucked back up through the pressure cap. A faulty pressure cap will cause the coolant to evaporate instead of re-entering through the cap. Without enough coolant to maintain safe temperatures, your engine can be damaged