On the surface, your coolant expansion tank cap may seem like a simple, ordinary part. It is often not given much thought or consideration, especially given your vehicle's maintenance schedule. Brake pad replacement, oil changes, tire rotation, and about a dozen other procedures are all closer to the forefront of your mind when protecting the longevity of your car or truck. But if your coolant reservoir cap is not performing its job as it is supposed to, then your car will suffer catastrophic cooling problems that will quickly lead to overheating as well as physical damage to engine components. This cap is designed to hold a certain amount of pressure (up to 15psi), and the reservoir can handle temperatures far above water's boiling point. But when the cap cannot hold pressure, then this could result in a burn-off of the coolant in the tank. If that level drops, so does the rest of the coolant in the engine and it will overheat. So don't be fooled by a coolant expansion tank with a low level of coolant, and simply top it off. It usually means something else is going wrong.
Liquid-cooling for internal combustion engines became very popular during WWII as engineers witnessed its efficiency when keeping engines in their military vehicles cool. Soon, automakers would adopt the technology and outfit civilian cars and trucks with it. The liquid that is used for coolant is an ethylene glycol and water mixture. Because it is kept at above-ambient pressure, the boiling point is raised so it still has the capacity to absorb heat. There are two places in this closed-loop system where the coolant pressure is maintained, and those are the radiator cap and the coolant expansion tank cap. They are the last line of defense against a total eruption of the coolant as it moves through the system, and they also make sure none of it is boiled away and lost.
There is nothing difficult about replacing your coolant overflow tank's cap. It usually unscrews with a couple of turns. Some will have a vent pipe attached, some may have a pressure relief valve. However, those are typically jobs for the radiator cap. Remember, when changing your coolant reservoir cap you must make absolute sure your engine temperature is low. This should never be done right after your car is shut off... and obviously not when the car is running.
Accept no substitute when ordering your replacement coolant expansion tank cap. These are important devices that serve a function vital to maintaining your engine. If you just put any old cap in its place, it won't work. If you buy the cheapest coolant recovery reservoir cap you can find online, it might fail under pressure. At Car Parts Discount, we will only carry top quality parts for your cooling system that meet the standards of original equipment. That means we will sell either the genuine part or one manufactured by a quality aftermarket brand. Either way, CPD has what you need and a price that will make you happy.