The radiator cap acts as more than just a lid for your radiator; it keeps your engine cool by sealing and pressurizing the coolant inside. And not all caps are the same--they are made to maintain a variety of pressure, depending on the size and type of engine. How do you know the pressure rating of your cap? It's written on the top of the cap. When you replace it, make sure you get one that has the same rating. When you buy a new one, take the old cap into the store with you if you aren't exactly sure.

• You will notice that the cap has three main features. The wider top of the cap, the smaller seat at the bottom (with a rubber seal) and a spring coil in between. This spring is what seals the cap to maintain pressure. If you squeeze the seat and the top together, and it's very easy to compress the spring, then replace the cap. It isn't doing it's job.
  
• If your cap is rusted or the rubber seal is dried out, it should be replaced.
  
• Here's a good rule: if you haven't replaced the cap for two years, make this small investment and you'll avoid a larger problem.
  
• Set aside the cap on a clean spot...You won't need it until you're done with the flush.