For the brief time my car has been mobile, I've found that it's not uncommon for the coolant to overflow after driving and shutting off the car. I made sure that the coolant level was just covering the cores inside with at least an inch of empty space at the top of the radiator. I figured the radiator cap was in need of replacement, although it was only five years old.
When I replaced it, I found that the current cap was a 13# unit and the replacement was listed as 13# as well. This means as the pressure builds within the cooling system, 13 pounds of pressure will cause the cap's spring to let some coolant past the cap and into the expansion tank (on newer cars) or pee out the overflow tube at the bottom of the car on mine...
The replacement cap showed up and it was 14#, which surprised me, but the car hasn't overflowed again since I've been using it. So that extra pound made a difference. In the photo the 14# cap is marked with the number "14" on it, and the 13# cap is below with the number "13#" on it...
