Originally Posted by TheKirbyMan
What would happen if you ran a cap with too low of a rating?
A 50/50 mixture of antifreeze and water boils at 227 deg F at atmospheric pressure. As you increase the pressure, you increase the boiling point. The pressure vs boiling point looks like this:
0 psi 227 F
4 psi 236 F
8 pis 248 F
12 psi 259 F
16 psi 267 F
So, if you run a lower pressure cap, you coolant will boil at a lower temperature. About the only time you would notice this is when your engine overheated for some reason, or when you shut the engine off and the coolant in the engine block heated up.
I have always run 7 psi caps on most of my cars, including cars with 205 degree thermostats. But, I am willing to live with the possible consequences. For example, if for some reason my engine overheats, it is going to blow off coolant. I keep several gallons of water/antifreeze in my car at all times. I also make sure my overflow line to the overflow tank is in good condition and is not leaking. When I shut the engine off on a really hot day, sometimes some coolant will belch into the overflow tank. When the engine cools, it draws it back into the radiator again. It will only do this if the entire cooling system, including the overflow hose, is well sealed.
The strain on hoses and water pump seals is a lot lower at 7 psi. Since switching to 7 psi caps (20 years ago) I have never had a hose blow out on me or a water pump shaft seal leak.
If you run a lower pressure cap, be prepared for the occasional emergency.