So im having this problem where when my jeep gets warm it starts hissing out of the radiator cap, which i think is subsequently causing it to over heat. My current radiator cap is a 13 psi cap and im looking at replacements that are either a 16 or 18 psi. Which one do i need? Or does it really make a difference?

I'm running a 13 lb cap and have no problems. I think your problem might be further down the cooling system.

raising the pressure at which your radiator cap releases is raising the boiling point of the fluid in your cooling system. Obviously either one will accomplish that, but to different amounts.

it would also depend on what motor you have, if you have a jeep 4.0 if its an open system or closed. you may want to check and see if your overflow hose/tank is plugged if you have one.

OEM is 18 lbs. and it is a recovery cap, meaning it sends overflow to the recover/overflow tank and as it cools it recovers/sucks the overflow back into the radiator.

so you will want an 18 lb recovery cap. They don't always say "recovery" but if you google for 18 lb radiator cap you'll find those are mostly for the TJs

here's a good 18 lb cap...

http://www.quadratec.com/products/52449_1452.htm

and check the overflow hose, etc as suggested previously to be sure its not clogged or crimped closed. If the overflow can't escape then its going to go somewhere, hopefully not split the plastic top on the radiator

cool thanks for the info guys. Part of the problem is how much i have to use the skinny pedal when im up here in denver... im so low on power that im full throttle through alot of things and im still barely turning the tires.