If coolant blew out of a 15# cap, that is a good indication of an overheated condition. My guess is that you have a cooling problem which could also indicate a cracked or warped head. The simplest way I have found to determine if the head needs pulling is by filling the radiator as full as possible, remove all the spark plugs, roll each cylinder to a compression stroke one at a time and apply 150# shop air to the cylinders as they are on their compression stroke. This will show up as bubbles in the radiator if you have a cracked or warped head. If you have to remove the head and it is not cracked, be sure to check it for being straight. Overheating causes heads to warp.
