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For every lbs of pressure it raises the boiling point 3*. Your typical cooling system will run about 15 lbs so you have a potential of 45*. If you have a leak you can be loosing a lot of cooling capability. That can be a big portion of your overheating problem. When is it overheating? At a stop or while on the road. Typically if you are overheating at a stop, that also indicates that you don't have enough fan and if you are overheating while moving(say freeway) you don't have enough radiator. When looking for a radiator a common misconception is that more cores is better. What you want to look for in a radiator is how many fins per square inch. What you want is the most surface area. If two radiators being equal thickness say 3 inches, and one is a 4 core and the other is a 2 core each with the same fins per square inch, the 2 core will outcool it. The core will be fatter giving more surface area to cool and the 4 core will have a gap between each row with less surface area. Typically you will only find a fatter core in aluminium radiators because aluminium is stronger than the copper core though copper is better at dissapating heat.
This time of year you shouldn't need the raise of boiling temp due to system pressure.

You need to fix or at least bypass the heater core, but I don't think it'll completely fix your overheating problems.
had the exact same problem....coolant leaking all over the floor, ruined carpet and the underlay....pretty ugly. had a bad heater core, switched it out for a new one and problem solved.

yah, when uve got coolant everywhere but ur radiator, temps gunna go way up... had to pull over several times. also check ur thermostat too, might need to be swaped out. the bolt on the underside of thermostat housing is a pain in the arse to get to, cant use a ratchet. I had to bend my oil dipstick to get to it....little turn by little turn. make sure u put ALOT of sticky blue stuff back on the gasket when u loosin the therm. housing, and make sure the gasket it perfectly centered, or else ur coolants going to be leaking out of there too.