Myself I like to run a low pressure cap to ease the overall strain on the system. Even if it's a new system, hoses, radiator, gaskets, heater core, etc. A 15-16 psi cap with straight water will raise the boiling point to something like 250 deg's. Now add a 50/50 mix of
antifreeze and that raises it again. I would say boiling point would be pushing 270 deg, or more depending on the mix.
Well if 15 lb cap will do the job why would I want to go higher? Surely your going to do SOMETHING (pull over, shut off the engine, get a tow) before you let it overheat to 270 degrees!
225 you say? Not bad say I. Seem's like most folks around these parts think anything over 220 is bad news, I disagree. Would I like it lower? Sure, but I wouldn't loose any sleep over 225 either, which no doubt, is well below your current boiling point. So you can rule out that as a factor as to why it won't come back down. Something else going on...
It would likely only vent when you pull over and shut if off, and the motor is really hot. That's when the engine water temp could exceed the boiling point, build up enough pressure to blow some out. Hopefully to your CATCH CAN and the water is then sucked back in when the motor cools. If the motor 'blows off' at 15 psi, again, I think you've got bigger problems than the pressure cap causing it. Higher pressure cap will only mask the symptoms, not get to the cause.
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