13psi on the radiator cap, and just because that's what the package says doesn't mean it's right... ALWAYS test new thermostats and radiator caps... it's too critical of a system not to.
I'm doubtful of it being the thermostat myself.
Test the coolant for acidity too, if it's a blown HG (which doesn't necessarily mean the oil is getting contaminated) the coolant system may be getting pumped full of exhaust gasses.
Check the radiator for blockages.
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