Closed Cooling System Mystery- Need Help
 

502 EFI. closed cooling system: After running for 1/2 hr. or more the starboard engine overflow reservoir will get filled or even overflow but the engine cooling system always maintains enough coolant to keep the engine within 5 or so degrees of normal operating temp. When I shut the engine down the heat exchanger /reservoir will maintain positive pressure- even if I open the cap days later, the system is still pressurized. Therefore the coolant from the reservoir never gets sucked back into the heat exchanger so of course its a few qts low. I swapped caps with the Port engine- no difference. I warmed the engine up to operating temp with the cap removed- ran it up to 3000 RPM and confirmed that the thermostat is open and coolant is definitely pumping through the system. I opened the bleed valve on the thermostat housing and antifreeze flowed out. What could possibly be causing over-pressurization of the system??