what is the preferred pressure rating for the coolant system?

16 LBS PSI ....Check it isn't coming out the tail pipe.

You should be able to lower the pressure to 14 psi which may or may not help if you have a slight leak. I would try to find small leaks and possibly do a leaking head gasket test.

I think what F_O_R is referring to " leaking head gasket test " Is a chemical test to see if exhaust fumes are getting into the cooling system. This is a test using something like turkey baster syringe that has a chemical in it and when you draw coolant from the vehicle coolant system it will change color telling if exhaust fumes are present in the cooling system. Combustion leak test kit . Harbor freight has the kit for about $24.00

If it does indicate exhaust are present in the cooling system then you have a blown head gasket between the cooling ports and exhaust ports. hence coolant is leaking into the exhaust system. Another tell tail sign is smoke coming from the exhaust pipe or a smell of coolant coming from the exhaust pipe.

Can you smell coolant? If you can it could be the heater core. Another tell tale sign is the windshield fogging up. Hope that helps! Jim

also check the passenger floor board for coolant leak from the heater core.