1999 Kenworth (CAT C12) High Pressure in Coolant Resevoir

WhiteCloud ancient Indian Spirit that live in oil pan.
Going to need a priest.

 

Well you have a few different possibilities, but I would lean towards a head gasket / cracked liner. One of your cheaper ways of testing if you insist on doing it yourself would be to drop your oil pan and pressurize your coolant system. Leaks or more likely drips / bubbles will start to show themselves at the culprit.

 

Not necessarily if you're sitting there just letting it idle. If you go out and attempt to work it on a long hard pull more often than not you will notice an overheat condition. This repeated overheating / cooling process is what leads to the demise of aluminum parts and especially heads. I highly suggest if you have excessive pressure buildup in your coolant system an oil analysis to back it up that you need to get your motor to a mechanic or cat house soon or your repair bill is just going to keep climbing.