I was intrigued with this problem and went out to the shop and got my manual.
Cummins Shop Manual B3. 9 & B5. 9 Engines
Group 8
Cooling
Page 8-3
Flow Diagram, Cooling system
The following is a quote from the manual;
" When the engine is below operating temperature, the thermostat is closed, and the coolant flow bypasses the radiator and goes to the water pump inlet through internal drillings in the block and cylinder head. "
"When operating temperature is reached, the thermostat opens, blocking the bypass passage to the water pump and opening the outlet to the radiator"
End of quote:
So the coolant is does not see the radiator until engine temperature is reached, thus the pressure cap does not come into play, until T Stat opens. Ok, I understand, but also wonder if blown plugs may be on some engines where they were not installed properly ?
Another good reason to keep the block heater plugged in during cold weather. I keep mine plugged in all the time when not running, and then a short warm up and Ii have heat. Plus the T Stat is starting to open, thus the Pressure Cap can handle excess over pressure.
WM300