Does anyone know a way to MAKE a northstar run cooler by 20 degrees or so?
There is absolutely no reason to make a Northstar run 20 degrees cooler. Why ??? .......... and consider than many emissions sensors and monitors don't even go closed loop until 160 degrees coolant temp (depending on ambient temps) and you have all the makings for destroying the cat and O2 sensors as well as running an overly rich mixture = fuel consumption = emissions codes = SES light = fail emissions testing.
Without typing a novel the size of War and Peace, go up a couple of threads and read the sticky posts explaining the Northstar cooling system, gauge operation, and fan protocols. Then go way up ^^^ to the top black bar and study the entire Cadillac Technical Archive - most of those articles were written by the GM design engineers responsible for the systems.
The thermostat is set to begin at 188 and fully open at 206, so the normal operating temps run between 190 - 210. The fan controls sticky will explain how and when and why fans run.