I found this thread but figured I would start a new one.
https://www.thirdgen.org/forums/ltx-...ing-issue.html

The swap is pretty much done. Sitting at idle the car will heat up to temp about 220 F turn the fans on and regulate temp like its suppose to. As soon as we drove it, pretty cool night, the temps went way down on the gauge. The thermostat and water pump are brand new, went in with the swap. The thermostat is made to the water neck. I took it out anyway last night and it was closed. Put it back together and filled her back up through the top radiator hose. Started the car with the rad cap off and kept filling until it would take no more, then put the cap back on. I verified the gauge on the dash while logging in HPtuners, the gauge is correct. We didn't get to drive it last night, but pretty sure everything is working like it should. Anybody else ever seen this?

Could it be that the thirdgen radiator is cooling to good? We have the heater core hooked straight to the water pump, no valves. Fan temps are stock 4th gen fbody settings.

It seems to me that the thermostat should regulate the temp no matter what. I am stumped.