RE: over heating out of ideas
 

bottom rad hoses collapse at high flow, air in the system creates pump cavitation,
Dex-Cool gels blocks passages, hot exhaust leaking into cooling system boils coolant,
stuck thermostats stop flow, backwards thermostats prevent flow, and the best ?
water pump impelelrs installed BACKWARDS dont pump well - yes, the assy people do screw up.

have you tried block & check?

https://www.cadillacforum.com/m_7193/tm.htm

RE: over heating out of ideas
 

check your heater core hoses when your car is at operating temp. if one is much hotter than the other one your heater core is plugged. I had 1998 deville 4.6 that no one could solve the overheating problem in after changing water pump, radiator, thermostat, had it flushed it still over heated. everyone said it was more than likely the head gasket, but it wasn't using fluid. as my last resort I unhooked the lower heater core hose and bypassed the heater core to see if it would solve my problem, IT DID ! For some reason if the heater core is plugged it doesn't allow the coolant to circulate properly in the engine, causing it to overheat. You can bypass the heater core by removing both hoses off of the heater core and connecting them together with copper pipe or plastic coupler, it's the easiest way to see if it solves your problem, if it does then replace your heater core.

RE: over heating out of ideas
 

check your heater core hoses when your car is at operating temp. if one is much hotter than the other one your heater core is plugged. I had 1998 deville 4.6 that no one could solve the overheating problem in after changing water pump, radiator, thermostat, had it flushed it still over heated. everyone said it was more than likely the head gasket, but it wasn't using fluid. as my last resort I unhooked the lower heater core hose and bypassed the heater core to see if it would solve my problem, IT DID ! For some reason if the heater core is plugged it doesn't allow the coolant to circulate properly in the engine, causing it to overheat. You can bypass the heater core by removing both hoses off of the heater core and connecting them together with copper pipe or plastic coupler, it's the easiest way to see if it solves your problem, if it does then replace your heater core.

Disconnect surge tank purge line at tank and at thermostat housing. Clean both hose fittings and blow out purge rail. If clogged you will see a gloppy liquid come out. That was the problem for my 2000 Deville.