My 1991 LS400 ran hot on me one day and every time after that it runs hot after about 15 minutes. I thought I blew a head gasket and put in some gasket sealer but it's still doing the same thing. Today I did a compression test and all 8 cyl. are close to 175psi. The fan clutch is new, the water pump is new, the thermostat is new, I put a garden hose through top radiator hose and ran water (thermostat out) through the top radiator hose through the engine and back out the bottom radiator hose and it seems to be flowing free. Am I missing something here

 

there is still air stuck in the cooling system. there is a certain way to remove the air, see below on how to properly bleed the cooling system:
http://www.lexls.com/tutorials/cooling/coolant.html

 

Check water control valve (located center top of firewall engine compartment.) for leaks..


Had a 1993 LS400 running a little hot...placed a paper towel underneath it and was stained red from coolant.

Also, check front of fan clutch for dust/dirt build-up...if the bimetal coil is clogged then it may cause it to not engage to cool engine..