Bought mine with dexcool + green coolant.... sludge caused this and messed up the tstat.
Straight dexcool will sludge too. You need to flush out the system. Radiator could be clogged in some areas too, use a laser temp reader to check that. Coolant passages in the engine could be clogged too with sludge.
No more GP 04 Impala SS Indy:
Gen V 3.4”|FWI|N* TB lq4 MAF|1.9 Modded Rockers|F&R Plogs|ZZP 3" Catted DP|ZZP 2.5" Catback|