Overheating. on 2002 Oldsmobile Intrigue

After driving 4 Miles engine gets hot. Removed thermostat,have water flow through radiatoor, heater, and engine block. Heater does not blow hot air when on high temp. New radiator, problem still exists. No water leakage, oil not affected. Exhaust normal. Engine runs smooth and powerful. Code P1258. Thanks Glen

Asked by for the 2002 Oldsmobile Intrigue
put the thermostat back in. you will cause internal engine damage from removing it. that may have been the whole problem.