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I just did the same service on my 300C 5.7 HEMI. When the engine is cold, remove the screw plug from the thermostat housing and replace it with a brass radiator drain valve ($3.00) available at auto parts stores. Loosen the upper radiator hose clamp and pull the hose off the thermostat housing. If it is dry, the radiator is not full. Lift the hose and point the open end up and pour fresh coolant into the hose until it overflows. Your radiator is full. Stab the hose back on to the thermostat housing and slide the clamp back into place. Crack open your new vent valve to verify there is no air. Verify the overflow tank is filled to the COLD FULL mark. Start it up and set the heater to maximum heat to circulate through the heater core while warming up the engine until the thermostat opens. Your temperature gauge should never exceed the 50% mark (203F). If it does, check for air again. Repeat this check when the engine is cold. You may have to add coolant to the overflow reservoir after a complete heat up/cool down cycle to maintain the COLD FULL mark. Repeat as necessary over the next day or two until the level remains at the mark when it is cold. You must completely purge all the air form the system or it will air lock in the radiator, block or heater and even cause your water pump to lose suction & flow result in severe engine damage from overheating. Under no circumstances should these engine be allowed to get so hot they hit the red mark on the temperature gauge. The aluminum heads will warp and the head gaskets will leak water into the oil and destroy your engine. If it is getting above the 50% mark on the temp gauge, shut it down and save it. Never run it if the system is air locked or the water pump is running dry. It is a bit time consuming to properly purge the system after a coolant change, but if you do not do it, you will be sorry.


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