Originally Posted by F18

Joe, I replaced my radiator last fall. It did take me a while to get the whole system air free.....there is no bleed valve on my 98 E300TD like I've had on some of my other vehicles. Its suppose to be self purging/bleeding. It is but it takes its time doing it!
Run the engine at high idle with the pressure cap off the over-flow tank and the heat setting at high. ....it does not matter if fan is on high or not.
Watch coolant level and top it off if needed. After engine has reached operating temp. put the cap on the over-flow tank and take the vehicle out for a highway spin with heat controls still on high....open your windows if needed. Park it and when cool check coolant level again. If fluid level dropped and or the heat started working then air finally was purged from the heater core.
Some guys pull the top hose partially off the radiator to purge the system before the coolant gets to hot handle...but that just creates a mess!
Good Luck!