This is a very common problem on all 5.7 and 6.4 ( thermostat failure). When you drain the cooling system on these you have to be patient and bleed the system several times. It will take a while but its not a huge deal. I am curious about something though- I've changed thermostats on 2 separate 5.7 and a 6.4 and never had to bleed the system. I saw a clever backwoods way to do it. Just crack the housing off on the corner and hold a plastic ziplock bag under to collect coolant. When it stops running out ( yes, you have to clamp rad hose)
you can throw the new stat in, refill and be on your way.
Does it make a rushing water noise on start up ( noise from behind dash)?
DId same thing on my Magnum with 5.7 . . . just took the bolts off, got the housing loose and let coolant drain into ziplock bag . . . pulled old stat, put in new stat (making sure gasket around stat is in place correctly), put on housing, bolted up, and added the coolant from ziplock bag to overflow . . no issue.
