Replaced the thermostat and housing in a 2006 3.5 because it was leaking. Spent 3+ hours trying to bleed air out.
Temp gauge used to be at halfway. Now it stays at 1/4 while driving. Dips below 1/4 when you turn the heat on.
After a 30 minute ride, if you sit and idle at 1500, the gauge will go up to the halfway mark, but no further.

Is there still air in the system?