The past few weeks my I6 hasn't been reaching normal operating temp like it should. Of course this happens when it starts to get cold out. I've gathered all the parts and decided to get it done this weekend before it gets any colder. Everything came from NAPA (fan clutch, water pump, thermostat, coolant, and hoses). I got lucky and borrowed the pneumatic tool to remove the fan clutch from a friend. All went pretty well, but I had to fight fan shroud/clutch assembly in and out. I filled it back up with coolant a little differently than in the past. After installing the lower hose, I only connected the upper to the engine and then filled it up through the hose. After it was full I did the radiator. This seemed to get most of the air out as I only had to top off a solo cup's worth (I alternate back and forth with coolant and water then just throw it away). Now it warms up so much faster it's scary. Plus it's way quieter with the new fan clutch.
So it turned out my problem was this:
The bottom is the crappy Oreilyzone part and the top is the Napa/ACDelco part. Notice the difference? The rubber seal came off and jammed up the thermostat.
Don't buy crap.