OK, I have next to 0 experience with 6.0's, but I will share with you what I've learned from years of wrenching nearly everything else. You've got air trapped in the system, be VERY careful, air trapped in the heads can quickly overheat them and take out the gaskets and crack the castings, and apparently HG/bolt problems seem to be rampant with 6.0's. Go ahead and get you a new gasket for your thermostat housing, along with some anti-seize(you won't need much and I love the stuff!). Now, you said the T-stat wasn't very old, did someone change it recently? Why didn't they flush and refill the system then? Might want to just go ahead and get a new T-stat, I'm going to share a trick with you that makes this process way easier. After you take out the old T-stat, clean the housing and block thoroughly, gotta have a good, flush surface to seal on. As long as the T-stat doesn't recess into the housing(you'll see a groove machined around the hole, some use a rubber o-ring as a gasket, if so, disregard this part) I like to glue my gaskets, one side only, to the T-stat housing(gooseneck for old schoolers) with 3M yellow gasket glue, don't use RTV. Then take your T-stat and just above where the valve flange flattens back out, lay the flat portion onto a 2x4 block to stabilize it, drill a 1/16" hole through it front to back. Wear gloves so if the bit spins the T-stat it won't cut your fingers. Clean off any shavings and tag ends from around the hole. What you are making is an air bleed hole, so that as you fill your cooling system, bottom to top^, it pushes the air through the T-stat and out the upper radiator hose allowing the coolant to displace and fill the passages. It can still trap a little air in the back of the head passages but as it circulates a couple of times it will burp it out. Put your T-stat back in, with the hole pointed up toward the hood, anti-seize your bolts and re-install them, torque them to spec, about 12-15 foot lbs. Don't over tighten them, they will break. Yes, you may ask me how I know that. Fill your system with 1 gal of quality coolant, I know Ford recommends their Gold brand but I think someone else has posted something about that, I've always used PEAK, if I'm wrong someone please let him and me know. Anyway then put in 1 gal of distilled water, don't use tap or well water if you don't have to, it'll crap up your radiator and heater core from the minerals and galvanic reaction. Then fill 50/50 to the top, give it a couple of minutes to bleed the air out, crank her up and turn your heater to hot/floor and adjust the levels accordingly till it's gotten to temp and cycled the T-stat a couple of times. That should do it. IF FOR ANY KNOWN GOOD REASON I AM WRONG ABOUT ANYTHING HERE AS IT PERTAINS TO 6.0's, SOMEONE INTERVENE AND PLEASE DO NOT LET THIS GENTLEMAN MESS UP HIS TRUCK ON ACCOUNT OF ME. Also, make sure the truck is level or slightly uphill when you do this.
Last edited by porkchop6209; 04-24-2012 at 10:38 AM.
Reason: An additional thought