1999 Ford F-250 Super Duty 6.8 liter v10 is

You'll have to drain system down when cooled off, remove any brackets in your way such as power steering reservoir holding bracket, then remove 2 bolts holding the housing, and remove it. Rad hose should be o.k. to leave on, but if it creates a problem, remove it as well. I'd install a new thermostat and "o" ring complete,then a new gasket as well. Clean all surfaces well before re-installing. Replace any bracket(s) you removed to access housing, after housing is installed. Remember, air valve on edge of thermostat goes in 3 oclock position when re-installed. Also a reminder as well, spring on thermostat goes down when re-installed. After all is re-assembled, re-fill coolant and start engine and run with heater on to bleed out all air. Leave rad cap off and pay attention to temperature gauge and when you see it getting to normal operating temperature, watch flow in rad for increased flow, indicating thermostat opened and coolant is circulating properly. Wait until you see that flow increase 2 or 3 times, and air should be out of system, and top up coolant in rad, put cap back on, and check that return bottle is at proper level as well. Keep an eye for any leaks, and re-check return bottle periodically for first little while, just to be sure all is good. Note: some only use the "O" ring as a seal, and no gasket is required, but just check that at autoparts place where you get part.

I HAVE A 1999 FORD F250 6.8L V10 I HAVE OIL IN THE RADIATOR