I have learned the hard way, over the years, to leave a working thermostat alone, and I also learned to only use OEM thermostats. Ford used to use aluminum thermostat housings and I would remove one during a hose change to clean the corrosion from it. In the process I would replace the thermostat and a couple of thermostats later, I would go back to a OEM thermostat from Ford to fix the erratic operation. Finally put an aftermarket cast iron thermostat housing on and didn't have any issues after that, as I quit messing with it.
Recently got stupid again. Was flushing the Cummins cooling system after I bought it because it had unknown green coolant in it. You cannot just remove the thermostat and run it, the seal is part of the thermostat itself. So I took the thermostat (original Cummins) and gutted the working parts out of it and used it to flush with. Then I bought a NAPA thermostat to replace it. Engine ran about 10 degrees hotter than it should, and then after a couple of thousand miles, started sticking, then would pop open. Made the trip to the Cummins distribution center about 40 or 50 miles away and bought their thermostat, $53! that was in 2018. Has worked perfectly ever since.
Charles
