2005 F150, 4.6L with 211k. In preparation for a coolant flush, I replaced my thermostat with OE Motorcraft. Ran the truck and burped the system to eliminate air. Never had over heating issues this was strictly to be a preventative maintenance service. My heat works great as well.

Shop calls me to let me know they couldn’t flush the system because the “thermostat wouldn’t open”. They said the upper radiator hose was hot while the bottom one remained cool (isn’t this normal operation?!)

Thinking maybe I had a defective NEW thermostat, I put the old one back in to see if that made a difference. With the old one back in I notice the same result….upper radiator hose remains hot while the bottom one remains cold. Now I wonder how long I’ve been driving around like this. Never had overheating issues and gauge indicates the same, comes up to operating temperature and remains right in the center where it should. I’m stumped. shop wanted $350 for a new thermostat and $260 to flush the system. Any ideas on what this is means? No coolant leaks or anything that might indicate a bad water pump. Thanks!

 

Shop is lazy. You should remove the thermostat to do a flush. If the top hose is hot, the thermostat is opening.