My 2003 F150 began overheating suddenly one morning on the way to work. Had driven approximately 3 miles.
After letting it sit for 5 min. I turned on key to check heat level. Temp gauge showed normal. I started engine and drove the remaining 10 miles to work. About 2 hrs later I drove the Truck to my mechanic, 15 miles away. It did not overheat. Mechanic suggested thermostat was sticking. He changed it. I drove back to work and parked it about 3 hrs, till time to get off. I drove it about 3 miles and it began to overheat again. I stopped, waited 5 min, restarted, temp was normal. I drove home, no problem. The next morning, at about the same place, it temp began to climb. Went thru same procedure. Temp rose again about a mile further, till almost in red, then fell rapidly to normal. This happened again 2 times on way back to mechanic. He replaced thermostat again. This time he let it sit for 3hours before test driving. It was still overheating. He changed the cap, not it. He replaced the water pump, though he said my old one didn't look that bad. He flushed the radiator and had it checked for blockage. There was some, so ended up replacing radiator. Hoses are good. No leaks. Ran a block test 3 times. The last time he bought new chemicals, in case the other had "gone bad". All test negative. There is no miss. Engine runs great. No codes being thrown. No coolant in oil or water in exhaust. He is stumped as am I. Truck has 143000 miles. Drives about 30 miles a day. Again this was a sudden change. Truck never ran hot more than 20 seconds. Mechanic been in business 35 years. I've used him for all my Automotive issues. Yesterday he said the truck was possessed by a demon. Anyone out there have any ideas or suggestions since we are clueless.

 

Was it actually boiling the coolant? Could it possibly be a bad gauge? I wouldn't think so but it would explain the situation.

The only other possibility I could think of is that the block coolant passages are plugged up.

 

No the coolant wasn't boiling. There was no steam released but did have the coolant smell. Mechanic is using an infrared thermometer. Truck will get to 250 degrees. After a few minutes the temp gauge will move to normal. I don't know about flow passage. This was a sudden occurrence. Always ran coolant mix 50/50. Alway used distilled water. Alway changed coolant yearly. Mechanic noticed something and I concur, the temp begins to rise after a turn, and on acceleration after a stop. I would let the motor idle and never got hot. Also, after the truck was good an warmed, after a couple of temp increases, temp would maintain and not overheat. Mechanic ran moter with thermostat out, and moter stayed at 150 degrees -166 at a/c vents . Also, the last thermostat he put in is an OEM Mopar thermostat. It has a hole in it supposedly. I had suggested drilling a 1/8 inch hole in the thermostat is how the subject came up.
Thank you for your reply.

 

If it doesn't overheat with the thermostat out and does with it in.......... I'd replace it again, maybe with a motorcraft.

You can also take the thermostat out and boil it in a pot on the stove with a thermometer in the water to test it to see if it opens when it's supposed to.

I did that with the two thermostats in my yamaha V6 outboard when one side was giving an overheating alarm. They both opened properly. My outboard problem was a clogged fuel injector. A couple bottles of Gumout for high mileage engines poured into the gas tank cured my miss and overheating problem