2005 F150 XLT
Truck starts and drives. At idle no issues, temp comes up and at the center temp mark the stat opens, recently installed a new stat and radiator. Highway speed the temp gauge spikes to max and I get a message to check gauges not warning lights and no limp mode. Pullover and idle temp goes down, and stays down unless I get up to higher speeds again. Nothing out of the ordinary with reservoir level, no mixing with oil.
Advice on how to do:
A device plugged into the OBDII which will display the temperature for engine coolant, engine head, transmission fluid, etc. A lot of people are downloading software online and buying an adapter. Some people buy very expensive tools like shops and dealerships use.
You need to be able to read the truck’s computer. You want to see what the temperature is for your coolant, and compare that with a point and shoot thermometer.
When the needle on the dashboard is high, pull over and start checking the temperature. Check the temperature at the cylinder head, thermostat, and the hoses. See if the fan is turning on. Look at the water pump.
It could be the water pump.
It could be just a bad sensor.
With this generation of trucks, the cooling system was not that complicated.
Thermostat opens to allow hot coolant out of the engine. If this does not work, the truck overheats.
Hot coolant travels through the upper radiator hose to the radiator. Airflow cools the fluid. At highway speeds, airflow is simply from your truck traveling so fast that air is flowing in through the grill. At lower speed, the cooling is assisted by the fan. Cool fluid goes back to the engine through the lower radiator hose. The water pump keeps this cycle going.
Since we know that at low speed, the truck is not overheating, we can believe that the thermostat is opening, the fan is working, and the water pump is doing it’s job. Now to figure out if the truck is actually overheating at highway speed. The needle on the dash is rising because the car’s computer is reading a high temperature signal. So check to see what temperature the computer is reading. Now check to see what the actual temperatures are. Your truck may not be overheating at all. If the water pump, fan clutch, or thermostat failed, you would overheat at low speed. While you are working on your cooling system, it doesn’t hurt to replace the belts & hoses, and pressure test the system. While you are at it, I would exchange the radiator fluid. Flush out the system and start with new coolant.
Credits to @Fifty150