Okay I have a very strange overheating problem. It started about 6 months back on a 400 mile trip, about 50 miles in the temp gauge began to rise above normal and hit halfway between normal and red. Pulled off turned the truck off turned it right back on and everything was normal. Checked fluids and everything was normal. This has happened probably 5 times since, it only happens at highway speeds. I had chalked it up to an electrical issue because one morning it happened and when I got back in the truck it wouldn't start due to a loose battery cable. Unfortunately it happened again this week. Every time this happens I can then the truck off and right back on and the temperature is normal. I do not drive this truck often so it hasn't been a priority to fix but I've got some free time so I decided to look into it. My 3 guesses would be a collapsing radiator hose, a water pump, or thermostat. But would any of these cause for a higher temperature but as soon as I restart the vehicle it would be back normal? Even if I just turn the key to the on position after this it is back to normal. On long trips it always seems to happen within the first hour, then never again after that. When this happens all other guages are normal. This is a 2000 f150 4.2 5 speed manual. I should also mention I have checked compression which was good and have also done a block test on it and it always checks out. There is no coolant loss except for a small amount out of a crack in the reservoir that I coated with silicone but has started leaking again but only loses about half a inch a month due to the drip.

 

That the reason it is over heating at speed. It cant pressureize the cooling system and the water is begininning to boil. You need to replace the coolant expansion tank.

 

I had considered that but wouldn't it cause this to happen all the time and not so sporadic? I need to do it but at the time it happened I couldn't find a replacement bottle where I was living, only those junky generic ones that don't work.

 

No because the amount of load on the interstate is higher than around town.