This summer while towing a trailer with an antique car through the mountains, my 2006 F150 XL with a 4.2 L v6 and a 5 speed manual transmission, 168,000 mi, the temp gauge buried to the right, buzzed and threw a PO1299 code. I pulled off the road and it did not appear that the engine was overheating. The coolant was full. It was warm, but not overly hot. When I turned the engine off and turned it back on, the gauge went to normal, but after a bit would indicate it was overheating. Prior to that I was having some engine misfires and had the coil, wires and plugs replaced. When I got home, the problem pretty much went away, but it shows up periodically since then. I have had the truck in the shop more than once for this problem and the shop has not been able to replicate it. I was getting other codes later and had some intake work done and the temp sensor was replaced. That has not eliminated the problem.

Last summer I towed the same trailer and car to New Hampshire from MD and out to the Shenandoah Mts. without a problem of any kind. I did have a real overheating situation earlier last summer and added antifreeze which made the problem go away. I wasn't driving the truck at the time, but provided the antifreeze when it got hot.

I am about at the end of my rope on this and have thought about taking it to he dealer to see what they can figure out. I have read on line that maybe changing the antifreeze might help. I have not done that, but may do that next week. If I mixed types of antifreeze might that be the source of my problem? This has been great truck, but this problem is driving me crazy. It does not seem to do it under any special circumstances, i.e. driving up a steep grade, etc.

 

You have the kind of mileage that makes it time to start going thru your cooling system. Drain it take out the thermostat put the thermostat housing back on without thermostat. Take off top radiator hose stick water hose in radiator turn on hose flush out till water runs clear then stick water hose in the other way and reverse flush. Check flow thru radiator look thru the hole you can see the first few rows of cooling tubes leave the radiator drain plug open stop the water let it drain a little then observe the flow. I would replace the radiator anyways with that many miles. there small in size anyways if you get tubes starting to clog especially pulling a trailer it will overheat. I would change the water pump also I guarantee you'll have pitting on the fins. But mainly your at the mileage your weep hole is likely to fail.
After flushing check thermostat in a pan of boiling water if good put back in thermostat housing with new gasket. NOTE when flushing don't have high water pressure going thru engine ! Just a nice even flow.. If you want more detail on how to do any of this feel free to ask.