F-150, heater will blow hot, then cold and then heat gauge will rise to extreme high, will boil over then gauge will drop to normal. Just had it flushed and no change. After driving for about 10-15 minutes at highway speed it doesn't do it.
If it is only overheating while idling or in heavy traffic I have a few suggestions. Is the fan shroud still in place it is vital for moving air at low speed. With the radiator cap removed allow it to come to operating temperature. Looking into the radiator you should see the coolant flowing through the radiator. If the coolant is not moving check the thermostat (as jzd suggested). If the flow looks good Check the fan clutch, if equipped. It should spin fairly easy when the engine is cold, but there should be some resistance after the engine has warmed. You of course are doing this test with the engine off. If the flow through the radiator seems sluggish or only a trickle it may be a failing waterpump. I have seen the impeller fins rust to the point they produced little flow but the pump didn't leak.
I had something similar happen to me after I changed the thermostat and flushed out the cooling system. There was air trapped somewhere in the system and wouldn't show up until I had been driving for around 20 to 40 minutes. The random nature of the issue threw me until I found out for my van: 1996 Ford Windstar, you have to take the cap off the coolant overflow and let the air work it's way out. That was what I read in my manual.
I also found out some older cars have a bleed screw for this. In any event, that fixed it!
This is exactly what will happen if there is air trapped in the cooling system (especially the heater core). There are other possible causes, but this is common after a poorly done coolant change. If you had a shop change the coolant, they should have purged ("burped" I guess) the air from the system before returning it to you. (It sounds like you already checked that the thermostat isn't upside down, but I don't think it even fits in the wrong way on at least the modern 5.4 liter engine.)
Here's a swell video of how to do this, if you are doing it yourself: http://youtu.be/zUpXgAJ1gjU?t=1m10s
- Use a freestanding funnel to fill radiator and overflow tank, using bleeder screw if it exists. (I don't think it does on F-150s).
- Turn the heater control to the highest temperature.
- Keeping a funnel full of coolant on the radiator, run the engine at 2,500 RPM for 10 minutes or so. It should suck down some more coolant during the run.
- After turning off the engine, make sure the radiator is still completely full, and the overflow tank is filled only to the "full" mark. Check (you can use your hand, carefully) the temperature of the upper and lower radiator hoses, and the temperature of the incoming and outgoing heater core hoses. They should all be hot. If one is cool, the coolant isn't circulating somewhere.
- Cap it up and drive happily.
Some people will say that the overflow tank should take care of this during normal driving, but having attempted quickie coolant changes on a wide variety of vehicles, I've found that to often not work. The above procedure has worked for me on dozens of cars, except on one horrible Japanese car where I had to loosen the heater outlet hose to get all the air out. Sheesh!
I am familiar with many F-series Eco-boost Fords that trap air in the top of the radiator. If the coolant is replaced without purging that air, all kinds of temperature excursions will occur, including very high gauge temperatures at times. Looking at the system, you will notice a vent line from the top of the radiator to the expansion tank. Ah, you say, the radiator will vent all by itself. But IT WILL NOT because THE VENT LINE IS A DUMMY! It is blocked! You cannot blow air through it! It came from Ford that way and YOU MUST NOT REPLACE IT WITH A HOSE THAT HAS NO BLOCKAGE. All you have to do to get the air out of the top of the radiator is to temporarily remove the "vent hose" from the top of the radiator, wait until coolant emerges from the opening, and then replace the "vent hose". Why the fake vent line from Ford? My guess is that after all the expansion tanks got made with the vent line nipple molded in, Ford discovered that the cooling system could not function properly with an authentic vent line in place. By blocking the vent line, Ford could cosmetically attach it to the expansion tank and still have a functioning cooling system.