Change your hoses... to me sounds like a circulation problem. like a collapsing hose.. Under pressure, the bulge on the inside of the hose will get bigger until it blocks flow. shut it off, pressure subsides, especially when on the hot side of the thermostat, and it is closed.. as soon as the engine stops spinning, no more pressure. the coolant backfills the reservoir, the bulge goes away enough to allow flow.. engine cools rapidly on next restart due to holding large amounts of cool coolant in the radiator.

Just a thought..

 

Its the Stat , replace it with a Motorcraft only, parts house ones wont work..

 

Update: So after changing the thermostat and flushing the coolant with no success. I decided to take my truck in to a mechanic. The mechanic drove it and said his opinion was small head gasket leak. However he was going to do a block test this morning. He just called me and said the block test passed so he is now thinking temp sending unit. I guess he hooked up the truck to a scanner and said the temp-sending unit was going ape sh*!. Those were his words exactly. I will update if it is works. 600 bucks to replace and trouble shoot. Thanks guys for all the help

 

Did you use a motorcraft stat ?

 

So the truck is back and seems to be fine. I installed a scangauge on it this morning and drove around to see what the coolant temp was. It ran between 200 and 205 around town. I got on the throttle up a pretty good hill and the temp got up to 209 for a sec and then went back to 200. Does this sound right? The temp outside was 70 in case that makes a difference. Also I never hear the fan kick on. Should I be hearing it come on. I know my MH that also has the V10, the fan kicks on all the time.

 

For what it's worth, I have a '99 F250 superduty 4WD with the V10. I only hear the radiator fan kick in and roar when I am towing my travel trailer. When I am just tooling around unloaded, I never hear it.

 

A scanner will only read the sensors and if a sensor is bad, It will show a bad readout. Put a thermometer in the degas bottle and see what it actually is in there. (borrow a candy thermometer)

 

If everything is working properly you shouldn't hear the fan unless you have a load. Sounds like the t stat is working properly with the temps you gave...enjoy

 

I know that this reply is late. I just went through this same thing myself with a 4.6 L engine. I will say up front that I am not a mechanic, but I do do all my own service work for all my vehicles with continued sucess. I just replaced the heads on this 4.6 engine and was experiencing the same issues. What I first thought was I got a poor head gasket seal but knew that I did not have these problems prior to the repair. The engine has never over heated since I have owned it, What I have learned from this experience is not enough antifreeze, forgetting to put the cap back on or tight enough ( twice i am embarrassed to say) all contributed to the over heating problems over the past 4 days. The Tritan engine has a protect mode. It will go into the P1299 code and run the protection program to keep the motor from frying. In this mode it will shut the air conditioner off, shut fuel off to cylinders, the temp gauge will go to max reading ( as a visual it is in the fail safe mode) and the oil pressure light and check engine lights may come on. This all works to keep the motor cool enough to get to safety and service. The key selling feature of the Tritan engine was that it could be run without water and not be ruined. The computer would shut the engine down if it got to that point. Trapped air in the system would make a sensor read wacky but sensor would have nothing to do with overflowing. But dumping the antifreeze to replace the sensor and refilling the system bleeding all the air out in the process would correct the air pocket problem. Pressurized system raises the boiling point along with the antifreeze added to the water. When you remove the pressure off the closed cooling system it will lower the boiling point and it will over flow the tank. Also any trapped air will push antifreeze out of the tank as it escapes. Air pocket do not cool engines and would also give false readings to the sensors. I did find several online sights that tell you how to test sensors before replacing them. I my experience every time I have been told it was a bad sensor it has never been true. Do not be fooled by part replacing mechanics. Hope this helps anyone that finds this thread looking solve their problem. My advice is if every thing has been normal and you do not see a leak, check to see that the coolant level is not low and that the cap was not loose enough to loose coolant to evaporation. I would replace the cap as the first part. as it is under 10 dollars and an easy fix and monitor for changing levels in the coolant as it should never change unless there is a leak!