I have a 2002 F-150 4.2 liter auto 80k.
From cold start reaches 200 degrees in 5 minutes, over the next 25 minutes it slowly but steadily rises to 245, when the light comes on I shut it down. I have been fighting this problem for several weeks now, have searched here but dont find any help.
New stat and cap, flow tested rad and pump. when I pulled the radiator the front was clogged with debris as well as the condenser, cleaned, tested and reinstalled, I thought this would cure the problem. no change. Any help would be appriciated.

 

How did you do your rad flow test? How about a faulity fan clutch.

 

Most likely your fan clutch is bad.Check it cold and warm.
Cold; before you start the engine spin the fan.It should turn easily.
Warm; start the engine and let it warm up.Shut off the engine.Spin the fan.Some resistance should be felt.If it turns easily replace the clutch.

 

Have you checked the lower radiator hose? I've had cars overheat on the highway (hose collapsed, no spring in hose, etc), but run cool at idle. I suppose due to the water pump suction (output) is sufficient to collapse lower hose. It's worth looking into. Good luck!

 

rad flow test done by radiator shop-hose not collapsing-fan clutch good. overheats regardless of driving condition, hiway,surface streets or long periods at idle. I appriciate your continued responese.

 

How about checking the cooling system for exhaust gases ( head gasket problem) sounds like everything in the cooling system has been addressed.

 

Grasping at a straw: could the sending unit/gauge be bad? You can verify the temp with a thermometer that will register up to that temp (grocery store may have some type of cooking thermometer). You stated you can see the temp at 210/245; you might consider verifying the temp with the gauge set.

 

I don't mean to sound like a smarty but are there specific instructions on reservicing the cooling system after doing maintenance on the system? I had a MKVIII and the 4.6L in my truck; they both have to be serviced through a port in the cooling system. I had left the reservoir cap off of the Mark and it overheated quick! The owners book showed the access point for ensuring all air was out of the system. You may be encountering pump cavitation; check the book. Good luck.

 

After I fill my system I always squeeze the upper hose to push the air out of the system,and it works! You can watch your de-gas bottle as the air is burped out of the system and coolant introduced!
Its possible you have an air bubble in the system somewhere.