ford 2002 f150 4.2 v6
 

i have a 2002 f150 4.2 v6.. my temp gauge in my truck slowly fluctuates up and down then buries itself in the red, not right away but after driving it for awhile, i can turn the truck off for a few minutes and the temp gauge returns to normal but then driving it, it goes back up..i've replaced the thermostat , flushed radiator and cooling system had it scaned at autozone codes came up with bad cooling system sensor and IATtemp sensor..replaced both of those sensors and still came back with a bad IAT sensor but i still think its a cooling sensor that i cant find..research that ive found says its behind the drivers side valve cover on the block which i did find but does not look like the cooloing sensor that im finding on-line picture wise,,the info im getting from ford says i have 2..a sending unit and a sensor but they r saying there both near the thermostat housing next to the radiator hose which is not right cause the only sensor there is a 3 prong sensor which is for the crankcase or cam..any help would be apreciated..im not loosing any antifreeze so i dont think its a head gasket and its not the clutch fan or water pump cause i flushed the cooling system with engine on and water was flowing out of it.....HELP ME PLEASE!!!

What were the codes that showed up?bad as in open circuit? need the codes
Does it seem to be running hot or is the gauge the only symptom?

The IAT code was p0113 I changed it but still registered replace sensor and I'm not sure what the code was for the cylinder head sensor but I replaced that one and the code never came back so it was ok..when the temp gauge buries in the red I have pressure built up in the reservoir tank and top radiator hose..I don't know if that's normal or not

You should run a scan with live stream data to determine if the IAT temp reading is close to the temp at the sensor.Could be a connection/short/ground problem ect.The scan can also compare cylinder head temp to coolant temp.
Don't rule out a head gasket because you're not loosing coolant.There still may be combustion gases entering the cooling system and causing these symptoms.
If there is air at the coolant temperature sending unit it can fluctuate the gauge.Can also cause the thermostat to fail to open at the proper temp.
Look for air bubbles in the coolant recovery tank,no heat and gurgling are also symptoms but you're not likely going to notice that in these temps.:)
You say there is pressure in the reservoir(plastic) tank or do you mean the rad itself?

The recovery tank is what I meant..that's we're the pressure is..I can slowly open the cap to the tank real releasing slowly and u can hear the pressure and the water is hot.. I can't find cooling sensor or the sending unit to try and replace those parts..looked at books in auto part stores and it shows my engine and a 5.4 v8 bn the same with cooling sensor and sending unit next to thermostat housing and on my truck the only sensor there is a 3 prong sensor which is for the cam or crankcase

The coolant temp sensor should be between the thermostat housing and the inlet pipe for a coolant hose.The temperature sending unit is to the right of that.You might have to push the smaller hose out of the way to see it.
I am just looking at a picture in a manual but I hope that helps.

I had mentioned that I replaced the cooling sensor..I did not it was the cylinder head sensor that I replaced. I camt find the cooling sensor or the cooling sending unit

Thanks Vaper I will check again. If there is a bad head gasket wouldn't that set off a cylinder head sensor which I replaced

According to my manual the engine coolant sensor is just to the right and back a inch or so from the thermostat housing.
The coolant temperature sending unit for the gauge is to the right of that on the intake just where it angles down a bit.It points at a 45 degree angle to the right .
You could be just getting some early symptoms of a firing ring allowing combustion gases into the coolant system but not bad enough to allow coolant into the cylinder.

These can be difficult to diagnose opposed to a major head gasket leak.There is a detector that can tell if there is combustion gases in the cooling system,they cost about 50 bucks.Or take it to a shop that has one.

I think I'm going to ford tomorrow where than can do a diagnostic on it and do a compression test..$100.00

I just looked in another section and it shows the sending unit is the one on the left closest to the thermostat housing.Hopefully someone can confirm this for you.

Thanks for everything Vaper