My 2001 F150 Temp Guage will slowly start to rise and continue to rise all the way to hot only after I have been driving it for about 30 mins and then when I sit in traffic, If I am going down the highway it will not get hot or if I start it up and let it idle (with ac on) it will not get hot. I have shut it off when the temp is almost all the way to hot,slowley released the pressure through the radiator cap,After about 5 mins, started it back up, and the temp will be normal and stay there when idleing. I have replaced the thermostat with no effect on it. Is it air in the system? (if so how do I bleed it?) Faulty sensor? (where is it located on the 4.2 V6?) or is something else causing this? I am baffeled!!! Looking foward to your replies. Thanks, Bob

 

I have an 02, 4.2 V-6 and it is very hard to make sure the coolant level is full as the recovery tank feeds into the lower hose. Keep running the engine with recovery tank cap off, heater on and it will purge the air if it is present. Cooling system operation the basics. Slow engine speeds like city driving, stop and go stuff, the fan pulls air across the radiator to cool the water. High speeds, ram air cools the water. This means, high speeds like 40 MPH and more and getting hot points toward radiator being at fault (plugged up). Overheating in traffic or at stops, points toward not enough air pulled across the radiator (fan clutch). Your starting the engine and AC on and not getting hot is most likely because the idling engine is really not generating enough heat to get that hot. With engine off, spin the fan, there should be resistance to its turning. If it spins easily the clutch is bad. Hope this helps.