I went on a long trip and the truck started to overheat going over a 2000 ft mountain pass. I was towing a 9000 pound travel trailer. I pulled off the road and put it in neutral and revved up the rpm to 3000. It cooled down quickly in a few seconds to normal temp. Happened twice til I made it over the hill. Traveled the coast and made it up the grape vine. Crossed the valley in 100 degrees going 65 and no problems with AC running. As I got home with in the last few miles it over heated again. Took it in and had 25 percent blockage. Replaced radiator TS. Next day hauling dump trailer on slight grade and overheated again. Just replaced water pump and fan clutch yesterday. Over heated today not carrying a load. I was driving at low rpm around 1100 at 50mph up a grade on purpose to test it. It got hot then just dropped in temp. Got hot again and then got cool quickly. Hot again and I punch it into passing gear and it cooled down. I notice if the rpm's are above 3000 it will not over heat. I did not hear the fan kick in but also had two teens talking in the cab. I also hear trickle sounds from behind the glove box. Told it was normal but never heard it til a few months ago. I have had this truck since it was new. Also read it could be the radiator cap or valves. Truck runs great as I also replaced the spark plugs for first time and I change oil regular. No smoke or water leaks. Could it be the gauge or?
Thanks in advance!

 

It can always be a bad gauge, but that's really the only guess I have. I'm gonna move this one down to the 6.8L V10 forum and stay subscribed for the solution.

 

Do you have any air flow blockage behind the grille?

 

First off if you are refering to the stock gauge on the dash it is worthless. You need to monitor the actual temp with a scan gauge 2 or similar device. This will be much more acurate. 2nd, what brand Therma Stat and temp Range did you put in? There are many posts about the Therma Stats being a problem causing overheating. It almost sounds like you have one that is rated too highf for your vehicle. Thats why when it gets hot and you rev it up the temp drops. I could be wrong but it sounds like a Stat to me.
The only other thing that concerns me is you said about the drip. That sounds like iy could be your heater core. Do you smell coolant in the cab? Are you ever low on coolant? Have you tested your mixture to see if it is mixed correctly?

 

No loss of fluid and no smell. Replacing the cap today with a new one from the dealer, another T/stat and a new sensor. The old sensor looked overheated. Hopefully that will solve it. The radiator guy checked the temp with his gun and it was fine while the truck was reporting hot. No blockage as new radiator.

 

I have now put in new Ford t-stat 195, new radiator(Car Quest), new water pump(Napa), new Ford sensor and have now added an electric fan (Lincoln Continental) in line. I control the switch for the fan. Have not pulled since the fan has been added as I wanted to be sure it didn't cause air flow blockage and defeat the purpose. Before adding the fan or anything else for that matter mine cools down if I sit at a light. So getting off the freeway and using the gun to check the temp does no good as by the time I am stopped it has cooled down already. I am certain mine is air flow but i have cleaned the ac coil and the radiator is new so that should not be it. I too have tried pushing the rpm's but that does not cool the thing down. Sitting in traffic with 10k being pulled with the air on THAT cools it down!! Go figure. Eager to hear if you find a solution. Oh yeah New Ford cap and degauss tank also.

 

Problem solved. It was the t/stat or the cap or sensor.

 

What sensor?

 

Bergy, If you're asking me what sensor, Then the one I changed is not the cylinder head temp but the one next to the t-stat housing.

 

Mine was next to T/Stat. Yesterday the Fuel pump went out. I was hoping for no more repairs for a while.