i drive my truck all over the place when i am south and it is really hot and only when i am going over 70 mph engine temp starts going higher and higher unless i let go of throttle it wont go down but its fine in dead stop traffic and at speeds below 70 and this is only in hot weather what shouls i check for or do i need an exhaust , its a 99 f350 dually psd.


thanx to all !!!

My guess is yout thermostat is not opening, that would be my first place to look.

I doubt it's a thermostat problem, or it would have showed up way before 70+ MPH... How many miles are on the truck? I am thinking you are running into the limits of your cooling system. I ask milage because it's possible that your cooling system needs some attention. Could be the rad is semi plugged up either the water side or dirt on the front of it, the antifreeze/water mix could be off, (more glycol will actually make an engine run hotter), or your water pump could be rusted out on the impellers (doubtful).


For your truck to behave normally in every other aspect, but get strange at higher speeds means you have most likely a minor problem that only shows itself when the truck is really pushed. The other fairly obvious thing is the fan clutch, is it engaging at all? If it is you should hear a pretty good roar from the fan. I'm guessing that at 70+ you're turning at least 3000 RPM. If there isn't enough air coming through even at 70, there may not be enough to take the heat off. Lemme know.