I was a Cummins mechanic back in the 1980s so I dont know if any of these suggestions is your problem.I probably worked on a 8.3 but my memory is not so good about it.Seems like there is a float or a ball with a spring on it or some kind of valve in the Radiator.That can give you lots of trouble with overheating and what it did on the newer motors was made like a overflow tank inside the radiator.Seems like it was on the first air to air trucks.Im sure that was on big cam 4 855 trucks.You should get a break down view of your radiator if there is one and see if maybe that is something causing your trouble.The reason I say that is this,somewhere there is a switch to make the fan come on when the water gets hot.If you had something like that in your radiator,and Im not saying you do,if it was stuck and the water was actually heating up where the water temp sensor was but not where the fan switch was.I have driven quite a few trucks that would get hot on a big hill and the way the mechanics fixed it was put a manual switch on the fan.All the driver has to do is turn the switch on for real hot weather,pulling a long hill,whatever is not kicking the fan on soon enough is maybe complicated or expensive and a switch is cheaper,I dont know and cant remember right now what the deal was.I seem to remember a wiring harness you could buy just for that purpose to hook up a fan switch.I could be wrong and I never was a parts man anyway.
Other things that caused me trouble on a Cummins was when they switched to a low flow cooling system they switched to 2 thermostats for that weird radiator I was talking about.If somebody replaced the dual thermostats if you have them,there is a piece of brass like a pipe that fits up into the thermostat housing.I think it comes on the thermostat.If you pull the old thermostat out and that piece doesnt come with it,then put a new one in,that thermostat wont ever close completely with that in there.Hope this helped.I dont even know if that stuff is on a 8.3 Cummins or not.Also there is a seal that goes around the thermostat up inside of the housing.If you replace the thermostat you should replace the seal.If that seal doesnt seal its the same as the thermostat being stuck open.I seem to remember a 8.3 has dual thermostats.If it has air to air,dual thermostats and a radiator with a built in overflow tank,Maybe this stuff could help.I know N14 Cummins motors have all of that stuff and more,but thats the newest Cummins owned an 1998 N 14.Other than that those switches that turn the fan on come in different temperatures.If you replaced that,and put the wrong one on,or if it had another one that was damaged and nobody caught it,could maybe be your trouble.I hope you tell me how you fix it,I would like to know.I worked on and owned Kenworths but very few Peterbuilts.However the truck with the weird radiator was a Peterbuilt and so was the truck with the thermostat jammed in wrong and stuck open was also a Peterbuilt.I dont remember if this was only on Peterbuilt trucks or not.Somehow I think everybody had a version of that weird radiator,and all Cummins motors went to low flow water system and 2 thermostats I think, and Peterbuilt was the only truck I ever drove where it caused a problem.Sorry thats about the best I can do for you with what I can remember.Some other big truck mechanics are on here sometimes and maybe they will know more.
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