1990 running hot need some help...
 

howdy!
i have recently done a body swap on a 1990 w250 5 speed 4x4 12v with a 1977 d200 body with the 1990 dash. I installed a mishimoto aluminum rad, electric fan ( the mechanical fan would not fit between the rad and the engine). I installed a 1994 thermostat housing/ elbow to run the newer style cross flow rad and installed a new thermostat. My issue is that as im driving down the highway the temp goes just under the H, then the thermostat opens and then the temp goes just above the cold and cycles back and forth between screaming hot and then back to cold. it never stays between. I am pulling my hair out. i have tried everything from bleeding air out multiple times and changing the coolant type from glycol to extended life. does any one have any guesses to what might be going on?

Bad gage / sensor?

Is it a genuine Cummins thermostat? and is it installed correctly?
Is the fan on a temperature switch to cut in 5-10 degrees above the thermostat?

i finally figured it out. i blocked the by pass port from the thermostat housing fromt he 94 engine to the head. i rigged up some brass fittings and a some silicone hose and a T for my thermostat switch for the fan, problem solved! but now i think the rad is way to big for the ole 12v.