Have a early 80's 55b with a cummins 378 v6 just fixed temp sensor and hooked up heater core. So figured will drain coolant and replace it. The temp sits at 140 degrees F. But when I rev the engine it goes to 220 F. But if i let it idle it will come back down .Has new temp sensor so I tried a new gauge same thing. Tried bleeding any air no help. Radiator doesn't seem to be getting hot it stays pretty cool. If i let it run long enough the top will get hot but not the bottom and I think the thermostat is in the bottom hose.? Temp gauge never worked before so not shore if this is a new problem or old I just bought it about a year ago but never really used it. Seems likes a lack of flow either water pump or restriction(possible radiator ). Just not to familiar with this machine. Was going to pull bottom hose off and try and look in and see if blocked. Also heat is not good unless engine is at high idle. Just want to get some ideas Thanks
If the rad is hot at the top and cold at the bottom, that is due to lack of flow. Could be water pump, Suction hose collapsing at high idle, rad core plugged. Normally if it is pump, with the belt off, if you can side to side movement in the pully, the pump bearings are bad and there maybe coolant coming out of the weep hole. The suction hose can get soft over time and collpase at hi idle. the rad core can be plugged, Normal rad is hot at the top and maybe 10 to 15 Degrees cooler at the bottom, if there is a greater temp difference the rad could be plugged. Best estimate is to check the easiest things first and leave the rad until last