1993 Oldsmobile Cutlass Diesel heater blows cold and engine overheats

1993 Oldsmobile Cutlass Diesel mileage: 100,206. The heater is only blowing out cold air and the car is overheating. I've had the radiator boiled and cleaned, changed the water pump, put a new thermostat in and got a new heater core. I've also changed all the belts. But the car is still overheating and the heater is still blowing cold air. Any idea's what to check next? (Also, I've bled all of the lines and checked for blockage. I am at the end of things I can think of to test.)