2007 Mack CJ - MP8 - High Coolant Temp - No Leaks

Question

Owner-operator here with an MP8. Truck runs fine empty, but as soon as I get a full load of gravel in the box, the temp gauge starts climbing past 220F. The fan clutch is engaging (I can hear the roar), and I've tried changing the thermostats twice now.

I don't see any bubbles in the surge tank and there's no oil in the coolant. Radiator looks clean on the outside, I hit it with the air hose and got some dust out but nothing major.

ReganRegen · Answer

✅ Confirmed fix: It was the water pump impeller. On the early MP8s, the plastic impeller can actually loosen on the shaft. When the engine is cold or at low RPM, it turns fine. But under high load/RPM, the impeller slips and doesn't move enough volume. Replaced with the updated stainless steel impeller version and now it stays at 190F all day.

GrandmaGeartooth · Answer

Get a thermal camera and check the radiator after a pull. You might have internal clogging in the tubes that isn't visible from the outside. If the bottom of the radiator is significantly colder than the top, it's restricted.

2007 Mack CJ - MP8 - High Coolant Temp - No Leaks

Suggest a repair