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Fluctuating Coolant Temperature

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Totally normal, a waste of time and money to fix.

When these trucks were sold new the dealer was supposed to tell the new owner the following and a sheet came with the owner's manual that said:

NOTE: THE COOLING SYSTEM USED ON THE CUMMINS DIESEL ENGINE EQUIPPED VEHICLES PROVIDES THE CAPACITY AND PROTECTION OF THE HIGHER GVWR AND GCWR RATINGS AVAILABLE ON THESE VEHICLES. OWNERS SHOULD BE ADVISED OF THE NORMAL SLOWER WARM UP AND LOWER ENGINE OPERATING TEMPERATURES ASSOCIATED WITH DIESEL ENGINES.

THE COOLANT TEMPERATURE GAUGE WILL FLUCTUATE ON DIESEL ENGINE EQUIPPED VEHICLES. IT IS INFLUENCED BY VEHICLE LOAD, OUTSIDE AMBIENT TEMPERATURE AND EXTENDED IDLE TIME. THIS FUNCTION IS CONSIDERED NORMAL

GAUGE FLUCTUATION WILL BE NOTICED MOST OFTEN UNDER STEADY STATE (HIGHWAY) DRIVING.

PLEASE ENSURE THAT YOUR CUSTOMER IS AWARE OF THIS NOTE PRIOR TO RELEASING THE VEHICLE.

In 1998 after years of complaints Cummins electronically turned the coolant gauge into basically an idiot light.

The temp still fluctuates, you just can't see it. :lol:

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I've found that most aftermarket thermostat (Napa) have a smaller hole to help with getting a steady temperature compared to Cummins thermostats that typically wander too much because of the larger opening.

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