How to Change a Cummins 12 Valve Thermostat

by Aram Khayatpour

The thermostat in your Cummins diesel engine allows it to sustain optimal operating temperature for a majority of the time. Without a well-performing thermostat your truck may run poorly, produce excess emissions, or overheat while towing (a common problem in the Cummins 12 valve). To avoid this it is best to replace the thermostat whenever symptoms of wear and poor performance are present, or as directed by your truck's owner's manual.

Place a large, empty fluid container under the drainage plug on the bottom of the radiator.

Open the radiator cap at the top. Then open the drainage plug at the bottom.

Remove the section of hosing that leads out of the radiator and into the metal elbow that bolts into the engine block.

Remove the three bolts that attach the metal elbow to the engine block. Then remove this piece to expose the old thermostat.

Extract the old thermostat and discard it.

Clean the thermostat housing thoroughly with a clean terrycloth towel.

Put the new thermostat into the thermostat housing.

Place some high temperature silicone sealant around the base of the metal elbow in a circle and bolt it back into place.

Reattach the hose to the metal elbow and the radiator.

Screw the drainage plug back into place.

Refill the radiator with coolant and screw the radiator cap back on.

Turn on the engine and let it run until it reaches the normal operating temperature.

Let it cool. Then open the radiator cap and top off the fluid level.


  • check Consider doing a radiator flush at this point, as you will be draining the system and doing most of the work involved in a radiator flush anyway.


  • close Never perform this job when the engine is hot; allow adequate time for the engine and the liquid inside to cool. Neglecting to do so could result in severe burns from very hot coolant.

Items you will need

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