Cummins Truck Thermostat Replacement: Step-By-Step DIY Guide

​Have you ever had a vehicle that just wouldn’t warm up in the cold? Is your cabin heat weak and it takes forever for the temp gauge to climb? These are very common symptoms of a bad thermostat. We drive a 2010 Dodge Ram 3500 and this article will show you where the thermostat is located in the Ram, and how to perform a Cummins thermostat replacement yourself.

What Happens When a Truck Thermostat Goes Bad?

After 200,000 miles on our truck, we started to notice it was not getting up to temperature unless we were towing the RV. Around the same time, the truck also started to overheat on us when making very long climbs up mountains with the RV. 

By overheating I mean the coolant temp would continue to climb even after the engine fan turned on and we had to stop twice over the summer to let it cool when coolant temps reached 230F. We never let it go too far that it would have caused engine damage. 

ALWAYS watch your coolant temps when climbing while towing or driving a large vehicle. ​Since we were experiencing temp issues both high and low we safely assumed that the thermostat in the engine was not functioning properly. Time for the Cummins thermostat replacement.

Suggested Reading: What Kinds of Engines Are in RVs?

What Is an Engine Thermostat?

The thermostat is a device that controls coolant flow based on temperature. When the engine is cold it keeps the coolant in the engine, when warm it lets some into the radiator and when hot it forces most of the coolant into the radiator to help get the engine cooled down.

We ran with the truck acting like this for almost a year but finally decided we needed to replace it, and of course, we decided to do it ourselves. We ordered a genuine Cummins thermostat as we were impressed that the first lasted as long as it did. You can get aftermarket units but we have heard that a lot do not hold temp well. This is the one we ordered.

How To Perform a Cummins Thermostat Replacement

On the Dodge Cummins 6.7 engine or any engine for that matter, the thermostat is pretty easy to find. Follow the coolant hose from the top of the radiator to the engine and the thermostat is located underneath where it connects.

On the 6.7 Cummins, all we have to do is unbolt 3 bolts that hold it down to the block to get at it, but unfortunately, there are a few things in the way.

Step 1: Drain the Coolant

Before you start taking anything apart, we actually needed to drain some of the coolants from the system.

This is not required, but if you don’t do this step, when you remove the radiator hose to expose the thermostat you will spill coolant that is in the system above the thermostat all over the engine. We decided to drain about a gallon of coolant out of the system to prevent this.

To drain the coolant, find the coolant drain located on the driver’s side bottom of the radiator and twist it open. Also, open the radiator cap a crack to allow air into the system. Catch the coolant in a bucket or pan and dispose of it properly. If you are on a municipal sewage system you may be able to dump it down the toilet, but check with the city first.​

Step 2: Gain Access to Thermostat (The Hard Part)

​While the coolant is draining, you can start taking things apart to get at the thermostat. Don’t forget about the coolant however and drain your whole system, unless you are going to replace all the coolant at the same time.

2.1 Remove Plastic Shroud

The first thing that needs to be taken apart is the plastic shroud on top of the engine.  Remove the dipstick and unbolt it from the top of the engine. Then remove the plastic cover.

2.2 Remove EGR Cross Tube

For a Cummins thermostat replacement, we need to remove the EGR cross tube due to the design of the engine. Next is the EGR cross tube. This is the exhaust gas recirculation tube that connects the exhaust to the intake.

There are two spring clips on each end of it that need to come off to remove it. It may have electrical connections that need to be removed as well. Once the electrical connections and spring clips are removed, there is one small 8mm bolt on a bracket located underneath the tube that you will need to get with a ratchet and extension. Once removed, take the EGR tube off.

​2.3 Remove Electrical Connector, Exhaust Pressure Sensor Tube, and Heat Shield

With the EGR tube off you can easily see where the thermostat is located. The cast metal housing that it is connected to has an electrical connector and the exhaust pressure sensor tube that needs to be removed as well. Also, remove the heat shield (I think that is what it is) connected on the top of the thermostat housing.

Step 3: Plop the New Thermostat In Place (The Easy Part)

Once all this is disconnected the thermostat housing can easily be removed from the engine by removing the 3 bolts holding it down.

After removing the 3 bolts lift up on the housing to expose the original Cummins thermostat. If you have drained the coolant it should not leak. To remove the thermostat just gently pry it out with a screwdriver!

Prying the old thermostat out ​and comparing the old Cummins thermostat with the new one, it was easy to see that the gasket material that kept coolant from flowing when cold was worn away. Too much leakage was causing the warming-up problem. There was no way to tell if the overheating issue is thermostat-related but let’s put it in and test it!

​To install the new thermostat, just plop it in the same spot the old one was and bolt it back together. Reverse the order you took the EGR apart and refill with coolant.

Step 4: Reassemble and Refill

​To get the truck full of coolant there is a “bleeder” port near the EGR cooler unit you need to crack open with a hex driver.

Slowly pour coolant into the system until it bubbles out of the bleeder port. When it sits full to the top put the screw back to the port and cap the radiator. Make sure the coolant reservoir is full and start the truck.

Check for any coolant leaks around the thermostat housing and let the truck warm up to operating temp. Keep an eye on the coolant level for the next few operating cycles and add to the reservoir if needed.  Use only HOAT coolant with a Cummins thermostat replacement. ZEREX G-05 is what I recommend.  ​

Cummins Thermostat Replacements Make Happier Trucks

After a few months of testing our new thermostat seems to have solved all our problems and is working perfectly! Overall this job was pretty easy and anyone DIY-minded can tackle this with a basic ratchet set.

As winter descends and we are facing colder temps we are very happy to once again have a warm truck! 

Did you replace your truck’s thermostat? Did this help? Leave us a comment and let us know!

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