How to Flush a Diesel Radiator

A truck with a diesel radiator.
  • 1-3 hours
  • Beginner
  • 30-60
What You'll Need
20 gallons of distilled water
Antifreeze: follow engine's manufacturer instructions for type
Large bucket/collection container
3' length of 1/4" tubing

To keep your diesel radiator running at peak efficiency, it's important to flush and refill it on a regular basis. Many people wait until the antifreeze looks badly contaminated, but if you anticipate this and monitor it on a regular basis, you can keep your coolant system in peak condition. Check your vehicle's manual for suggested time periods. It is not difficult to learn how to flush a diesel radiator. One rule of caution is to always work on your radiator when it is cool. For your safety, do not work on it while it's still hot.

Step 1 - Prepare Vehicle

If at all possible, park your vehicle on a slight incline, facing downhill. It is helpful to turn the wheels facing away from the hill in order to prevent it from rolling downwards while in park. Put on your emergency parking brake on at the same time.

Step 2 - Allow System to Cool

Allow your vehicle to completely cool down, at least one to two hours.

Step 3 - Drain System

Place the bucket under the radiator drain and open it with a pair of pliers. You may find it handy to use a 3-foot length of ¼ inch tubing to direct the flow into the bucket, although this is not necessary. Open the radiator cap.

Step 4 - Refill System with Water

Once all the antifreeze has fully drained, close the radiator cap and tighten the radiator drain plug. Using distilled water, fill the system to its capacity. Start and run engine to circulate coolant.

Step 5 - Turn off Vehicle

Turn off the engine and allow it to cool again.

Step 6 - Repeat Steps 2 to 4

You should repeat steps 2 through 4 of draining and filling with water, then allowing the engine and heater to run, until it drains clear. How dirty your antifreeze is will determine how many times you need to repeat this procedure.

Step 7 - Refill with Antifreeze

Once the water is clear and free of debris, it's time to refill it with fresh antifreeze. There is no concern if there is a little water left in the system. Be sure that the radiator plug is firmly seated. Refill the system with the amount of antifreeze that your engine's manufacturer recommends. Only dilute the antifreeze per instructions on the bottle or by the manufacturer of your vehicle. Some antifreeze requires an additive, so be mindful when you are purchasing it that you have what you need. Replace radiator cap.

Step 8 - Start Vehicle

Start your vehicle and turn your heater on its highest setting. Allow to run until it reaches its normal operating temperature. At this point, your vehicle is ready to drive.

Check the coolant in your diesel radiator after running for a few days to be sure that it is at its proper level and there are no leaks.