How to Do a Diesel Coolant Flush

Diesel coolant does more than prevent freezing; its additives protect against cavitation on wet-liner engines, inhibit corrosion, and keep passages clean. Over time those additives deplete and deposits form, which is why a periodic coolant flush is essential maintenance, not optional. A proper flush removes old coolant and contaminants and refills with fresh, correct coolant. Here is how to do it thoroughly.

Why coolant condition matters

• Depleted additives allow corrosion and, on some engines, liner cavitation
• Silicate dropout and scale plug narrow passages like the 6.0L oil cooler
• Mixing incompatible coolant types can form gel or sludge
• Old coolant loses its ability to protect against overheating and freezing

Step-by-step flush

1. Let the engine cool completely, then place a large drain pan under the radiator or block drain.
2. Open the drain and let the old coolant out fully. Dispose of it responsibly; it is toxic to people and animals.
3. If the system is dirty, fill with clean water or a proper cooling-system flush, run the engine to operating temperature with the heater on hot, then drain again. Repeat until it runs clear.
4. Close the drains and refill with the correct coolant type and mix for your engine. Never guess the type; using the wrong coolant causes deposits or gel.
5. Bleed the air out using the bleed screws, running the engine with the cap off where the system allows until the level stabilizes.
6. Run the heater on hot to purge the heater core, and top off as needed.
7. Recheck the level and inspect for leaks once cool.

Use the right coolant

Diesel coolants come in several chemistries (conventional, OAT, HOAT, and others), and some engines require a supplemental coolant additive (SCA) or a nitrited formula to protect cylinder liners from cavitation. Using the wrong type, or mixing types, is a common and expensive mistake. Match the coolant to your engine's specification, and if your platform uses SCAs, test and maintain them on schedule.

Getting a complete flush

A partial drain leaves old, depleted coolant behind, so a thorough job matters:

• Drain the block as well as the radiator where a block drain exists, since a lot of coolant hides in the block.
• Flush with clean water or a proper flush product until it runs clear.
• Bleed the system completely so no air pocket causes a false overheat.
• On engines prone to deposits, such as the 6.0L Power Stroke, consider a coolant filtration kit to keep the new coolant clean.

Fresh, correct coolant, bled properly, protects the water pump, radiator, EGR and oil coolers, and the block for years.

Frequently asked questions

How often should I flush diesel coolant?

Intervals vary by engine and coolant type, but every few years or per the manual is typical. Sooner if the coolant looks dirty or additives have depleted.

Can I mix coolant types?

No. Mixing incompatible chemistries can form gel or sludge. Match the coolant to your engine's specification and do not mix types.

What are SCAs and do I need them?

Supplemental coolant additives protect cylinder liners from cavitation on some diesels. If your engine requires them, test and maintain them on schedule. Need coolant or a filtration kit? Email sales@coredieselparts.com.