The 6.7L Cummins engine holds 12.5 quarts (11.8 liters) of coolant at full capacity.

The 6.7L Cummins diesel engine requires 12.5 quarts (11.8 liters) of coolant for a complete fill, including the block, radiator, and reservoir. Always use a 50/50 mix of distilled water and high-quality extended-life coolant (HOAT-type) to prevent corrosion and overheating. Flush and replace coolant every 3-5 years or 150,000-300,000 miles, depending on usage.

Coolant Capacity Breakdown

  • Total system capacity: 12.5 quarts (11.8 liters)
  • Engine block only: ~8 quarts (7.6 liters)
  • Radiator & hoses: ~4 quarts (3.8 liters)
  • Reservoir (expansion tank): ~0.5 quarts (0.5 liters)

Coolant Type & Specifications

  • Recommended type: HOAT (Hybrid Organic Acid Technology) coolant (nitrite-free for newer models).
  • Mix ratio: 50% distilled water / 50% concentrate (pre-mixed coolants are also acceptable).
  • Avoid: Traditional green (IAT) or orange (OAT) coolants-can cause gel formation or corrosion.
  • pH level: Should be between 7.5-10.5 for optimal protection.

Coolant Change: DIY vs. Professional Service

Method Cost (USD) Time Required Tools Needed Pros Cons
DIY Drain & Fill $50-$100 1-2 hours Drain pan, wrench, funnel, gloves Low cost, full control over coolant type May leave old coolant residue; air pockets possible
DIY Flush (Garden Hose) $70-$120 2-3 hours Flush kit, hose, drain pan, T-fitting Removes more contaminants than drain-only Risk of water dilution if not purged properly
Professional Machine Flush $150-$300 30-60 mins None (shop handles all tools) Thorough flush, no air pockets, warranty-safe Higher cost, dependent on shop quality

Step-by-Step Coolant Replacement Process

  1. Prepare: Park on a level surface, let the engine cool completely, and gather tools.
  2. Drain old coolant:
    • Open the radiator drain valve (bottom driver's side).
    • Remove the engine block drain plug (if accessible).
    • Collect fluid in a drain pan (expect ~10-11 quarts).
  3. Flush (optional but recommended):
    • Reinstall drains, fill with water + cleaner, run engine 10 mins.
    • Drain again, repeat with clean water until runoff is clear.
  4. Refill:
    • Close drains, fill radiator with 50/50 HOAT mix until full.
    • Top off the reservoir to the "Cold Fill" line.
  5. Bleed air:
    • Start engine, let idle with heater on high until thermostat opens (~10 mins).
    • Rev engine to 2,000 RPM briefly to purge air, then top off coolant.
  6. Check for leaks: Inspect hoses, drains, and water pump for seepage.

Signs Your Coolant Needs Replacement

  • Discoloration: Rusty, brown, or milky coolant indicates contamination.
  • Overheating: Engine runs hotter than normal (check gauge or warning light).
  • Sweet smell: Coolant odor inside/outside the cab signals a leak.
  • Low reservoir level: Frequent top-offs suggest a leak or evaporation.
  • Gelling or sludge: Thick, gelatinous coolant means it's broken down.

Common Mistakes to Avoid

  • Mixing coolant types: Combining HOAT with OAT/IAT causes chemical reactions.
  • Using tap water: Minerals in tap water accelerate corrosion.
  • Skipping the bleed step: Trapped air leads to overheating or false low-coolant warnings.
  • Overfilling: Too much coolant can cause pressure buildup and leaks.
  • Ignoring the thermostat: A stuck-closed thermostat prevents proper coolant flow.