What Happens When DEF Contaminates Hydraulic Fluid

[MikePhua]

The Nature of DEF and Its Chemical Risks
Diesel Exhaust Fluid (DEF) is a solution composed of approximately 67.5% deionized water and 32.5% urea. It’s designed to reduce nitrogen oxide emissions in diesel engines equipped with Selective Catalytic Reduction (SCR) systems. While harmless in its intended use, DEF becomes highly corrosive when introduced into hydraulic systems. Urea, a nitrogen-based compound, can degrade seals, corrode metal surfaces, and destabilize hydraulic oil chemistry. Even small amounts—such as one gallon in a skid steer’s hydraulic tank—can trigger widespread contamination.
Terminology Notes

• DEF (Diesel Exhaust Fluid): A urea-water solution used in emissions control.
  
• Hydraulic Fluid: Oil used to transmit power in hydraulic systems, sensitive to contamination.
  
• Polishing Filter: A fine filtration system designed to remove microscopic contaminants and water.
  
• Cylinder Retraction: A method to reduce fluid volume in hydraulic cylinders during draining.
  

Symptoms and Immediate Response
In one incident, a CAT 262D skid steer received a gallon of DEF into its hydraulic tank due to misidentification. The machine continued to operate briefly, but milky fluid appeared in the tank—an early sign of water contamination. The team drained the tank, replaced the filter, and added fresh hydraulic oil. However, only about 85% of the fluid was removed, and the new oil quickly turned milky again.
Key symptoms included:

• Milky hydraulic fluid
  
• No immediate performance loss
  
• Persistent contamination after fluid change
  

Why DEF Is So Damaging
Urea is water-soluble and corrosive. It can attack:

• Pump internals: Causing pitting and erosion
  
• Valve bodies: Leading to sticking and leakage
  
• Seals and hoses: Accelerating degradation and swelling
  
• Metal surfaces: Promoting rust and electrochemical corrosion
  

Unlike diesel or water alone, DEF’s chemical profile makes it harder to neutralize. It doesn’t simply evaporate or separate—it binds with water and oil, forming emulsions and residues.
Recommended Remediation Steps

• Cycle Cylinders Fully: Retract and extend all hydraulic cylinders to purge contaminated fluid from both sides.
  
• Disconnect and Flush Lines: Remove hoses at cylinder ports and flush with clean oil while keeping the tank full.
  
• Install Polishing Filters: Use water-stripping filters for several operating hours to extract residual moisture and urea.
  
• Drain and Repeat: Multiple fluid changes may be required. Each cycle removes a percentage of contamination.
  
• Use External Power: Move cylinders with another machine to evacuate trapped fluid without running the contaminated system.
  

Operator Anecdotes and Field Wisdom
One technician recalled a Quadtrac tractor that required a full hydraulic system replacement after 2.5 gallons of DEF were added. Another mechanic noted that even after multiple flushes, urea residues persisted, requiring lab sampling to confirm system cleanliness.
A field mechanic in Pennsylvania emphasized that filler caps are color-coded and embossed with symbols—blue for DEF, black for hydraulic, green for fuel. Despite this, misfills continue to occur, especially in early mornings or under pressure.
Preventive Measures and Long-Term Solutions

• Lock Hydraulic Filler Caps: Restrict access to trained personnel.
  
• Label Tanks Clearly: Use large, color-coded decals and signage.
  
• Train Operators on Fluid Identification: Include DEF handling in onboarding.
  
• Sample Fluid After Cleanup: Send oil to a lab for urea and water analysis.
  
• Use Filter Carts for Deep Cleaning: Circulate oil through external filtration systems.
  

Conclusion
DEF contamination in hydraulic systems is a serious and increasingly common issue. Its corrosive nature and chemical persistence make cleanup difficult and costly. Immediate action—draining, flushing, filtering, and sampling—is essential to prevent long-term damage. Prevention through labeling, training, and restricted access is the most effective defense. In the age of emissions control, knowing your fluids is no longer optional—it’s operational survival.