January 17, 2020

Accidents Happen: Detecting DEF Contamination Using Oil Analysis

Accidents can happen – even experienced professionals make mistakes from time to time. Take for example the accidental addition of Diesel Exhaust Fluids (DEF) into engine, transmission or hydraulic oils. While not as common as DEF in diesel fuel, we have experienced cases where this has happened. This is an error that could lead to significant damage to your equipment if not resolved quickly.

Diesel Exhaust Fluid (DEF) is a water-based urea solution used to reduce pollution in newer Tier 4 Final diesel engines. This liquid is introduced into the exhaust gas to neutralize harmful nitrogen oxides (NOx) before they enter the atmosphere. DEF fluids have been used for some time now in on-highway truck engines, and more recently in off-road mining and construction equipment.

What happens when oils are contaminated with DEF?

DEF is a water-based fluid, which does not mix well with oils. Introducing DEF into engine, transmission or hydraulic oils can lead to an emulsion or separation of layers within the oil reservoir. This can cause inefficient oil flow, or damage to the oil pump or system components. Water-based fluids also tend to flash off into steam under pressure and high temperatures, which means that they provide poor lubrication, causing systems to run ‘rough’. Over time, this can lead to deposits or excessive wear. DEF contamination will also lead to increased rates of corrosion, which will be most obvious (and damaging) on soft metals such as those found on the clutch plates in a transmission. Finally, the presence of water in any oil will promote oil oxidation, which means that the oil will degrade faster than normal.

Besides water, DEF also contains a chemical called urea (DEF is essentially 67.5% purified water / 32.5% urea). This is a basic chemical (somewhat similar to urine) which neutralizes acids. Oils that have been contaminated by a small amount of urea would probably become a bit more basic, but probably not enough to notice significant degradation. You could notice a change in acidity levels if DEF was added over time, but most issues are from one-time accidents.

How can you detect DEF contamination using oil analysis?

The most obvious sign that DEF has contaminated an oil system is the presence of elevated water levels in your oil sample results. Water contamination can be verified using either the (more basic) Crackle test or the (more sensitive/accurate) Karl Fischer test.

Unfortunately, neither test will not conclusively say whether the water is from DEF contamination, or from some other source of water such as condensation, worn seals, excessive idling, or running an engine too cold. However, if the oil sample has two distinct layers (i.e. free-standing water at the bottom), then it may be possible to confirm the water/urea mixture by using a refractometer.  

What should you do when oils are contaminated by DEF?

If you suspect that your engine, transmission or hydraulic oil has been contaminated by DEF fluid, options include:

  1. Drain off some oil from the bottom of the tank to get to the water layer – This will remove some of the DEF (i.e. bleed and feed approach)
  2. Collect an oil sample and submit for testing – This will help determine problem severity  
  3. Review when your next oil change is due – and schedule an oil change sooner if needed
  4. Review and correct the reasons for the problem – Additional training or modified work processes

Mistakes do happen, but with early detection and proper oil analysis testing, corrective action can be taken to mitigate the effects of DEF contamination.  If you would like to discuss this topic in more detail, please reach out to Fluid Life.


Mark Shierman is the Corporate Director Of Client Services at Fluid Life with over 25 years of experience in oil analysis, lubricants testing, reliability and predictive maintenance. Mark has a BSc in Chemistry and industry certifications including CRL, CLS, and OMA. Mark and his team of experts look forward to assisting you and answer any questions you may have.