Case Studies

Be DEFinite About Your Mercedes, Land Rover, Audi, VW, Porsche and BMW Diesel DEF Fluid

Be DEFinite About Your Mercedes, Land Rover, Audi and BMW Diesel DEF Fluid

DEF (Diesel Exhaust Fluid) – The Other Engine Fluid
Along with oil, antifreeze, transmission fluid, brake fluid, and fuel, modern diesel engines also need DEF to run properly. All North American Mercedes Sprinters built after 2010 have DEF tanks, same with Audi, BMW, Land Rover, Porsche and VW.

DEF helps with emissions control and the computer in your vehicle will stop you from driving if it detects an empty DEF tank. But don’t panic if the DEF light on your dash comes on. You’ve got plenty of time to fill up.

So, what is DEF, why does your engine need it, and what’s the best way to fill the DEF tank?

What is DEF?
Diesel Exhaust Fluid (DEF) is a mixture of urea and water. Basically the same ingredients as urine. But don’t try peeing into the DEF tank to refill it – DEF is a specific proportion (32.5% urea, 67.5% water) and pretty pure. The impurities in your pee would mess up the SCR catalyst. Also, be sure the DEF fluid you select is compatible with your vehicle,

Urea is nasty stuff – it’s corrosive and will eat through copper cables. In the exhaust system, its job is to convert nitrogen oxides (NOx) into water and nitrogen. The DEF is injected into the exhaust pipe just before the selective catalytic converter (SCR system). The exhaust has already been treated several different ways by this point. The DEF breaks down into ammonia in the hot exhaust gasses. The SCR helps this ammonia to react with nitrogen oxides. This reaction produces water vapor and nitrogen – neither of which cause pollution issues.

The DEF Dash Light/Display
How much DEF you use will depend on your driving style, but it’s only a tiny proportion of the amount of diesel you use.
Some 2016+ vehicles have a DEF level indicator in the dash display (if you have the high line display with the steering wheel buttons). Everyone else pretty much as to guess how full their DEF tank is, or just wait for the dash display to light up.

When there’s 1.5 gallons of DEF left, the vehicle will warn you to refill.

DEF dash light
DEF dash light – display will also say “DEF Check”

If you have the high line LCD display, it will tell you to “Check Additive” (not a useful message) or “Check Diesel Exhaust Fluid” on newer vehicles. Ignoring the warning isn’t a good idea. When the DEF level gets to 0.8 gallons, the check engine light will also come on, and the van will tell you that you only have a certain number of starts left. After that, it will go into limp home mode and you won’t be able to drive at speeds above 5 miles per hour.

The sensor in the DEF tank seems to still be an engineering work-in-progress. It’s not uncommon to hear reports that it has failed, even though they’ve filled the tank up, part of this is due to the corrosive nature of the DEF fluid itself. The NOx sensor in the exhaust system also fails occasionally, and that can lead to DEF warnings too. The dash light won’t go out as soon as you add more DEF. It will take a couple of starts for the van’s computer and sensors to update.

What Does “Poor DEF Quality” Mean?
The dreaded warning messages, they always lead to the same thing, lost revenue and unexpected expenditures. So, you are driving, and you receive a message on the dash reading “Poor DEF Quality.” What does this mean? What should you do about it, or not do? By the end of this post, our hope is that you will have a clear understanding as to what this message is suggesting and how you can go about getting yourself back up and cranking out the miles as efficiently as possible  “Poor DEF Quality”…may not always mean your DEF quality is bad.

To start, “Poor DEF Quality” is highly suggestive and worded in a way that can be misleading. I believe the best way to word this message would be “High NOx Emissions Detected” or “Poor SCR Efficiency” (some mfg use this terminology). Either of these would be a more accurate representation of what is wrong and better encompasses its potential causes. In short, this message means that the NOx emissions measured at the outlet NOx sensor are higher than the computer expects for a given situation.

What can cause this message to come up?   The potential causes are a more extensive list than the dashboard message suggests.
While it is common that the root cause is poor DEF quality, it is not a guaranteed cause by any means.

Potential causes beyond bad DEF fluid include: 

• Bad NOx sensors
• Contaminated SCR box
• DEF dosing issues

• Potential EGR issue

Methods of Testing DEF
Technicians can use various methods to test DEF quality, such as a refractometer where you take a sample of the DEF, place it on the sight glass, close the lid, and hold it up to the light. Ideally, you expect to be at a level of 32.5%. Investing in a refractometer can pay great dividends if it can prevent even one roadside breakdown caused by an engine derate.

Another test option is putting test strips in the DEF tank. Test strips are the most cost-effective and easiest test solution for technicians. If the strip stays the same color, DEF is in good shape. However, it is crucial to know exactly when to test the DEF.

When to Test DEF
There are a few indicators that a technician should test DEF quality, including:

  • Fault code(s) indicate poor DEF quality. An engine light will typically appear, and technicians should test DEF and perform a comprehensive diagnosis of the aftertreatment system.
  • A vehicle sat for an extended period of time. If your fleet has a truck experiencing longer downtime, you may want to test DEF before dispatching the vehicle as DEF can reach temps over 160 degrees, shortening the lifespan.
  • DEF has a noticeable odor. Typically, DEF may have a slight ammonia or sulfur smell, but a noticeable, pungent scent could indicate poor quality. When testing, technicians should smell the DEF tank to ensure there isn’t cross-contamination.

Overall, the interval for testing DEF quality will depend on factors such as the vehicle’s usage, operating conditions, and manufacturer recommendations. Technicians shouldn’t wait for other signs that the DEF has gone bad.

Diagnostics At Your Fingertips
Diagnosing through the list can be tough if you have nothing else to work with. But if you have any recent trouble codes, they could give you some direction as to the best place to start your diagnosis.

If this message arrived shortly after a DEF fluid refill, the easiest thing you can do is to test your DEF fluid. Is it good? If it is good, what should you do next?

The next thing to do is a thorough visual inspection. Look over the entire exhaust and DEF systems, including the EGR all the way through to the SCR. Any visible leaks, excessive corrosion, or unusual wear typically suggests a problem area.

If your truck has not received all its factory recommended maintenance, a thorough diesel tune-up would make sense. A new DEF filter, EGR tune-up, and a fresh set of NOx sensors can go a long way. It can eliminate the potential of skewed sensor readings or poor EGR performance, and it can assure you that you’ll have the correct information feeding the computer for future diagnosis if the problem continues.

After a diesel tune-up, a recent code history could help if you had some recent issues, but no items replaced, it may be the root of your headaches. For example, if you had a DEF pump code recently, it might make sense to diagnose the DEF injection system. Beyond that, you will want to start digging into the individual components and testing each one.

While there is no way to eliminate all the potential issues, maintenance does wonder in the prevention of unexpected problems. During the diagnosis process, we use a very sophisticated diagnostic computer be used to put our vehicle into a forced DPF regeneration, which is often a part of diagnosing aftertreatment issues.

An Ounce Of Prevention
If you’re an Atlantic Motorcar client, we’re going to be keeping an eye on this for you.
If you’re not, make sure it gets checked, kind of like that old bromide about voting, “early and often”. 😉

Our Goal For You and Your Car
Our goal is to save you money, not spend it. This is why each and every car that we service gets a free Courtesy Maintenance Inspection during its first visit. Our goal is to let you know about the small problems before they become big ones. Right now, we have a number of customer cars with well over 200,000 miles, and several approaching 300,000! And these cars are not just limping along – most look and drive pretty much the way they came out of the showroom.

Proper maintenance is an investment in the life of your vehicle.
Knowing, not just doing, but actually knowing and understanding the difference, makes all the difference.
Be sure your car is properly loved, our professionals will attend to both you and your car’s needs.
Atlantic Motorcar…Extraordinary Service for Extraordinary Cars, just a phone call away, (207) 882-9969.


Accessibility Toolbar