Pesky Diesel Exhaust Fluid

This may seem obvious to some but for the rest of us mere mortals let me explain the virtues of Diesel Exhaust Fluid, also called DEF (pronounced “deaf” ) or Add Blue.   Interesting, in my experience the Mercedes dealerships and service individuals are vastly unaware of the true details of DEF.   Most will try to placate you with superlative answers but when pressed they will truly admit they don’t know.

Diesel Exhaust Fluid (DEF) is a non-hazardous solution, which is 32.5% urea and 67.5% de-ionized water. DEF is sprayed into the exhaust stream of diesel vehicles to break down dangerous NOx emissions into harmless nitrogen and water.

Ultimately it doesn’t matter.  When your Mercedes-Benz Sprinter get’s low on DEF the vehicle computer will tell you and warn you that you only have a certain number of starts remaining.  That’s right, once you hit zero you will not be able to start the vehicle.   Done.  You are stuck.

No need to panic.  You are close to a resupply of DEF.  Merely visit any gas station that sells diesel fuel and you’ll find inside a box or jug labeled Diesel Exhaust Fluid.  This will run you about $7-10 per gallon.

BUY THE FUNNEL!  Yes, spend the $3 and buy the funnel and keep it in your vehicle for next time.  It makes it super easy to pour the big box of fluid into the reservoir.  Your owners manual will give you the diagrams and pictures to get it in the right spot.  Tip… Its the blue cap labeled DEF.

Once you restart your vehicle the computer will register that you added DEF to the reservoir and your life altering countdown to destruction will end.

Few things I learned along the way

1. My salesperson told me that I wouldn’t need to worry about DEF and they would add it when I came in for an oil change at 15K miles.   I have to add DEF about every 6K miles.  The dealership service manager did find the precise calculation on usage and he and I determined this would be a reasonable consumption rate.  There is a big caveat that it depends on how you drive.
2. In the trucking world, these drivers fill the gas and fill the DEF.  No, Sprinter owners don’t need to do this.  Just wait for the fabulous machine to tell you to add it.  You could proactively check if you’re going on a big trip of course.
3. DEF Fluid is incredibly corrosive.  Don’t know for sure but I don’t take any chances.  I always rinse the area with lots of water.  Some Sprinter forums share stories of DEF eating through everything.
4. My friendly dealership in Pennsylvania charged me $50 to fill my DEF tank – I didn’t know it was so simple and no one could tell me.  The kind folks at the Marietta Mercedes-Benz charged me $85 to “check it” when I had an oil change.  It is a $7 bottle of liquid.  You can do this yourself.
5. The vehicle computer provides a mileage countdown until DEF will need to be added.  No idea where this number comes from.  It will usually show a reading of 20K miles but trip the engine start shortly thereafter.  A full reservoir puts the reading at 27K or so miles.   I have pointed the error out to MB service techs to no avail.