The story begins in the early 2000s, when the government laid down the law on a new generation of emissions standards, requiring manufacturers to make drastic improvements in fuel economy and emissions. Manufacturers responded by looking at a wide range of new technologies, including Gasoline Direct Injection, electric cars, hybrids, fuel cell vehicles and Advanced Electronic Common Rail Diesels. Over half of the vehicles in Europe are diesels, which have excellent fuel economy and low HC/CO emissions, so German manufacturers chose to refine them and bring them to the U.S.
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There was just one problem with that plan, diesel engines tend to produce very high levels of NOx emissions. NOx limits in the U.S. were always stricter than those in Europe, but the new laws slashed the limit by 90 percent! Engineers at Mercedes, BMW, Volkswagen and Bosch put their heads together and began searching for the ideal NOx solution that would revolutionize modern diesels.
Research focused on three different types of solutions. The first — decrease the amount of NOx coming out of the engine; the second — use a NOx Storage Catalytic Converter to filter it out; and the third, Selective Catalytic Reduction (SCR). SCR injects a liquid urea solution (called Diesel Exhaust Fluid, or “DEF”) into the exhaust where it breaks the NOx down into nitrogen and water. Each technology has different tradeoffs. Reducing the engine’s NOx production is less effective and decreases power. Using a NOx Storage Catalytic Converter is more effective but requires extra fuel to be injected to burn the stored NOx off which reduces fuel economy. SCR is extremely effective and doesn’t reduce fuel economy, but it requires many expensive parts and space for a large storage tank for the DEF. Another disadvantage of the SCR system is the requirement for maintenance; periodically the DEF must be refilled or the system will quit functioning.
Mercedes-Benz and BMW ultimately decided SCR was the way to go. At the time, though, the government didn’t like SCR — they preferred the NOx Storage Catalytic Converter technology VW was promoting because it was maintenance free. U.S. officials were concerned that SCR systems would not be maintained and that the cars would run wild with untreated NOx emissions. The EPA made sure that the Ultra Low Sulfur Diesel (ULSD) needed for these new emissions technologies would be available everywhere by the time the new laws phased in but there was concern that the supply and distribution infrastructure for DEF wouldn’t be ready.
Manufacturers were required to disable the vehicles if the SCR system is not working. Ordinarily, the driver doesn’t even know the system is there, but if the DEF level runs low, warnings appear in the cluster in stages and ultimately the vehicle goes into limited starts mode and counts down. When the count hits zero, most vehicles will not start again. Make sure to refill the tank as part of your regular service on all SCR equipped diesel vehicles!
Diesel Exhaust Fluid is 62.5 percent deionized water and 32.5 percent urea by definition, and the industry standard for its quality is ISO 22241. You can buy DEF from any manufacturer and use it in a Mercedes system if it meets the ISO standard, which should be on the product label. AdBlue® is a German Auto Industry brand name for DEF, and I will use the term “AdBlue” when referring to DEF going forward.
Although you don’t have to buy it from Mercedes they sell a special bottle I really like. You screw it onto the fill port, push it down and the fluid will flow into the tank until it is full. Then you release and unscrew it for a drip free job. It also makes it perfectly clear the tank is full without accidentally overfilling it and making a mess. AdBlue is corrosive so always use gloves and safety glasses and keep it away from painted surfaces. Clean up is easy, just wash it away with water.
An important first step is to make sure that the fluid is good quality. AdBlue has a limited shelf life (at least one year when stored properly) and it is also vulnerable to contamination. On Sprinters, the AdBlue fill port is under the hood and commonly gets water or washer fluid in it instead. On the 166 chassis, it is right next to the diesel fill port, so sometimes it is contaminated with diesel, and vice versa (Figure 1).
Mercedes recommends using a refractometer to measure the quality of AdBlue. Refractometers work by measuring the way light bends as it passes through a liquid. When the chemicals or their concentrations change, the way they affect light changes as well. You want a refractometer made specifically for DEF testing because it will show a scale corresponding to the percentage of Urea; an OTC 5025, for example. Regardless of which one you use, be sure to follow the instructions for proper calibration and testing. Mercedes simply specifies the exact standard of 32.5 percent, but good fluid generally tests within 32 percent to 33 percent (Figure 2). If the urea percentage is incorrect, the fluid is low quality or it was contaminated. In either case, it will be necessary to flush the system and replace the fluid. It’s also not a bad idea to verify the quality of your own shop supply to avoid creating unnecessary contamination problems for yourself and your customer.