In 2010, diesel engines sold in the U.S. are required to meet new Environmental Protection Agency (EPA) emissions requirements. To meet the standards for nitrogen oxide (NOx) reduction, the new Duramax Diesel engines in full-size GM trucks and vans feature a Selective Catalyst Reduction (SCR) system (Fig. 1) that uses automotive-grade urea -- known as Diesel Exhaust Fluid (DEF) -- that is mixed in the exhaust stream, causing a chemical reaction that converts the gases into mostly nitrogen and water.
(Fig. 1)
The DEF in the vehicle must be refilled periodically, similar to the fuel tank, but not as often.
Diesel Emissions Control
The SCR after-treatment system on the new Duramax Diesel engines causes NOx reduction reactions to occur in the exhaust stream. It's called "selective" because it uses ammonia as a reductant to reduce NOx levels within a catalyst system.

(Fig. 2)
The after-treatment system (Fig. 2) features a:
A. Diesel Oxidation Catalyst (DOC)
B. DEF injection and mixer
C. Diesel Exhaust Fluid (DEF)
D. Selective Catalyst Reduction (SCR)
E. Diesel Particulate Filter (DPF)
F. Exhaust cooler
The Engine Control Module (ECM) controls the DEF delivery system. Based on the engine NOx emissions level communicated to the ECM by the NOx sensors, the ECM will send a command to the DEF injector to dose a given quantity of DEF. The injected DEF combines with the exhaust gas with the help of mixers before contacting the SCR. The SCR brick stores the ammonia and through a chemical reaction with the NOx in the exhaust gas produces nitrogen (N2), water (H2O) and small amounts of carbon dioxide (CO2), which are natural elements common in the air.
Diesel Exhaust Fluid
DEF is a colorless, clear solution of water and urea that has 32.5% (+/- 0.7%) of urea by weight.
DEF also is referred to by other names such as AdBlue, ClearBlue, Urea, and Aus32 (Aqueous Urea Solution).
The recommended temperature for storing DEF is between 12°F - 86°F (-11°C - 30°C). To prevent DEF decomposition, it should not be transported or stored at temperatures above 77°F (25°C). The shelf life of DEF depends on storage temperature and exposure to sunlight. At a constant ambient storage temperature of 50°F, DEF may have a minimum shelf life of 36 months. But at a constant ambient storage temperature of 86°F, it may have a minimum shelf life of only 12 months. DEF also freezes below 12°F, but it may be reused once it thaws.
Before using DEF, be sure to check every batch of fluid for potential evaporation.
Driver Requirements
The operation of the after-treatment system for NOx reduction is transparent to the driver. There is no change in vehicle performance or driveability.
The only requirement of the driver is to replenish the DEF when the instrument cluster prompts to do so or at regular oil change intervals, whichever comes first. DEF usage is heavily dependent on drive cycles and will vary from vehicle to vehicle.
The DEF fill location (identified by a blue cap) on GM full-size vans is next to the fuel filler and on GM full-size trucks is on the right rear side of the engine compartment. The nozzle diameter is smaller than the diesel fuel nozzle.
Use the J23688 refractometer to test for DEF dilution. If DEF is contaminated with diesel fuel, the DEF system must be replaced.
If the diesel fuel is contaminated with DEF, the water-in-fuel sensor should detect the contamination. Follow the appropriate diagnosis and repair procedures in the Service Information.
Driver Warnings
A number of driver warnings will display on the Driver Information Center to warn the driver that the DEF level is getting low and should be refilled. Several messages indicate the DEF mileage range that the vehicle will be able to travel before refilling.
A new DEF warning symbol also will flash on the instrument cluster to communicate DEF system warnings. (Fig. 3)
(Fig. 3)
The Driver Information Center messages include:
· Exhaust Fluid Range: XXX - displays available mileage range when the DEF is getting low
· Exhaust Fluid Low Speed Limited Soon - displays when the DEF range falls below a specified range; speed will be limited when the DEF level becomes critical
· Exhaust Fluid Empty Refill Now - displays when the DEF is empty; speed will be limited and other messages will display that provide more information.
If these warning are ignored, additional warning messages will display and the vehicle speed will continue to be severely limited.
When adding DEF to an empty or very low tank, always add at least one gallon (3.78L) of fluid to release the vehicle from speed limitation.
DEF Availability
The DEF infrastructure is expected to grow over the next few years. DEF will be made available at dealerships, fuel stations, truck stops (pumps as well as small containers), and other locations that sell automotive fluids.
DEF is available from ACDelco in one gallon, 10-4022 (19286291), and 2.5 gallon, 10-4023 (19286292), packages. (Fig. 4)
(Fig. 4)
- Thanks to Mike Militello
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