DEF and EGR Systems

DEF is a precisely mixed 32.5 percent urea and 67.5 percent deionized water solution. It is injected into the exhaust stream upstream of the SCR (Selective Catalytic Reduction) catalyst. Inside the hot SCR catalyst, the urea breaks down into ammonia, which reacts with NOx emissions and converts them into harmless nitrogen gas and water vapor. Without DEF injection, the SCR catalyst cannot reduce NOx — and diesel NOx emissions are a serious health hazard.

DEF concentration must be exactly 32.5 percent. Contaminated, diluted, or degraded DEF poisons the SCR catalyst — and a poisoned catalyst is a multi-thousand-dollar replacement. Never put anything other than approved DEF in the DEF tank. Never store DEF in metal containers — use only approved HDPE plastic. DEF degrades above 86 degrees Fahrenheit over time, so do not store it in direct sunlight or hot environments. DEF freezes at 12 degrees Fahrenheit. The vehicle's DEF tank has a heater to thaw it, but storage containers in unheated areas will freeze in winter. Frozen DEF expands and can crack containers not designed for it.

The DEF system includes a supply module in the tank with a pump, a filter, and a level/quality sensor. A heated supply line carries DEF to the dosing injector mounted in the exhaust. The ECM commands the injector to spray a precise amount of DEF based on exhaust NOx levels, temperature, and flow rate. A NOx sensor downstream of the SCR monitors the effectiveness. Running out of DEF triggers a progressive derate — first a warning, then a speed limitation, and eventually the vehicle will not restart after shutdown until the tank is refilled. The ECM enforces this by law. The DEF quality sensor detects diluted or contaminated fluid and will set codes and derate for bad DEF as well.

EGR recirculates a portion of exhaust gas back into the intake manifold. Exhaust gas is inert — it does not burn. Mixing it with fresh intake air lowers peak combustion temperatures, which reduces the formation of NOx. Lower combustion temperature means less NOx coming out of the engine, which means less work for the SCR system downstream.

Diesel EGR systems include a cooler that reduces the temperature of the recirculated exhaust before it enters the intake. The EGR cooler uses engine coolant as the cooling medium. EGR cooler failure is a serious concern. A cracked cooler allows coolant to enter the exhaust or the intake. Symptoms: white smoke from the exhaust, unexplained coolant loss with no visible external leak, and potentially hydrolocking if coolant accumulates in a cylinder. If you find coolant loss with white smoke on a diesel, pressure test the cooling system and specifically test the EGR cooler for leaks.

The EGR valve controls how much exhaust is recirculated. On diesels, soot and carbon buildup on the valve is inevitable over time, especially in vehicles that do a lot of low-speed, low-load driving. A valve stuck partially open at idle causes rough idle, misfires, and excessive smoke. A valve stuck closed under load causes high NOx codes. Carbon buildup is the most common EGR valve failure on diesels. Some can be cleaned. Some must be replaced. Inspect the valve with the engine off and command it open and closed with a scan tool to verify full range of motion.