Duramax Diesel Exhaust System Fluid Injectors

The new 6.6L Duramax diesel engine (RPOs LGH, LML) that debuted in the 2010 Express and Savana and 2011 Silverado and Sierra is equipped with an advanced exhaust aftertreatment system to reduce emissions. The aftertreatment system features two new diesel exhaust system injectors. 

The Diesel Exhaust Aftertreatment Fuel Injector, also called the Q57 Indirect Fuel Injector or the Hydrocarbon Injector, supports Diesel Particulate Filter (DPF) regeneration -- reducing particulate matter, or soot, in the exhaust -- by adding fuel to the engine exhaust system. On the previous Duramax diesel engine (RPO LMM), this was accomplished using the cylinder injectors via post injection. Now, the Exhaust Aftertreatment Fuel Injector sprays fuel into the turbo downpipe.

The other new injector is the Diesel Emission Reduction Fluid Injector, also called the Diesel Exhaust Fluid (DEF) injector. It injects DEF into the exhaust gases to suppress oxides of nitrogen (NOx) emissions. The DEF injector is located downstream of the Diesel Oxidation Catalyst (DOC) and upstream of the Selective Catalyst Reduction (SCR) system/DPF. 

Diesel Exhaust Aftertreatment Fuel Injector

The Diesel Exhaust Aftertreatment Fuel Injector, or Hydrocarbon Injector (HCI), is a new type of dedicated diesel fuel injector used during DPF regenerations only. The fuel injector threads into the turbo downpipe tube. (Fig. 4)

Fig. 4

The HCI is commanded on by the Engine Control Module (ECM) and injects fuel directly into the engine's exhaust gases downstream of the engine's turbo. Fuel to the injector is supplied from the low-pressure side of the high-pressure fuel pump. The injector's control valve is located over the right rear cylinder head (Fig. 5). 

Fig. 5

The HCI supplies a measured quantity of fuel into the exhaust gas only during enabled regeneration events. The DOC converts this added fuel into the heat that's needed to regenerate the DPF by incinerating accumulated soot. DOC temperatures are monitored during regeneration by two Exhaust Gas Temperature sensors (EGT 1 and EGT 2). If temperatures are too low, DTC P0420 will set.

The HCI system operates only when enabled (regen enable). On 2010- 2011 model year vehicles, the system isn't used during service regenerations. The service regeneration cycle is driven by post-injection from the engine's eight diesel fuel injectors (just as on the 2007-2010 Duramax engine; RPO LMM). On 2012 model year vehicles, the HCI system is used for both enabled and service regenerations.


Successful on-road DPF regeneration relies on proper HCI function. For Duramax diesel DTCs such as P0420, P24A0, or P2463, the indirect fuel injector should be diagnosed for proper function as described in the Service Information - Indirect Fuel Injector Diagnosis (document I.D. 2400107). The Tech 2 cannot at this time energize the HCI directly to cause fuel to spray, so a coil test tool is needed (J 39021). However, the Tech 2 does have an output control that tests the HCI coil for an open circuit in the key on, engine off mode.

TIP: It is imperative to test the HCI for proper flow quantity prior to replacing the DOC to avoid misdiagnosis of the problem.

If the HCI isn't injecting enough fuel, the regeneration-measured exhaust temperatures (as determined by EGT 1 and 2) may be too low and set DTC P0420. Prolonged HCI difficulty may also set DTC P2463 or P2459.

During the fuel injector diagnosis procedure, the HCI is first purged of air using several six second bursts of fuel spray activations using the J 39021 coil test tool. When air is present within the injector, the fuel spray pattern may appear as individual streams. As air is purged from the valve, the streams should merge into a uniform sheet (cone) of fuel spraying 360 degrees about the nozzle exit. 

One reason DTC P0420 may set is if the HCI isn't providing enough fuel to obtain adequate regeneration temperatures. To determine proper HCI function, perform the Indirect Fuel Injector Diagnosis procedure (document I.D. 2461331), referenced in step six of DTC P0420 (document I.D. 2400107) in the Service Information.

Diesel Emission Reduction Fluid Injector

The Diesel Emission Reduction Fluid Injector, or Diesel Exhaust Fluid (DEF) injector, (Fig. 6) sprays DEF into the exhaust for distribution into the SCR catalyst. The DEF injector is mounted just downstream of the DOC canister on the DOC exhaust pipe.

Fig. 6

TIP: To help maintain DEF injector integrity, let the vehicle idle for 10 minutes immediately after a service regeneration before turning off the engine. This allows time for the DEF injector to cool. 

When servicing the DEF tank and DEF injector, do not overstress the DEF injector's plastic inlet nipple. Make sure the emission reduction fluid exhaust supply pipe retains slack. The nipple can fracture if overstressed. (Fig. 7) 

Fig. 7

When removing the DEF supply pipe from the DEF injector, it may be necessary to flush the connector with water to ease release of the supply pipe from the injector nipple. 

Diesel Exhaust Fluid

DEF (urea) is a clear solution of approximately 32% ammonia and 68% water. When the water evaporates from the fluid, white crystalline deposits (some deposits may appear darker depending on soot incorporation) of urea remain. (Fig. 8) Since this fluid travels through the DEF injector, it is common for these deposits to form at the exit nozzle of the injector and, in some cases, inside the injector. 

Fig. 8

The presence of deposits over the DEF injector's exit nozzle alone shouldn't prompt an injector replacement. Rather, it is very important that the DEF Quantity Test -- refer to the Emission Reduction Fluid Injector Quantity Test in the Service Information (document I.D. 2472450) -- always be performed when diagnosing the DEF injector for performance issues. 

The urea deposits are usually soluble in water. Once the DEF system begins spraying fluid again, these deposits dissolve and clear from the nozzle exit. This includes the crystals that may form on the inside of the DEF injector as well as at the exit nozzle.

If these deposits interfere with DEF injection, perform the quantity test a few times to allow for the DEF crystals to dissolve.  

It may be possible to see the three separate DEF injector outlet ports. (Fig. 9) During the emission reduction fluid injector quantity test, check that the DEF fluid stream from each of these ports form a triangular spray pattern. 

Fig. 9

Temperature also helps dissolve the crystals deposits. Urea melts at about 135°C. Since the quantity test is performed when the vehicle is off, it may take longer for the crystals to dissolve than it would on a running vehicle. This is why it's important to perform the quantity test several times before diagnosing the DEF injector spray and flow performance.

The DEF quantity test should be performed when DTCs P204F, P207F, P20EE, or P202E are set. 

TIP: To help maintain the integrity of any DEF injector removed from a vehicle and returned to the Warranty Parts Center for analysis, install the yellow inlet tube and outlet nozzle caps before shipping.  

- Thanks to Brian Fuller


Does the Diesel Exhaust Fluid get injected into the exhaust during system regeneration?

For Diesel Particulate Filter regeneration, diesel fuel is injected into the exhaust system ahead of the Diesel Oxidation Catalyst to raise the temperature of the exhaust needed for regeneration.

The aftertreatment system, which reduces emissions, uses Diesel Exhaust Fluid (DEF) to react with NOx to convert exhaust pollutants into nitrogen, water and trace amounts of CO2.

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This page contains a single entry by Blog Post published on September 28, 2011 5:57 PM.

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