Diesel Exhaust Fluid Contamination in the Fuel System

Poor diesel engine performance conditions, such as hard starts, rough running or hesitations and low fuel pressure, may be the result of fuel system contamination. The most common sources of contamination are other automotive fluids, including gasoline, engine coolant and engine oil. In diesel engines, one other culprit of fuel system contamination is Diesel Exhaust Fluid (DEF), which may be mistakenly added to the fuel system by owners. (Fig. 1)


Fig. 1


These fuel system contamination issues may be found on many GM vehicles equipped with diesel engines (RPOs LM2, LUZ, LH7, LGH, LWN, LML, L5P, L5D), including 2010-2022 Express, Savana; 2011-2022 Silverado; Sierra; 2014-2019 Cruze; 2016-2022 Colorado, Canyon; 2018-2019 Equinox, Terrain; 2019-2022 Silverado 2500HD/3500HD, Silverado 4500HD; Sierra 2500HD/3500HD; and 2020-2022 Tahoe, Suburban, Yukon and Escalade models.

Fuel system contamination can affect the performance of the fuel pump module, especially the spring and strut components, fuel level sensor and the fuel pump. (Fig. 2)


Fig. 2


Filling the DEF Tank and Fuel Tank

Some customers may add DEF to the fuel system in error due to the location of the DEF fill port. For example, on some full-size trucks, the DEF fill and fuel fill are both located behind the fuel fill door. It’s important to show customers the owner’s manual and other product materials that include details about the correct way to fill the DEF tank and how to distinguish the fill port for DEF from the fuel fill.

To ensure the correct system is being filled, only remove one cap at a time. The DEF fill cap and the fuel fill cap (if equipped) should not be removed at the same time. With the capless fuel fill system, extra caution should be used. The DEF system uses a blue cap while the fuel fill is green.

Fuel System Cleaning

If diesel fuel is mixed with DEF, or if DEF or gasoline is added to the fuel tank, it may lead to repairs that may not be covered by the New Vehicle Limited Warranty.

If the fuel tank is accidentally filled with DEF:

  • Do not turn on the ignition
  • Do not start the engine
  • Do not drive the vehicle

As soon as the ignition is turned on, the fuel pump will be engaged and contaminated fuel will be sent into the fuel system, which may potentially damage the entire system. Inform customers to have the vehicle towed to the dealership or a qualified service facility.

DEF is a corrosive fluid that tends to form deposits on components. If the in-tank fuel pump has been run, then contaminated fuel has been pumped through both the low-pressure system and high-pressure pump.

  • If the engine has been cranked, the entire fuel system from the fuel fill pipe to the high-pressure fuel pump, fuel rail, injectors, injector return fuel lines, high-pressure fuel pipes and low pressure return lines must be replaced.
  • If the engine has not been cranked, the system from the fuel fill pipe to the high-pressure fuel pump as well as the high-pressure pipe from the pump to the rail must be replaced along with the low-pressure return lines.
  • If the in-tank fuel pump has not been run, the fuel fill pipe, tank and fuel pump module should be replaced.

Refer to Fuel System Cleaning in the appropriate Service Information for complete details.

Fuel Pump Module

GM has seen a number of failed diesel fuel pump modules that have been returned under warranty due to DEF contamination of the fuel system. DTC P1029 (Fuel Pump Phase U-V-W Circuits Open) set in the Engine Control Module (ECM), which indicates a loss of continuity with the fuel pump, is a potential sign that DEF was added to the fuel tank.

Other signs of contamination can be seen on the fuel pump module itself, where there may be corrosion of the steel and aluminum components. (Fig. 3)


Fig. 3


DEF contamination also may leach between the plastic portion of the fuel pump and the terminals, leading to fractured terminals and terminal failure. (Fig. 4)


Fig. 4


Recognizing DEF Contamination

Without removing the fuel pump module, it may be difficult to recognize DEF in the fuel tank. White crystals can appear in the form of residue (similar in appearance to salt) when it dries on components. (Fig. 5) Of course, the residue cannot be seen in the contaminated fuel.


Fig. 5


Taking a fuel sample may be one of the most effective ways to determine if there is contamination. Taken from the bottom of the fuel tank or fuel pump module reservoir, the sample may show a cloudy mixture when it’s shaken. Any DEF in the sample will usually separate from the fuel within 20 seconds and sink to the bottom of the sample. (Fig. 6) The same reaction occurs on a larger scale in the fuel tank, where the fuel pump picks up the contaminated fuel.


Fig. 6


TIP: New testing procedures for contaminated fuel are currently being reviewed. Look for additional details about fuel testing coming soon to Service Information.

When servicing a vehicle with contaminated fuel, also check the fuel filter, which has a reservoir that collects water from the filter. The fuel filter reservoir should be drained.

DEF and Fuel Don’t Mix

It should be stressed to customers that if a mistake was made and DEF was added to the fuel tank, the vehicle should not be driven to the dealership for service. The vehicle ignition should not be turned on and the vehicle should not be started. These actions will help limit any damage to the fuel system from contaminated fuel. The New Vehicle Limited Warranty may not cover any fuel system repairs related to fuel contamination.

– Thanks to Rodney Lopez



Cradle Brace Mounting Rattle
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  1. Terrie Ansley says:

    #1. DEF nozzle should not fit in diesel tank. The nozzle should be elongated or squared so that it will not fit the round diesel tank.
    #2 Bad design being right beside each other and the DEF nozzle fitting into the diesel tank. There should at least be an automatic cutoff sensor to prevent further damage.
    #3. Bad color choices for colorblind individuals
    #4. There should be a warning in red letters to DO NOT SART truck if accidentally adding DEF to fuel.

  2. Having a green and blue cap does not seem like a good color combination if a person is color blind.

  3. With DEF in a gasoline engine that has been cranked, the DEF will contaminate and corrode the fuel system. It will be necessary to follow the same basic cleaning procedure, including replacing the low-pressure fuel system as well as the high-pressure system on the engine.

  4. What about def in a gasoline vehicle. had a customer pull up to a card lock station and the bulk diesel nozzle would not fit in their gas fill port so they used the bulk def and filled their tank and made it 1km before the fuel pump died there is no listing what to do if a gasoline system is contaminated with def

  5. I just replaced an entire system on a 1500 3.0 to the tune of $16,000.

  6. ROBERT COOK says:

    A diesel techs new best friend we get one a month around here. That owners manual is the most unread
    book in America

  7. Well, with gigantic block lettering saying DEF I don’t know what more they could have done. Some people can’t be taught.

  8. Chad Roberts says:

    Didn’t really see these issues until GM put the two fill ports right next to each other.

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