What About The Persistent Problems Of The Independent Gas Drive Urea Injection System?

Guide: with the implementation of national V emission regulations, the vast majority of medium and heavy commercial vehicles on the market are equipped with SCR post-treatment system to meet the requirements of escalating national regulations. At the same time, the pain of card users also follows… Such as: “why is the emission lamp (MIL) always on?”, “Don’t burn urea again!”, “The engine is out of power again!” wait.
Based on the increasingly severe challenge of environmental pollution and the needs of national strategic development, the country has made up its mind to implement national V emission regulations. Although it has promoted the R & D strength of domestic automobile technology and improved the competitiveness of products, there is a large gap of maintenance service personnel for urea injection system in the automotive aftermarket, so there is a dilemma that no one can start in the face of such faults.
Therefore, Xiaobian will focus on the independent gas driven urea injection system of FAW liberation, focus on the “persistent diseases” of these systems, and try to “suit the remedy to the case”, so as to help all card friends’ vehicles return to normal, restore power, improve vehicle attendance, and make card friends earn more money and have a happy mood.
Not much to say, let’s first introduce the urea injection system independently developed by FAW Jiefang. A simple understanding of the structure and principle is also helpful to the use and maintenance of the vehicle. Only by knowing yourself and the enemy can we win by surprise. The introduction of the structure and principle of the urea injection system is relatively boring. Card friends can browse it quickly, save and collect it for follow-up research. However, in the later part of the introduction of urea injection system faults, please taste it carefully and welcome communication.

Structural principle and working process of gas driven urea injection system

The urea injection system structure and principle are shown in the figure below:

The air driven urea injection system is composed of highly integrated urea tank assembly (including pressure sensor, urea liquid level and temperature sensor), intake and exhaust solenoid valve assembly, air filter, electric heating urea pipe, urea nozzle, heating water pipe of urea tank, cooling water pipe of urea nozzle and other parts. The compressed gas generated by the air compressor is used as the air source. The ECU detects the pressure in the urea tank through the pressure sensor, controls the opening of the intake and exhaust solenoid valve to keep the pressure in the urea tank stable, controls the urea injection according to the actual working conditions of the engine, so that the urea can participate in the post-treatment catalytic reduction reaction to meet the emission standard, and delays the evacuation of the compressor gas in the urea tank after the vehicle is shut down. Moreover, in winter, the system can thaw and heat the urea in the urea tank, and electrically heat the urea in the urea pipe to thaw the urea. At this time, the urea in the system meets the normal injection function.
urea injection system working process:
(1) Determine whether the system needs thawing and heating
After starting the trouble free vehicle, judge the current urea temperature and status through the atmospheric temperature sensor and urea temperature sensor. If the temperature is lower than the lower limit, ECU controls the water heating solenoid valve to open, and the coolant at the engine outlet enters the urea tank through the solenoid valve to heat urea. When thawing is completed, the urea temperature T1 = – 5 ℃. At the same time, ECU controls the heating of electric heating urea pipe. Before the completion of urea thawing in urea tank, the urea in urea pipe also completes thawing. After that, the system enters the heating state. It also controls the opening and closing of the water heating solenoid valve to ensure that the water valve is closed when T1 > 5 ℃ and opened when T1 < 0 ℃. At the same time, ECU always heats the urea pipe by PWM.
The thawing and heating processes are carried out at the same time as the pressure building process. If the system does not need thawing and heating, it directly enters the pressure building state.
(2) Pressure building process
When the air pressure of the whole vehicle reaches 0.8MPa or 1.2MPa, the compressed air is directly connected to the air filter switch through APU, enters the air filter, and then passes through the air inlet solenoid valve to inflate the urea tank. The pressure in the tank is p = 0.715mpa. The air inlet valve is closed and the pressure building is completed. In this process, the exhaust solenoid valve is always closed.
(3) Urea injection process
The power source of liquid urea is provided by compressed gas, which passes through the urea filter element, enters the urea pipe, and then to the urea nozzle; ECU performs urea injection according to the actual working conditions of the engine. ECU detects the pressure in the urea tank at the same time. By controlling the opening of the intake and exhaust solenoid valve to ensure that the pressure in the tank is p < 0.685mpa, open the intake solenoid valve until P = 0.715 MPa, and then close the intake solenoid valve; P> At 0.75mpa, open the exhaust solenoid valve until P = 0.715 MPa, and then close the exhaust solenoid valve. In short, ECU will control the opening and closing of intake and exhaust solenoid valves in the whole process to ensure that P is between 0.685-0.715 MPa.
(4) System self check
When the pressure build-up of the system starts and the target pressure is not reached after a period of time, the system will report the failure of pressure build-up, the OBD fault light is on, and the engine torque is limited. After successful pressure build-up, the system will monitor the pressure in the tank in real time. If the pressure drop in the tank is greater than the limit value for a period of time, the system will report the system air leakage fault, the OBD fault light will be on, and the engine will not limit torque in a short time, but if P continues to be less than 0.6MPa, it will cause insufficient injection pressure of urea nozzle, poor urine atomization and less urea involved in catalytic reduction reaction, At this time, the system will report the fault that the nitrogen and oxygen emission of SCR system exceeds 5g / kWh (National V 3.5 g / kWh) or the nitrogen and oxygen emission of SCR system exceeds 7g / kWh, the engine fault light is on and the engine torque is limited.
(5) Emptying process
After the vehicle is shut down, the ECU controls the exhaust solenoid valve to open until P < 0.01 MPa. At this time, the exhaust solenoid valve is closed and the emptying time is T < 1.5min. The emptying function can be calibrated whether it is reserved or not, and the delay time and emptying time can be calibrated.
Reminder: do not turn off the main power switch immediately after the vehicle is turned off. Turn off the main power switch after the exhaust of urea tank is completed. The system cancels the urea reflux function and does not empty in winter. The pressure in the tank will fill the urea pipe with urea, and the frozen urea may expand and crack the urea nozzle.

After introducing the structure and working principle of the system, let’s summarize the failure modes of important parts in the system. Although they are all simple faults, they do affect the engine power. I hope you card friends must read them carefully.

Failure modes of components in the urea injection system

(1) Relevant fuse
Status: the fuses of urea nozzle solenoid valve, intake and exhaust solenoid valve, electric heating urea pipe and water heating solenoid valve are open circuit
Vehicle performance: the OBD fault light is on, the engine fault light is on, the engine torque is limited, and the power is insufficient.
Remarks: when the vehicle power is insufficient, first check whether the fuse is intact and not loose.
(2) Air filter
Status: the air filter is closed or the switch is blocked, leaking, etc; The air filter is blocked, leaking, not replaced regularly, etc.
Vehicle performance: OBD fault light is on, engine torque is limited and power is insufficient.
Remarks: replace the air filter after 50000 km.

(3) Intake and exhaust solenoid valve
Status: poor contact of intake and exhaust solenoid valve connector, water inflow at the connector, short circuit and open circuit of solenoid valve harness, internal open circuit of solenoid valve, stuck solenoid valve, blocked solenoid valve, frozen solenoid valve, etc.
Vehicle performance: there is a problem with the intake solenoid valve and the associated harness connector, the OBD fault light is on, the engine fault light is on, the engine torque is limited, and the power is insufficient. If there is a problem with the exhaust solenoid valve and the associated harness connector, the OBD fault light is on, and the engine will not limit the torque in a short time or immediately limit the torque.

Remarks: the resistance value of intake and exhaust solenoid valve at room temperature is 40-60?, When the intake and exhaust solenoid valve works, the power supply voltage is 24V and the stability maintenance voltage is 3.5V. The inlet and exhaust solenoid valves have different appearance and different functions, but the working principle and characteristics are the same. If necessary, the valve bodies can be interchanged.
(4) Integrated urea tank assembly
Status: leakage or loss of urea cover, open circuit of pressure sensor, short circuit of plug and harness of pressure sensor or urea liquid level and temperature sensor, open circuit or water inflow, stuck or damaged urea liquid level and temperature sensor or short circuit open circuit, cracking of urea tank, blockage or cracking of urea filter element, etc.
Vehicle performance: the OBD fault light is on, the engine fault light is on, the engine torque is limited, and the power is insufficient.
Remarks: ensure that the urea cover is tightened and the urea level is greater than 10%. Replace the urea filter element regularly (50000 km).

(5) Electric heating urea pipe
Status: bending and blocking of urea pipe, short circuit of resistance wire of urea pipe, ablation or short circuit of harness connector, cracking of urea pipe, urea pipe connector, etc.
Vehicle performance: the OBD fault light is on, the engine fault light is on, the engine torque is limited, and the power is insufficient.
Remarks: do not often pull and insert the connector between urea pipe and urea nozzle.

(6) Urea nozzle
Status: blocked urea nozzle, crystallization, ablation or corrosion of harness connector, short circuit or open circuit inside the nozzle, nozzle expansion crack, etc.
Vehicle performance: the OBD fault light is on, the engine fault light is on, the engine torque is limited, and the power is insufficient.
Remarks: when the urea nozzle is blocked by crystallization, it can be cleaned by microwave.

(7) Water heating solenoid valve
Status: the water valve is blocked, the water valve is damaged, the harness connector of the water valve is ablated or water enters, the internal short circuit or open circuit of the water valve, etc.
Vehicle performance: the OBD fault light is on, the engine torque is limited in winter, and the power is insufficient.
Remarks: the engine coolant contains iron filings and other impurities, which can easily block the filter screen of the water valve.

(8) Nitrogen oxygen sensor
Status: sensor harness connector open circuit or short circuit, sensor internal short circuit or open circuit, connector pin corrosion, loose connector or water ingress, etc.
Vehicle performance: the OBD fault light is on, the engine fault light is on, the engine torque is limited, and the power is insufficient.
Remarks: protect the connector of nitrogen and oxygen sensor to prevent water from entering the connector.

(9) SCR front row temperature sensor
Status: exhaust temperature sensor damaged, harness open circuit or short circuit, sensor raised, sensor series resistance, etc.
Vehicle performance: the OBD fault light is on, the engine fault light is on, the engine torque is limited, and the power is insufficient.
Remarks: ensure the factory state, and do not refit or remove the exhaust temperature sensor.

(10) Accelerator pedal, crankshaft sensor and other sensors that affect the normal operation of the engine
Status: accelerator pedal, crankshaft sensor, etc. are damaged or faulty.
Vehicle performance: the OBD fault light is on, the engine fault light is on, the engine torque is limited, and the power is insufficient. The failure of these sensors will affect the monitoring of engine speed and load by the urea system, and then affect the normal operation of urea injection.
When there is a problem in the parts of the urea injection system, the system can detect and report the corresponding fault urea injection system. We can also check the abnormal conditions of the parts or sensors according to the fault.

"suit the remedy to the case" - fault code analysis and inspection suggestions

(1) P205c: the temperature signal of urea tank is lower than the lower limit; P205d: urea tank temperature signal exceeds the upper limit
Vehicle performance: the OBD fault light is on, the engine fault light is on, and the engine will not limit torque in a short time
Recommended inspection:
① Check whether the connector of urea temperature sensor is loose, rusted, watered or damaged;
② Check whether the resistance value of urea temperature sensor is normal and whether the function fails;
③ Check whether the 5V power supply and ground wire of urea temperature sensor are normal;
④ Check whether the signal line of urea temperature sensor is short circuited to ground and power supply line;
(2) P204c: urea pressure signal is lower than the lower limit; P204d: urea pressure signal exceeds the upper limit
Vehicle performance: the OBD fault light is on, the engine fault light is on, the engine torque is limited after 50 hours, and the fault can be eliminated after three normal working cycles
Recommended inspection:
① Check whether the connector of urea pressure sensor is loose, rusted, water ingress and damaged;
② Check whether the resistance value of urea pressure sensor is normal and whether the function fails;
③ Check whether the 5V power supply and ground wire of urea pressure sensor are normal;
④ Check whether the signal line of urea pressure sensor is short circuited to the ground and whether it is short circuited to the power supply line;
(3) P203d: urea level signal voltage exceeds the upper limit; P203c: urea level signal voltage is lower than the lower limit
Vehicle performance: the OBD fault light is on, the engine fault light is on, the engine torque is limited after 50 hours, and the fault can be eliminated after three normal working cycles
Recommended inspection:
① Check whether the temperature sensor connector is loose, rusted, watered or damaged;
② Check whether the resistance value of the temperature sensor is normal and whether the function fails;
③ Check whether the 5V power supply and ground wire of the temperature sensor are normal;
④ Check whether the signal line of the temperature sensor is short circuited to the ground and whether it is short circuited to the power supply line;
(4) P0428: temperature signal voltage before SCR inlet exceeds the upper limit; P0427: temperature signal voltage before SCR inlet is lower than the lower limit
Vehicle performance: the OBD fault light is on, the engine fault light is on, the engine torque is limited after 50 hours, and the fault can be eliminated after three normal working cycles
Recommended inspection:
① Check whether the temperature sensor connector is loose, rusted, watered or damaged;
② Check whether the resistance value of the temperature sensor is normal and whether the function fails;
③ Check whether the 5V power supply of the temperature sensor is normal;
④ Check whether the signal line of the temperature sensor is short circuited to the ground and whether it is short circuited to the power supply line;
(4) P2200: nitrogen and oxygen sensor power supply failure; P2209: nitrogen and oxygen sensor heating failure; P2204: nitrogen and oxygen sensor nitrogen and oxygen signal failure; P2201: nitrogen and oxygen sensor oxygen signal failure
Vehicle performance: the OBD fault light is on, the engine fault light is on, the engine torque is limited after 50 hours, and the fault can be eliminated after three normal working cycles
Recommended inspection:
① Check whether the connector of nitrogen and oxygen sensor is loose, rusted, watered and damaged;
② Check whether the resistance value of nitrogen and oxygen sensor is normal (the resistance between can_h and can_l is 120? And 60? After being connected in parallel with the terminal resistance of ECU), Whether the function fails;
③ Check whether the 24V power supply and ground wire of nitrogen and oxygen sensor are normal and can_ H and can_ Whether l voltage is normal;
④ Check whether the harness of nitrogen and oxygen sensor is worn or short circuited;
⑤ Check whether the battery voltage is too low;
(5) P0073: atmospheric temperature signal exceeds the upper limit; P0072: the atmospheric temperature signal is lower than the lower limit
Vehicle performance: the OBD fault light is on, the engine fault light is on, and the engine will not limit torque in a short time
Recommended inspection:
① Check whether the connector of atmospheric temperature sensor is loose, rusted, watered or damaged;
② Check whether the resistance value of the atmospheric temperature sensor is normal and whether the function fails;
③ Check whether the 24V power supply of atmospheric temperature sensor is normal;
④ Check whether the signal line of the atmospheric temperature sensor is short circuited to the ground and whether it is short circuited to the power supply line;
(6) P181b: urea nozzle control circuit fault
Vehicle performance: the OBD fault light is on, the engine fault light is on, the engine torque is limited, and the fault can be eliminated after three normal working cycles
Recommended inspection:
① Check whether the fuse of urea nozzle is normal;
② Check the resistance of urea nozzle (12?) Whether it is normal and functional failure;
③ Check whether the connector terminal of urea nozzle is rusted and whether the connector is loose;
④ Check the urea nozzle harness for short circuit or open circuit;
(7) P3043: control circuit fault of water heating solenoid valve of urea tank;
Vehicle performance: the OBD fault light is on, the engine fault light is on, and the engine will not limit torque in a short time
Recommended inspection:
① Check whether the fuse of urea water valve is normal;
② Check whether the resistance of urea water valve is normal and whether the function fails;
③ Check whether the connector terminal of urea water valve is rusted and whether the connector is loose;
④ Check the harness of urea water valve for short circuit or open circuit;
(8) P3028: urea pipeline heating relay control circuit fault
Vehicle performance: the OBD fault light is on, the engine fault light is on, and the engine will not limit torque in a short time
Recommended inspection:
① Check whether the fuse of urea pipeline heating relay is normal;
② Check whether the resistance of heating relay of urea pipeline is normal and whether the function fails;
③ Check whether the connector terminal of urea pipeline heating relay is rusted and whether the connector is loose;
④ Check whether the harness of urea pipeline heating relay is short circuited or open circuited;
(9) P3014: air inlet valve control circuit fault of gas drive urea system
Vehicle performance: the OBD fault light is on, the engine fault light is on, the engine torque is limited, and the fault can be eliminated after three normal working cycles
Recommended inspection:
① Check whether the air inlet valve fuse is normal;
② Check whether the resistance of the intake valve is normal and whether the function fails;
③ Check whether the connector terminal of intake valve is rusted and whether the connector is loose;
④ Check the intake valve harness for short circuit or open circuit;
(10) P31824: control circuit failure of exhaust valve of gas driven urea system
Vehicle performance: the OBD fault light is on, the engine fault light is on, and the engine will not limit torque in a short time
Inspection suggestions:
① Check whether the exhaust valve fuse is normal;
② Check whether the resistance of the exhaust valve is normal and whether the function fails;
③ Check whether the exhaust valve connector terminal is rusted and whether the connector is loose;
④ Check the exhaust valve harness for short circuit or open circuit;
(11) P0420: low efficiency of catalytic system
Vehicle performance: the OBD fault light is on, the engine will not limit torque in a short time, and the fault will be eliminated after three normal working cycles
Recommended inspection:
① Check whether the urea solution is qualified (32.5%);
② Check whether the urea nozzle is blocked or crystallized;
③ Check whether the aftertreatment catalyst (silencer) is blocked;
④ Check whether the urea filter element is blocked or falls off;
(12) P0424: heating catalyst temperature too low
Vehicle performance: the OBD fault light is on, the engine will not limit torque in a short time, and the fault will be eliminated after three normal working cycles
Recommended inspection:
① Check whether the front row temperature sensor at SCR inlet is installed normally;
② Check whether the exhaust temperature sensor is raised or pulled out;
③ Check whether the exhaust temperature sensor is connected in series;
④ Check whether the exhaust pipeline leaks;
(13) P1d20: catalyst removed or replaced by dummy system
Vehicle performance: the OBD fault light is on, the engine will not limit torque in a short time, and the fault will be eliminated after three normal working cycles
Recommended inspection:
① Check whether the urea solution is qualified (32.5%);
② Check whether the urea nozzle is blocked or crystallized;
③ Check whether the aftertreatment catalyst (silencer) is blocked;
④ Check whether the urea filter element is blocked or falls off;
⑤ Check whether the post processor is removed or lost;
(14) P1900: urea nozzle blocked
Vehicle performance: the OBD fault light is on, the engine will not limit torque in a short time, and the fault will be eliminated after three normal working cycles
Recommended inspection:
① Check whether the urea solution is qualified (32.5%);
② Check whether the urea nozzle is blocked or crystallized;
③ Check whether the urea nozzle is stuck and atomized well;
④ Drive the urea nozzle with the diagnostic instrument and check whether the injection is normal;
⑤ Check whether the float of urea level sensor is stuck;
(15) P12047: urea nozzle normally open
Vehicle performance: the OBD fault light is on and the engine fault light is on. The engine will not limit torque in a short time. This fault will be eliminated after three normal working cycles
Recommended inspection:
① Check whether the urea injection system leaks urea;
② Check whether the urea nozzle is stuck;
③ Check whether the urea nozzle orifice is of normal size;
④ Check whether the urea nozzle is cracked or expanded;
(16) P1181e: abnormal interruption of urea supply
Vehicle performance: the OBD fault light is on, the engine torque is limited, and the fault can be eliminated after three normal working cycles
Recommended inspection:
① Check whether the urea injection system leaks urea;
② Check the urea tank and urea pipe for air leakage or urea;
③ Check whether the exhaust solenoid valve is not tightly closed;
④ Check whether the urea nozzle is separated from the urea pipe;
(17) P17d4: nitrogen and oxygen emission of SCR system exceeds 7g / kwh; p17d5: nitrogen and oxygen emission of SCR system exceeds 5g / kWh (National V 3.5 g / kWh)
Vehicle performance: the OBD fault light is on, the engine torque is limited, and the fault will not be eliminated until three normal working cycles
Recommended inspection:
① Check whether there are other fault codes in the system. If there are, check and eliminate them one by one according to the suggestions;
② Check whether the urea solution is qualified (32.5%);
③ Check whether the post-treatment catalyst is normal;
(18) P1d21: nitrogen and oxygen sensor removed u1307: nitrogen and oxygen sensor communication timeout
Vehicle performance: the OBD fault light is on, the engine torque is limited after 50h, and the fault can be eliminated after three normal working cycles
Recommended inspection:
① Check whether the connector of nitrogen and oxygen sensor is loose, rusted, watered and damaged;
② Check whether the resistance value of nitrogen and oxygen sensor is normal (the resistance between can_h and can_l is 120? And 60? After being connected in parallel with the terminal resistance of ECU), Whether the function fails;
③ Check whether the 24V power supply and ground wire of nitrogen and oxygen sensor are normal and can_ H and can_ Whether l voltage is normal;
④ Check whether the harness of nitrogen and oxygen sensor is worn or short circuited;
(19) P181f: low urea consumption
Vehicle performance: the OBD fault light is on, the engine will not limit torque in a short time, and the fault will be eliminated after three normal working cycles
Recommended inspection:
① Check whether the urea solution is qualified (32.5%);
② Check whether the urea nozzle is blocked or crystallized;
③ Check whether the urea nozzle is stuck and atomized well;
④ Drive the urea nozzle with the diagnostic instrument and check whether the injection is normal;
⑤ Check whether the float of urea level sensor is stuck;
⑥ Check whether the urea filter element is blocked;
(20) P1d19: urea exhausted
Vehicle performance: the OBD fault light is on, the engine torque is limited, and the fault will not be eliminated until three normal working cycles
Recommended inspection:
① Check whether the urea level is too low (less than 10%);
② Check whether the urea level float is stuck in the low position;
③ Check whether the urea tank and urea pipe leak urea;
(21) p0065: gas leakage of gas driven urea system
Vehicle performance: the OBD fault light is on, the engine will not limit torque in a short time, and the fault will be eliminated after three normal working cycles
Recommended inspection:
① Check whether the urea injection system leaks urea;
② Check the urea tank and urea pipe for air leakage or urea;
③ Check whether the exhaust solenoid valve is not tightly closed;
④ Check whether the urea nozzle is separated from the urea pipe;
⑤ Check whether the heating pipe in the urea tank is cracked;
(22) p1901: urea temperature is too high
Vehicle performance: the OBD fault light is on, the engine will not limit torque in a short time, and the fault will be eliminated after three normal working cycles
Recommended inspection:
① Check whether the water heating solenoid valve is normally open;
② Check whether there is antifreeze in the urea tank;
③ Check whether the urea temperature sensor is loose;
④ Check the urea temperature sensor for water ingress, corrosion or short circuit;
(23) p1902: pressure build-up failure of urea system
Vehicle performance: the OBD fault light is on, the engine torque is limited, and the fault will not be eliminated until three normal working cycles
Recommended inspection:
① Check whether the air filter switch is on;
② Check whether the intake and exhaust valves work normally;
③ Check the urea supply system for air leakage;
④ Check whether the urea pressure sensor is normal;
Caution: when the urea injection system fails, several fault codes often appear at the same time. The fault can be solved by patiently troubleshooting one by one according to the suggestions. Welcome card friends to leave messages and exchange. (text / special author of truck house: Jiefang R & D Division)

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