Classic Case:
Rongguang B12 engine recently appeared after the engine startup fault light on. After testing with a diagnostic instrument fault code: P1396 “wheel speed sensor dynamic failure” P0302 two cylinder misfire, P0301 one cylinder misfire. The car uses the Siemens engine management system.
Inspection and maintenance:
- Checked cylinder wires and spark plugs, no problem.
- Checked the fuel quality and injector, and no abnormality was found.
- Check the crankshaft position sensor and its wiring is not a problem, and the trigger wheel is not missing teeth, or a dirty phenomenon, replace the new crankshaft position sensor, the fault remains, and the light is still on.
- Replace the ECU, the fault will still occur.
- Check whether the carbon canister solenoid valve and EGR valve work properly. Use compressed air to check the smoothness of each interface of the carbon canister, and the work of the solenoid valve. The solenoid valve was found to be in a long open state. Replace the carbon canister solenoid valve, the problem is solved.
The carbon canister solenoid valve is damaged, and has been in a normally open state, at idle, if there are fuel vapors in the intake manifold, resulting in too rich a mixture, so that the engine combustion is not normal, resulting in part of the cylinder works poorly, the phenomenon of “misfire”.
When we encounter such problems, we can consider more from the data flow. For example, the data reflects the air-to-air ratio. For some problems, we may be able to find some breakthroughs.
Fuel vapor purification system working principle
Fuel vapors rise to the top of the fuel tank, and through the piping, vapors are collected in the activated carbon canister, under proper engine conditions, the electronic control unit commands the carbon canister solenoid valve to open, which sends the gas to the back of the intake manifold throttle, and then into the combustion chamber.
Solenoid valves for carbon tanks
Controlled by the ECU to open or close the passage between the carbon canister and the intake manifold, it consists of a solenoid coil, armature valve, etc.
Carbon Tank Solenoid Valve
One tube goes to the gas tank, one to the intake manifold, and one vent to the atmosphere.
(EVAP) When does the carbon canister solenoid valve start working?
- Fuel control mode is closed-loop control.
- The coolant temperature is above 40°C.
- Throttle opening in non-idling state.
- The engine is in a non-deceleration state.
- Battery voltage is 8-19V.
Imagine if the solenoid valve was normally open when it shouldn’t be working, what would cause the engine to fail?
Carbon canister solenoid valve failure phenomenon
EVAP solenoid valve normally open
The result is an overly rich mixture, fault light on, black smoke, driving off, most noticeable at idle, which tends to lead to unstable total speed, and acceleration will be affected to some extent.
EVAP solenoid valve normally closed
Oil vapors cannot be cleaned in time, increasing HC emissions and gasoline odor.
The large air holes in the carbon canisters are too dirty and clogged
This causes the fuel tank to be sucked flat and deformed, and the fuel tank rattles during driving.
Maintenance Methods
- Check the supply voltage of the solenoid valve, normal should be close to the battery voltage. If abnormal, check whether the power supply circuit has an open or false connection.
- Without unplugging the solenoid valve harness connector, connect one end of the diode test light to the solenoid valve control wire and the other end to the positive battery terminal.
With the car running and switching between idle and high engine speed, you should be able to observe the test light flashing. If abnormal, check the solenoid control wiring for open or short-to-ground faults. - Check the resistance of the solenoid valve, disconnect the connector of the solenoid valve of the carbon canister, and use a multimeter to measure the resistance value between the two terminals of the connector, the resistance is 22~30Ω at 20°C.
- Disassemble the EVAP solenoid valve, in the state that the solenoid valve is not powered, and the solenoid valve should not be able to both ends, to the solenoid valve power: the solenoid valve should be connected to both ends. If abnormal, replace the EVAP solenoid valve.
(EVAP) Carbon Canister Solenoid Valve Data Flow
Carbon Canister Duty Cycle
The electronic control unit controls the opening of the carbon canister solenoid valve using a duty cycle signal. The solenoid valve is opened under appropriate working conditions. The greater the duty cycle signal, the greater the opening of the solenoid valve and the greater the airflow for carbon tube blowing. The duty cycle is expressed as a percentage. (Take the Baojun 560 1.8 engine as an example)
At idle, the duty cycle is relatively small, the solenoid valve opening is relatively small, and less fuel vapor is introduced.
At high RPM, the duty cycle is larger and the solenoid valve opening is relatively large, introducing more fuel vapor.
Knowledge Expansion
1. How to determine that the solenoid is stuck in the normally open position based on the data stream?
This can be analyzed in conjunction with the target idle speed, actual engine RPM, and MAP values: oxygen sensor data, and fuel correction data.
2. The solenoid valve normally opens data flow will have the following changes:
- The actual engine rpm is higher than the target rpm.
- The MAP intake pressure value is high.
3. The voltage of the front oxygen sensor is high.
- The long-term fuel correction value is negative in the direction of correction.
- The above can be determined by the EVAP solenoid valve being stuck in the partially or fully open position.
- The same can be used to disconnect the pipeline and block the method of air intake to further confirm.
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