Carbon canister purge control valve

The purpose of this test is to check the actuation voltage controlling the carbon canister purge control valve’s operation.

How to perform the test

View connection guidance notes.

1. Use manufacturer’s data to identify the carbon canister purge control valve circuit and operating modes (i.e. the engine conditions required to trigger valve operation).
2. Connect PicoScope Channel A to the valve’s switched earth circuit.
3. Start and run the engine.
4. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
5. Start the scope to see live data.
6. Vary the engine and/or test conditions as necessary to trigger valve operation.
7. With your waveform on screen stop the scope.
8. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Note

The canister is operated by the Engine Control Module (ECM) using switched earth Pulse Width Modulation (PWM). There will be supply voltage at both terminals until the solenoid begins to operate.

Example waveform

Waveform notes

This known good waveform has the following characteristics:

• A switched earth PWM voltage, with a fixed cyclic period in which the voltage switches between battery positive, at around 14 V, and battery negative, at just above 0 V.
• The period the signal spends around 0 V relative to the total cycle period indicates the PWM duty. In this example, the low voltage period is around 4 ms out of every 100 ms cycle period, indicating a 4 % duty.
• The waveform cycles 5 times in a 500 ms period, indicating a 10 Hz base frequency.
• In this example, there is a (induced) voltage increase, up to around 15 V, each time the valve solenoid is switch off.

Waveform Library

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select Carbon canister purge solenoid voltage.

Further guidance

The carbon canister is a container filled with fuel vapor absorbent carbon granules. It stores harmful evaporative emissions until they can be delivered to the intake manifold for combustion by the engine.

The carbon canister’s inlet is connected to the fuel tank and its outlet is connected via a purge control valve to the intake manifold. The canister volume will also have a fresh air inlet via a one-way (or solenoid-controlled) vent valve.

Fuel vapors are purged from the canister when a vacuum is present on its outlet. Therefore, canister purging occurs when the intake manifold pressure is below atmospheric pressure, such as when the engine is idling.

The flow of vapor from the canister to the intake manifold is regulated by the ECM-controlled purge control valve. Generally, the valve is mounted within the engine compartment, although it can be mounted directly on the canister.

Typically, the purge control valve is a normally closed solenoid valve. It requires a circuit current to act to open the valve, whilst it works against a closing spring. The solenoid valve will have an ignition feed on one side and an ECM controlled switched-earth PWM voltage on the other. Therefore, the ECM can vary the solenoid circuit current and the degree of opening of the valve.

The ECM uses the exhaust oxygen sensor to monitor the enrichment of the mixture when the purge valve is opened. As such, fuel vapors will not be purged from the carbon canister until the engine is warm and lambda control is functioning.

The system allows fuel vapors to be burned when they are least likely to have an adverse effect on performance or emissions. The purge control valve opening protocols are pre-programmed into the ECM and will vary across manufacturers and applications.

System fault symptoms

• Malfunction Indicator Lamp (MIL) illumination.
• Diagnostic Trouble Codes (DTCs).
• Rough running.
• Drivability issues.
• Erratic idle.
• Smell of gasoline.
• Difficulty starting.
• Fuel trims out of tolerance.

Typical causes of failure

• Short and open circuits or high resistances in the solenoid or circuits.
• Leak or collapse of vacuum lines between the intake manifold and the canister.
• Leak or collapse of the tank vent lines between the tank and the canister.
• Seized (open or closed) purge control valve.
• Blockage of tank vent lines, vacuum lines, or purge control valve (e.g. from the breakdown and escape of carbon granules from the canister) [all may require flushing or replacing if the carbon canister has deteriorated excessively].
• Failure of ECM control, such as software or hardware (driver) issues.

GT030