How Nissan Evaporative Systems WorkPosted 2/15/2006
By Paul Kujawa
The evaporative system referenced in this article is used on many Nissan makes and models. On this system, fuel vapors from the fuel tank are allowed to flow through a vacuum cut valve into the charcoal canister to be stored and later purged. The vacuum cut valve acts as a one-way check valve and prevents vacuum from being able to enter the fuel tank during normal purge operation. Purging of the charcoal canister is done through the use of a canister purge volume control valve, an evap canister purge control vacuum solenoid and a diaphragm valve called the canister purge control valve.
To purge the fuel vapors from the charcoal canister, the engine control module (ECM) must first energize the canister purge control vacuum solenoid valve, which will apply vacuum to the canister purge control valve - causing it to open and apply vacuum to the charcoal canister. The system uses the canister purge volume control valve (a six-wire stepper motor) to regulate the amount of purge flow that is allowed at any given time.
The ECM controls the position of the canister purge volume control valve to vary the size of the orifice from zero steps (closed) to 64 steps (wide open). On light accel, engine warm, the canister purge volume control valve is controlled at five to 20 steps. At cruise, the canister purge volume control valve is controlled at eight to 30 steps and under heavy load, it is controlled at 25 to 64 steps. The evap canister purge control solenoid valve, canister purge control valve and the canister purge volume control valve are all located under the hood in the engine compartment.
During a canister purge, the canister has to be open to the atmosphere to allow fresh air into the canister, which will then bring fuel vapors with it into the intake manifold. This fresh air enters the canister through the canister vent control solenoid valve, which is normally open and closes when it is energized. The canister vent control valve is bolted to the canister.
When the ECM performs a leak check of the evap system, it energizes the canister vent solenoid to seal off the canister vent. The ECM energizes the evap canister vent control solenoid valve by grounding it. A check valve (evap breather valve) after the evap canister vent control valve prevents fuel vapors from being able to leave the canister through the vent if the evap canister vent control valve is off.
To test the system for leaks, the ECM uses the evap system pressure sensor, the vacuum cut valve bypass solenoid valve and the canister vent control valve. The evap control system pressure sensor is typically located in the rear of the vehicle by the charcoal canister and has three wires: a 5-volt reference from the ECM, a ground and an output voltage signal wire. The sensor output voltage is about 3.3 to 3.4 volts with key on, engine off (KOEO) with no vacuum or pressure in the evaporative system. With vacuum in the evap system, the output voltage should drop. The vacuum cut valve and the vacuum cut valve bypass solenoid valve are located in the rear of the vehicle by the canister.
To check the operation of the system, simulate the self-diagnosis leak test (see below) in the same manner as the ECM, using a digital volt ohmeter (DVOM) to monitor the evap system pressure sensor signal wire at the ECM (the voltage should read approximately 3.3 to 3.4 volts KOEO). While monitoring the evap pressure sensor voltage, ground the evap system solenoids with the engine running to see if the pressure sensor voltage drops, indicating vacuum in the system.
To simulate the test:
Step 1: Bypass the evaporative canister purge volume control valve by removing the hoses from the valve and connecting them together. This will ensure the engine vacuum is applied to the canister purge control valve during the test.
Step 2: Ground the canister vent control valve to seal the canister vent.
Step 3: Ground the vacuum cut valve bypass solenoid valve to bypass the vacuum cut valve to allow vacuum to be applied to the fuel tank.
Step 4: Momentarily ground the purge control solenoid with the engine running to apply vacuum and open the evap canister purge control valve. This allows manifold vacuum to be applied to the evap system, resulting in the evap control system pressure sensor voltage dropping (Note: grounding the purge control solenoid too long may result in a large enough vacuum leak to kill the engine; increasing engine rpm may be necessary).
When the evap control system pressure sensor voltage drops, if there is no leak in the evap system, the voltage should hold. If the voltage climbs back to the key on, engine off value, then a leak is indicated. Vacuum lines can be pinched off in the evap system to isolate portions of the system and then reintroduce vacuum into the evap system to see if the pressure sensor voltage drops and then holds, which will help isolate the leak.
Common evaporative system problems include:
Disconnected, plugged or cracked vacuum lines can also result in evaporative system codes being set.
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