Evaporative Emissions System: Description and Operation
PURPOSEThe basic evaporative emission (EVAP) control system used on all vehicles is the charcoal canister storage method. Gasoline vapors from the fuel tank flow into the canister through the inlet labeled "TANK." These vapors are absorbed into the activated carbon (charcoal) storage device (canister) in order to hold the vapors when the vehicle is not operating. The canister is purged by Powertrain Control Module ([1][2]PCM) control when the engine coolant temperature is over 60°C (140°F), the Intake Air Temperature (IAT) reading is over 10°C (50°F), and the engine has been running. Air is drawn into the canister through the air inlet grid. The air mixes with the vapor and the mixture is drawn into the intake manifold.
OPERATION
The EVAP canister purge is controlled by a solenoid valve that allows the manifold vacuum to purge the canister. The powertrain control module (PCM) supplies a ground to energize the solenoid valve (purge on). The EVAP purge solenoid control is pulse-width modulated (PWM) (turned on and off several times a second). The duty cycle (pulse width) is determined by engine operating conditions including load, throttle position, coolant temperature and ambient temperature. The duty cycle is calculated by the [1][2]PCM. The output is commanded when the appropriate conditions have been met. These conditions are:
- The engine is fully warmed up.
- The engine has been running for a specified time.
- The IAT reading is above 10°C (50°F).
A continuous purge condition with no purge commanded by the [1][2]PCM will set a DTC P1441.
Poor idle, stalling and poor driveability can be caused by:
- A malfunctioning purge solenoid.
- A damaged canister.
- Hoses that are split, cracked, or not connected properly.
A charcoal-filled canister captures and stores gasoline fumes. When the [1][2]PCM determines that the time is right, it opens a purge valve which allows engine vacuum to draw the fumes into the intake manifold.
Enhanced EVAP Control System
The basic purpose of the Enhanced Evaporative Emissions control system is the same as other EVAP systems. A charcoal-filled canister captures and stores gasoline fumes. When the [1][2]PCM determines that the time is right, it opens a purge valve which allows engine vacuum to draw the fumes into the intake manifold.
The difference between this and other systems is that the [1][2]PCM monitors the vacuum and/or pressure in the system to determine if there is any leakage. If the PCM determines that the EVAP system is leaking or not functioning properly, it sets a Diagnostic Trouble Code (DTC) in the PCM memory.
The enhanced EVAP system is required to detect evaporative fuel system leaks as small as 0.040 in. (1.0 mm) between the fuel filler cap and purge solenoid. The system can test the evaporative system integrity by applying a vacuum signal (ported or manifold) to the fuel tank to create a small vacuum. The [1][2]PCM then monitors the ability of the system to maintain the vacuum. If the vacuum remains for a specified period of time, there are no evaporative leaks and a PASS report is sent to the diagnostic executive. If there is a leak, the system either will not achieve a vacuum, or a vacuum cannot be maintained. Usually, a failure can only be detected after a cold start with a trip of sufficient length and driving conditions to run the needed tests. The enhanced EVAP system diagnostic will conduct up to eight specific sub-tests to detect fault conditions. If the diagnostic fails a sub-test, the PCM will store a Diagnostic Trouble Code (DTC) to indicate the type of fault detected.
(1) Vent Solenoid
(2) EVAP Purge Solenoid
(3) Throttle Body
(4) Fuel Filler Neck
(5) Fuel Tank
(6) Rollover Valve
(7) EVAP Canister
(8) Ignition Feed
(10) EVAP Purge Solenoid Driver Signal from [1][2]PCM
(11) Vent Solenoid Driver Signal from PCM
(12) Vent Filter
(13) Fuel Tank Pressure Sensor
(14) Fuel Tank Pressure Signal to [1][2]PCM
(15) 5 Volt Reference "A" Circuit from PCM
(16) Sensor Ground Circuit from PCM
(17) Fuel Level Sensor
(18) Fuel Level Signal to PCM
(19) 5 Volt Return
Electrical Components
The electrical components that make up the enhanced EVAP system are:
- Fuel Tank Pressure Sensor. The fuel tank pressure sensor is a three-wire strain gauge sensor similar to a common Manifold Absolute Pressure (MAP) sensor. However, the fuel tank pressure sensor has very different electrical characteristics due to its pressure differential design. The sensor measures the difference between the air pressure (or vacuum) in the fuel tank and the outside air pressure.
The sensor mounts at the top of the fuel pump assembly. A three-wire electrical harness connects it to the [1][2]PCM. The PCM supplies a five-volt reference voltage and a ground to the sensor. The sensor will return a voltage between 0.1 and 4.9 volts. When the air pressure in the fuel tank is equal to the outside air pressure, such as when the fuel cap is removed, the output voltage of the sensor will be 1.3 to 1.7 volts.
When the air pressure in the fuel tank is 4.5 in. H2O (1.25 kPa), the sensor output voltage will be 0.5 - 0.2 V. When there is neither vacuum nor pressure in the fuel tank, the sensor voltage will be 1.5 V. At -14 in. H2O (-3.75 kPa), the sensor voltage will be 4.5 - 0.2 V.
- EVAP Canister Purge Solenoid. Normally closed, the purge solenoid opens upon the [1][2]PCM's signal to allow engine vacuum to purge gasoline fumes from the canister. Mounted on top of the upper intake manifold assembly.
- EVAP Canister Vent Solenoid. Located next to the canister, the vent solenoid opens to allow air into the EVAP system. Fresh air is necessary to completely remove gasoline fumes from the canister during purge. The EVAP vent solenoid closes to seal off the evaporative emissions system for leak testing.
- Fuel Level Sensor. The fuel level sensor is an important input to the [1][2]PCM for the enhanced EVAP system diagnostic. The PCM needs fuel level information to know the volume of fuel in the tank. The fuel level affects the rate of change of air pressure in the EVAP system. Several of the enhanced EVAP system diagnostic sub-tests are dependent upon correct fuel level information. The diagnostic will not run when the tank is less than 15% or more than 85% full. Be sure to diagnose any Fuel Level Sensor DTCs first, as they can cause other DTCs to set.
- Manifold Absolute Pressure (MAP) Sensor. The [1][2]PCM compares the signals from the fuel tank pressure sensor and the MAP sensor to ensure that a relative vacuum is maintained in the EVAP system.
Non-Electrical Components
- Purge/Vacuum Hoses. Made of rubber compounds, these hoses route the gasoline fumes from their sources to the canister and from the canister to the intake air flow.
- EVAP Canister. Mounted on a bracket ahead of the fuel tank, the canister stores fuel vapors until the [1][2]PCM determines that engine conditions are right for them to be removed and burned.
- Fuel Tank. The tank has a built-in air space designed for the collection of gasoline fumes.
- Vacuum Source. The vacuum source is split between two ports, one on either side of the throttle body.
- Fuel Cap. The fuel cap is designed to be an integral part of the EVAP system.
System Fault Detection
The EVAP leak detection strategy is based on applying vacuum to the EVAP system and monitoring vacuum decay. The [1][2]PCM monitors vacuum level via the fuel tank pressure sensor. At an appropriate time, the EVAP purge solenoid and the EVAP vent solenoid are turned "ON," allowing the engine vacuum to draw a small vacuum on the entire evaporative emission system.
After the desired vacuum level has been achieved, the EVAP purge solenoid is turned "OFF," sealing the system. A leak is detected by monitoring for a decrease in vacuum level over a given time period, all other variables remaining constant. A small leak in the system will cause DTC P0442 to be set.
If the desired vacuum level cannot be achieved in the test described above, a large leak or a faulty EVAP purge solenoid is indicated.
Leaks can be caused by the following conditions:
- Disconnected or faulty fuel tank pressure sensor.
- Missing or faulty fuel cap.
- Disconnected, damaged, pinched, or blocked EVAP purge line.
- Disconnected or damaged EVAP vent hose
- Disconnected, damaged, pinched, or blocked fuel tank vapor line.
- Disconnected or faulty EVAP purge solenoid.
- Disconnected or faulty EVAP vent solenoid.
- Open ignition feed circuit to the EVAP vent or purge solenoid.
- Damaged EVAP canister.
- Leaking fuel sender assembly 0-ring.
- Leaking fuel tank or fuel filler neck.
A restricted or blocked EVAP vent path is detected by drawing vacuum into the EVAP system, turning "OFF" the EVAP vent solenoid and the EVAP purge solenoid (EVAP vent solenoid "OPEN," EVAP purge Pulse Width Modulate (PWM) "0%") and monitoring the fuel tank vacuum sensor input. With the EVAP vent solenoid open, any vacuum in the system should decrease quickly unless the vent path is blocked. A blockage like this will set DTC P0446 and can be caused by the following conditions:
- Faulty EVAP vent solenoid (stuck closed).
- Plugged, kinked or pinched vent hose.
- Shorted EVAP vent solenoid driver circuit.
- Plugged EVAP canister.
The [1][2]PCM supplies a ground to energize the purge solenoid (purge "ON"). The EVAP purge control is PWM, or turned "ON" and "OFF," several times a second. The duty cycle (pulse width) is determined by engine operating conditions including load, throttle position, coolant temperature and ambient temperature. The duty cycle is calculated by the PCM and the output is commanded when the appropriate conditions have been met.
The system checks for conditions that cause the EVAP system to purge continuously by commanding the EVAP vent solenoid "ON" and the EVAP purge solenoid "OFF" (EVAP vent solenoid "CLOSED," EVAP purge PWM "0%"). If fuel tank vacuum level increases during the test, a continuous purge flow condition is indicated, which will set a DTC P1441. This can be caused by the following conditions:
- EVAP purge solenoid leaking.
- EVAP purge and engine vacuum lines switched at the EVAP purge solenoid.
- EVAP purge solenoid driver circuit grounded.