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Introduction to Evaporative Emission controls
The Evaporative Emission (EVAP) system is used to prevent fuel vapor emission into the atmosphere. On most models, the vapor generated in the fuel tank and carburetor bowl vent (if equipped) enters the storage canister where the charcoal absorbs and stores the vapor. During certain conditions, vacuum is applied to the storage canister and the fuel vapor is drawn into the intake manifold to be burned in the combustion chambers. During this process, the storage canister is purged by air drawn through the filter located on the canister. On some models the fuel vapors are routed to the positive crankcase ventilation system and stored in the crankcase until the engine is started. Most systems use a sealed pressure/vacuum relief fuel filler cap.
History
The EVAP charcoal canister was first introduced in California for the 1970 model year. Its purpose was to capture raw fuel vapors (hydrocarbons) from the tank and carburetor bowl before they could escape into the air. It's been known for some time that activated charcoal readily adsorbs hydrocarbon vapor, holding it on the surface of each granule like a magnet. This is different from absorb, which sucks it in like a sponge. Since the HC is only on the surface of the charcoal, it can be easily blown off with a blast of fresh air.
On earlier carbureted engines, a non-venting gas cap and a simple vacuum operated check valve were used to keep the system closed with the engine off or at idle. With the engine running above idle, the valve opens and a hose between the canister and the throttle body allows fresh air to be pulled through the charcoal and mixed with the intake air. There is a calibrated orifice in this circuit to limit the total air/vapor flow, making this a calibrated vacuum leak. This simple system worked just fine when tailpipe HC emissions were regulated to 1.5 grams/mile. But according to the Bosch automotive handbook, if 1% of the total intake air is composed of fuel vapors from the charcoal canister, the air/fuel ratio can change by as much as 20 percent. Dealing with such a large variable forced the design of a closed-loop fuel trim system with a large range.
Enhanced Evaporative Control
On some models equipped with OBD II, the PCM monitors EVAP system vacuum level through fuel tank pressure sensor input. The EVAP purge valve and canister vent valve turn on at an appropriate time, allowing engine vacuum to draw a small vacuum on the entire EVAP system. After the desired vacuum level has been reached, the purge valve turns off, sealing the system. Leaks are detected by monitoring for a vacuum level decrease over a given time period if all other variables remain constant.
A small leak should set DTC P0442 or DTC P0456.
Large leaks should set DTC P0455.
The system should be inspected to ensure all canister(s), control solenoids and valves (VMV) , liquid/vapor separator, fuel tank pressure sensor (FTPT) and roll over valve are properly installed. Inspect all electrical wiring and vacuum and vent lines for disconnections, wear, damage or restrictions. The fuel tank(s) should also be inspected for proper filler cap installation. On some later models with OBD II, an improperly installed fuel filler cap may cause the Malfunction Indicator Lamp (MIL) to illuminate and a DTC to be stored in the control module memory.
Components
The EPA has an official list of components that defines the evaporative emission control system:
• Fuel tank
• Canister vent solenoid valve
• Fuel tank vacuum or pressure sensor
• Fuel lines
• Vapor lines
• Fuel tank cap
• Charcoal (EVAP) canister
• Purge lines
• Vapor Management Valve (Purge solenoid valve)
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On-board diagnostics must be able to verify proper operation of the system, record any failure in DTC memory and alert the driver when the system is not functioning properly.
Two parameters define proper EVAP function: system integrity, as in leaks or blocked lines, and proper purge airflow from the canister to the engine
At this time the basic strategy for checking system integrity is to use a pressure sensor; either built into the fuel tank unit or mounted somewhere else in the fuel vapor control system. At the appropriate point in a http://www.fordforumsonline.com/forum/engine-drivetrain/40-obd-ll-drive-cycle.html?nojs=1#links, the PCM will close the canister vent solenoid valve, use the engine to pull a vacuum of about 14 inches of water on the system, close the purge valve and measure how long it takes for the vacuum to decay to a specified level. In general, if it decays too fast, there is a leak. If it doesn't decay fast enough, there is a blocked line.
The Ford EVAP monitor requires:
1) The EVAP system must pull 8" H2O within 15 seconds with purge active and vent closed or DTC P0455 will be recorded, indicating a large Leak.
2) The EVAP system must maintain vacuum for 10 seconds with less than 4" H2O decay, or DTC P0442 will be recorded, indicating the possibility of a 0.040" leak, which is a small Leak.
3) The EVAP system must pull 7" H2O and maintain it or a DTC P0456 will be recorded, indicating a possibility of a 0.020" leak, which is a small Leak.
Note: The Fuel Tank Pressure Transducer (FTPT) operates as follows:
![]()
The Evaporative Emission (EVAP) system is used to prevent fuel vapor emission into the atmosphere. On most models, the vapor generated in the fuel tank and carburetor bowl vent (if equipped) enters the storage canister where the charcoal absorbs and stores the vapor. During certain conditions, vacuum is applied to the storage canister and the fuel vapor is drawn into the intake manifold to be burned in the combustion chambers. During this process, the storage canister is purged by air drawn through the filter located on the canister. On some models the fuel vapors are routed to the positive crankcase ventilation system and stored in the crankcase until the engine is started. Most systems use a sealed pressure/vacuum relief fuel filler cap.
History
The EVAP charcoal canister was first introduced in California for the 1970 model year. Its purpose was to capture raw fuel vapors (hydrocarbons) from the tank and carburetor bowl before they could escape into the air. It's been known for some time that activated charcoal readily adsorbs hydrocarbon vapor, holding it on the surface of each granule like a magnet. This is different from absorb, which sucks it in like a sponge. Since the HC is only on the surface of the charcoal, it can be easily blown off with a blast of fresh air.
On earlier carbureted engines, a non-venting gas cap and a simple vacuum operated check valve were used to keep the system closed with the engine off or at idle. With the engine running above idle, the valve opens and a hose between the canister and the throttle body allows fresh air to be pulled through the charcoal and mixed with the intake air. There is a calibrated orifice in this circuit to limit the total air/vapor flow, making this a calibrated vacuum leak. This simple system worked just fine when tailpipe HC emissions were regulated to 1.5 grams/mile. But according to the Bosch automotive handbook, if 1% of the total intake air is composed of fuel vapors from the charcoal canister, the air/fuel ratio can change by as much as 20 percent. Dealing with such a large variable forced the design of a closed-loop fuel trim system with a large range.
Enhanced Evaporative Control
On some models equipped with OBD II, the PCM monitors EVAP system vacuum level through fuel tank pressure sensor input. The EVAP purge valve and canister vent valve turn on at an appropriate time, allowing engine vacuum to draw a small vacuum on the entire EVAP system. After the desired vacuum level has been reached, the purge valve turns off, sealing the system. Leaks are detected by monitoring for a vacuum level decrease over a given time period if all other variables remain constant.
A small leak should set DTC P0442 or DTC P0456.
Large leaks should set DTC P0455.
The system should be inspected to ensure all canister(s), control solenoids and valves (VMV) , liquid/vapor separator, fuel tank pressure sensor (FTPT) and roll over valve are properly installed. Inspect all electrical wiring and vacuum and vent lines for disconnections, wear, damage or restrictions. The fuel tank(s) should also be inspected for proper filler cap installation. On some later models with OBD II, an improperly installed fuel filler cap may cause the Malfunction Indicator Lamp (MIL) to illuminate and a DTC to be stored in the control module memory.
Components
The EPA has an official list of components that defines the evaporative emission control system:
• Fuel tank
• Canister vent solenoid valve
• Fuel tank vacuum or pressure sensor
• Fuel lines
• Vapor lines
• Fuel tank cap
• Charcoal (EVAP) canister
• Purge lines
• Vapor Management Valve (Purge solenoid valve)
On-board diagnostics must be able to verify proper operation of the system, record any failure in DTC memory and alert the driver when the system is not functioning properly.
Two parameters define proper EVAP function: system integrity, as in leaks or blocked lines, and proper purge airflow from the canister to the engine
At this time the basic strategy for checking system integrity is to use a pressure sensor; either built into the fuel tank unit or mounted somewhere else in the fuel vapor control system. At the appropriate point in a http://www.fordforumsonline.com/forum/engine-drivetrain/40-obd-ll-drive-cycle.html?nojs=1#links, the PCM will close the canister vent solenoid valve, use the engine to pull a vacuum of about 14 inches of water on the system, close the purge valve and measure how long it takes for the vacuum to decay to a specified level. In general, if it decays too fast, there is a leak. If it doesn't decay fast enough, there is a blocked line.
The Ford EVAP monitor requires:
1) The EVAP system must pull 8" H2O within 15 seconds with purge active and vent closed or DTC P0455 will be recorded, indicating a large Leak.
2) The EVAP system must maintain vacuum for 10 seconds with less than 4" H2O decay, or DTC P0442 will be recorded, indicating the possibility of a 0.040" leak, which is a small Leak.
3) The EVAP system must pull 7" H2O and maintain it or a DTC P0456 will be recorded, indicating a possibility of a 0.020" leak, which is a small Leak.
Note: The Fuel Tank Pressure Transducer (FTPT) operates as follows:
- Nominal voltage is 2.4-2.8 volts.
- Voltage will decrease with vacuum.
- Voltage will increase with pressure.
