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Discussion Starter · #1 · (Edited)
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.

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.

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, 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.

According to Chrysler technical information, a 0.020" leak will emit approximately 1.35 grams/mile HC.


· Registered
4,353 Posts
Good job electro!
I added tags code P0442, and code P0455 and am mentioning them again here, so that searchers may find this information more quickly.

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Without being specific, I just wanted to add that all manufacturers had to "upgrade" their EVAP systems in about 2005 to meet new EPA standards. The system works basically the same as described, but the carbon canister used to store fumes is much larger. Additionally, the "vapor management valve" is more sophisticated and there are additional sensor(s) to monitor the system for proper operation.

These newer systems are more difficult to diagnose and therefore repair.

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944 Posts
Discussion Starter · #4 · (Edited)
Engine Off Natural Vacuum, EONV

Starting on selected models in 2005, and gradually covering all models is the newest Ford EVAP control system called Engine Off Natural Vacuum, or EONV. The theory consist of utilization of a phenomena known as Natural Vacuum.

Natural Vacuum EVAP system leak detection is based on the Gas laws. That is, pressure in a sealed container will change if the temperature of the gas in the container changes. As stated, this principle applies only to a sealed vessel. Any leak, even a small leak in a system, will allow the internal pressure to equalize with atmospheric pressure.

EONV EVAP testing is only done when the vehicle is turned off, and at rest. Typically this is done over a 30 minute time frame. Ford has chosen the use of EONV to perform the 0.020" leak check. Other EVAP leak checks continue to be done while the engine is running.

The Ford EONV Monitor consists of 5 stages.

  • Phase 0: Stabilization - With CVS open, Tank Pressure is allowed to stabilize. A fuel volatility test is performed and FTPT offset correction is learned if volatility test passes.

  • Phase 1: First Test - CVS is closed and pressure peaks below positive pass threshold, sending test to phase 2. If the positive pass threshold was exceeded, the test would have completed and a pass would have been recorded.

  • Phase 2: Transition - CVS is opened and a second stabilization phase occurs. A second FTPT offset is learned during this time.

  • Phase 3: Second Test - CVS is closed again and in a vacuum develops that eventually exceeds the negative pass threshold. When this occurs, the test proceeds to Phase 4.

  • Phase 4: Test Completion - CVS opens, results are recorded, and stay alive electronics shut down.

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