Edited: 4/8/2009 2:57:06 PM by DOWNSUB
I found this>>>>>>>>>> The enhanced evaporative emissions (EVAP) system uses two solenoids and a pressure sensor mounted on or near the fuel tank. The first solenoid is called the vapor management valve (VMV); the second is the canister vent (CV) solenoid. The pressure sensor is called a fuel tank pressure (FTP) sensor. The VMV, a normally closed solenoid, has three hoses and two wires going to it. Two of the three hoses are larger in diameter, usually three-eighths of an inch. The third hose is smaller, usually three-sixteenths of an inch, and usually has manifold vacuum on it. One of the wires has battery voltage with the key on and the other is grounded by the PCM to energize the solenoid. For the valve to allow vacuum into the EVAP system, the PCM has to energize the solenoid by grounding the electrical part of it, and also has to have vacuum applied to the small control vacuum port. The CV is a normally open solenoid with two wires going to it. One wire is battery voltage; the second goes back to the PCM to be grounded. Some systems also run a vent hose from the solenoid up to a higher point (the most common is up to the filler neck area) to be open to atmosphere. Generally, the solenoid itself is mounted directly to the charcoal canister. When the solenoid is at rest, the charcoal canister should be able to vent to atmosphere. When the PCM wants to check the system for leaks, it energizes the CV solenoid, which should close off the canister from venting and seal the system. The FTP sensor works very much like a typical manifold absolute pressure (MAP) sensor. It is mounted either in the top of the fuel tank or in the vapor line going from the VMV to the fuel tank and charcoal canister. When the PCM commands a vacuum into the system to check for leaks, the voltage changes according to the amount of pressure, or in this case, negative pressure in the system. An input found on some of the newer enhanced systems is a fuel-level input to the PCM, which allows the PCM to adjust its bleed-off rate calculation based on the amount of fuel in the tank. For the PCM to test the system, certain criteria must be met: 1) engine is running; 2) not overheating; 3) is at idle or part throttle; and 4) is either in open or closed loop (loop status will vary per calibration). Once all these criteria have been met, the PCM will start to duty cycle the ground on the CV solenoid to close off the system from atmosphere. After the CV solenoid is energized and the system should be sealed, the PCM will start to duty cycle the ground on the vapor management valve. If everything is working correctly, this will create a negative pressure, or vacuum, in the EVAP system. The FTP sensor voltage should drop as the pressure decreases. Once the PCM has determined that there is enough vacuum in the system (typically 7 inches H20), it will de-energize the vapor management valve and look for the voltage on the FTP sensor to change. If the system is sealed properly, the voltage should remain about the same until the PCM de-energizes the canister vent solenoid. If there is a leak, the voltage will start to increase while the CV solenoid is still grounded. The PCM knows how large the leak is, depending on how much and how fast the voltage changes. Leaks as small as .040 inch will cause a code to set. A code P0442 will set if there is more than about 2.5 inches H2O change in 15 seconds at approximately 75 percent fuel tank level. Starting in model year 2000, some calibrations actually started checking for leaks as small as .020 inch. A code P0456 will set if there is more than 2.5 inches H2O change in 45 seconds at approximately 75 percent fuel tank level. The amount of bleed-up allowed will vary, again based on how much fuel is in the tank. A code P0455 will set if there is more than 7 inches H2O change over 30 seconds. There are other codes related to the EVAP system, but these are the most common. Hopefully this information will give you a better understanding of the EVAP systems and how to test them when they fail.
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