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Evac system5/5/2023 ![]() The typical range of a gas vapor system is generally not greater than 3.0 to 4.0 in.-H 2O positive pressure and −10.0 in.-H 2O negative pressure and/or vacuum. To put this into perspective, 1.0 psi = 28.0 in.-H 2O, and 1.0 in.-Hg (inches of mercury) = 14.0 in.-H 2O. The system is designed to operate with very small pressure changes and is typically displayed in inches of water (in.-H 2O). The PCM relies heavily on the FTPS to validate the operation of the CPS and CVS and determine if any leaks exist in the gas vapor system. The FTPS is a three-wire pressure transducer that’s used to measure pressure changes in the gas vapor system. ![]() The PCM uses the CVS to seal the gas vapor system for leak integrity checks. The CVS is a basic on/off-open/closed solenoid that’s normally open when deenergized. The PCM operates the CPS to pull gas vapors from the charcoal canister, using intake manifold vacuum, while the engine is running. ![]() The CPS is a duty-cycle-controlled, normally closed solenoid that separates the engine from the gas vapor system. The canister purge solenoid (CPS), canister vent solenoid (CVS) and fuel tank pressure sensor (FTPS) are the heart of the system. The powertrain control module (PCM) uses three simple but robust components to purge gas vapors and test the gas vapor system for leak integrity. 1 on page 20 shows the typical layout of a vacuum-based system, which is commonly used by many vehicle manufacturers. This article will provide an overview of the different systems and, in the case of the vacuum-based system, the evolution of design enhancements used to comply with the. The most important step in any diagnostic process is understanding the system design, the specific components used and its theory of operation. The exceptions are its hybrid electric vehicles, which use an evaporative leak check monitor (ELCM) similar to the Toyota key-off vacuum pump system, and 2011-03 Ford Fiestas, which use a natural vacuum leak detection (NVLD) system similar to the one used by Kia and BMW. While some vehicle manufacturers have introduced very different designs along the way, Ford has primarily used a vacuum-based design as a foundation. The evolution of evaporative emissions systems has been driven by changes in emissions standards.
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