This canister vent valve is normally open, and on several models a clogged fresh air inlet has caused problems that turn on the malfunction indicator lamp (MIL), but more on that later. The purge control solenoid valve is used to seal the system at the intake manifold end, and another solenoid valve seals the canister’s fresh air inlet. In either case, leak-testing an EVAP system requires sealing the system at each end. Ford Motor Co., General Motors (GM) and some Asian models use vacuum tests, while DaimlerChrysler, some Asian and most European models use pressure tests. Leak-testing with pressure requires a pump and extra plumbing, but the drive cycle is less exacting. But because the vacuum comes from the intake manifold, the drive cycle (the engine operating conditions needed for the test) is very precise, so there’s no guarantee the test will actually run during any given trip. Vacuum testing is mechanically simpler, requiring only two solenoid valves and a fuel tank absolute-pressure sensor. There are two different ways for an OBD system to leak-test an enhanced EVAP system: pressurize it or put it under vacuum. Fortunately, they’re not hard to understand. There are several different ways to accomplish these tasks, and of course the automakers have used them all. The former is required by law to monitor the health of the EVAP system, and the latter is needed by the PCM to control tailpipe emissions and preserve driveability during canister purge. The two “enhancements” refer to the on-board diagnostic (OBD) system’s ability to detect leaks as small as 0.5mm and to monitor canister purge flow. Like the earlier systems, the enhanced EVAP system captures fuel tank vapors and sends them to be burned in the engine. Techs can tell the difference by looking for a service port cap under the hood, sometimes marked “EVAP.” Most new vehicles sold in all 50 states met those requirements that first year, and today they all do. The enhanced evaporative emission control system is a result of California emissions regulations that went into effect for the 2000 model year. But in addition to tools and training, troubleshooting EVAP codes also requires an understanding of how the system is supposed to work. Even better, now there is a service port that makes it easier to search for leaks using smoke. This is good news, because it means the PCM can provide useful diagnostic information for what may be the most difficult system to troubleshoot. The newest generation of enhanced evaporative emission controls features a powertrain control module (PCM) that monitors every component and function of the evaporative emission (EVAP) system.
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