If a vehicle is having difficulty resetting the EVAP monitors, try asking the customer to run at least a ½-full tank of fuel for a while. This is important for key-on and key-off testing because a certain amount of air volume in the tank is required. If a vehicle is driven around empty or full, chances are it will not be able to test the system. The fuel level in the tank is critical for some vehicles to test the integrity of the EVAP system. These conditions can range from how much fuel is in the tank to how long the vehicle sits overnight. In order for some operations and testing to be performed, certain criteria must be met. This is where it gets complicated with evaporative emission monitors and testing. This can be performed with the vehicle running or with the key out of the ignition. The valves can isolate the fuel tank, and the pressure sensor can measure changes in pressure over time, as either the fuel pump removes fuel from the tank or the fuel cools down when the car is parked. The valves and pressure sensor can also be used to perform leak tests. The ECM/PCM controls the valves, and some vehicles have dedicated EVAP modules that connect to a serial database. Purge solenoids typically move the vapor between the tank, canister and engine. Vent solenoids can be found connected to the tank and on some EVAP canisters. Vent solenoids allow outside air to enter the system to control the pressure differential (some vents even have small air filters). Some systems combine the valves into one component. The valves come in two style: vent and purge. The system regulates this flow with a series of valves. This is the main EVAP sensor used to detect pressure changes caused by leaks and actions of the solenoids in the system. Inside the fuel tank is a barometric-style sensor that senses changes in air pressure. The start of the journey is in the fuel tank. Instead of using gravity, the EVAP system uses engine vacuum to move the vapors from the tank to the engine. Think of the components of the EVAP system as locks in a canal. The system traps the vapor and moves the vapors using pressure differentials between the components and engine vacuum. A computer is constantly monitoring the results of the commands sent to the valves with the help of sensors mounted in the tank and EVAP canister. They are also very selective when system checks are performed. The PCM will select the right engine temperature and throttle position to purge the system. Modern EVAP systems are selective when they vent vapors into the engine. It all started in 1996, when every new vehicle sold had an Evaporative Emission Control System (EVAP) that could detect leaks thanks to OBDII.
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