The EVAP Running Loss system consists of a fuel tank, fuel filler cap, fuel tank mounted or in-line fuel vapor control valve, fuel vapor vent valve EVAP canister, fuel tank pressure (FTP) sensor, EVAP canister purge valve, intake manifold hose assembly, canister vent (CV) solenoid, powertrain control module (PCM) and connecting wires and fuel vapor hoses.
1. The EVAP Running Loss system uses inputs from the engine coolant temperature (ECT) sensor, the intake air temperature (IAT) sensor, the mass air flow (MAF) sensor, the vehicle speed sensor (VSS) and the fuel tank pressure (FTP) sensor to provide information about engine operating conditions to the PCM. The fuel level input (FLI) and FTP sensor signals to the PCM are used by the PCM to determine activation of the EVAP Monitor based on presence of vapor generation or fuel sloshing.
2. The PCM calculates a variable duty cycle based on the desired amount of purge vapor flow to the intake manifold for a given engine condition. The PCM can then output the correct duty cycle to the EVAP canister purge valve. The PCM uses the EVAP Running Loss system inputs to evacuate the system using the EVAP canister purge valve, seals the EVAP Running Loss system from atmosphere using the CV solenoid, and uses the FTP sensor to observe total vacuum lost for a period of time.
3. The canister vent (CV) solenoid seals the EVAP Running Loss system to atmosphere during the EVAP Running Loss Monitor.
4. The PCM outputs a variable duty cycle signal (between 0% and 100%) to the solenoid on the EVAP canister purge valve.
5. The fuel tank pressure (FTP) sensor monitors the fuel tank pressure during engine operation and continuously transmits an input signal to the PCM. During the EVAP monitor testing, the FTP sensor monitors the fuel tank pressure or vacuum build-up.
6. The fuel tank mounted fuel vapor vent valve assembly, fuel tank mounted fuel vapor control valve (or remote fuel vapor control valve) are used in the EVAP Running Loss system to control the flow of fuel vapor entering the engine. All of these valves also prevent fuel tank overfilling during refueling operation and prevent liquid fuel from entering the EVAP canister and the EVAP canister purge valve under any vehicle severe handling or rollover condition. The liquid/vapor fuel discriminator is part of the fuel vapor control valve assembly on Escort/Tracer (2V) applications.
7. The EVAP Running Loss system, including all the fuel vapor hoses, can be checked when a leak is detected by the PCM. This can be done by pressurizing the system using Rotunda Evaporative Emission Tester kit 134-00056 or equivalent and the leak frequency (ultra-sonic) detector included with the kit.
The Fuel Tank Vapor System -
Gasoline is extremely volatile in almost all environments, and even diesel is aromatic. Since these vapors can be flammable or noxious, they must be contained & routed to the engine to be burned. But they are produced even when the vehicle is unused for long periods, so a simple tube from the fuel tank to the engine would still allow them to vent out the air filter. Also, during hot weather or violent maneuvers, the quantity of vapor generated can exceed the engine's capacity at low RPM, so the vapors must be stored & their flow regulated.
The system begins in the fuel tank where one or more valves are used to vent vapor pressure, but also to exclude liquid from the vapor system due to overfilling, slosh, or rollover. There may also be a pressure sensor to monitor the system's operation & effectiveness, and/or a vent valve (CANV solenoid, or built into the cap) to allow fresh air [b]into[/b] the fuel tank or vapor system. As vapor exits the tank, it flows thru a tube to a canister containing carbon (activated charcoal), which absorbs the fuel vapor, but allows air to pass. Depending on the size of the fuel tank, there may be several canisters, or a larger canister. Older canisters are vented, but they're known to collect water, so most modern canisters are sealed. Another tube leads from the canister toward the engine's intake, but it may contain a regulator valve (CANP solenoid, or VMV). The vapor system may also combine with the PCV system at this point.
Being virtually a zero-maintenance system, most faults are simple valve failures, hose leaks, or mechanical damage (collision, road debris, etc.).
Faults in the evaporative systems are usually detected by the use of a special machine which pumps a non-toxic non-flammable high-visibility smoke into the vapor lines to make leaks evident. But a common source of evaporative codes on '97-04 vehicles is the operator not securing the fuel filler cap. Earlier vehicles didn't detect this, and later vehicles are designed to exclude this from turning on the CEL.