Evaporative Emissions System: Description and Operation

Evaporative Emission Diagram:

Evaporative Emission System Diagram:

PURPOSE
The Evaporative Emission (EVAP) system will prevent fuel vapor build-up in the sealed fuel tank.

OPERATION
The conditions that enable canister purge during various engine operating modes are a warmed up engine, wide open or part throttle position, no engine overheating, fuel control in desired mode (closed loop only or both open and closed loop) and EEC power relay on.

EVAP System Power Flow
The EEC power relay provides vehicle power to the Powertrain Control Module (PCM). The EVAP canister purge solenoid, Vapor Management Valve, VMV, Purge Flow (PF) sensor and Canister Vent (CV) Solenoid. The PCM provides reference voltage to the Fuel Tank Pressure (FTP) sensor. The ground signals EVAP canister purge , VMV, PFS, CV and Signal Return (SIG RTN), are controlled by the PCM. The ground signals, except the SIG RTN, are cycled off and on at a 10 Hertz frequency with a variable duty cycle. The duty cycle ramps up (electrical control) to slowly draw the the canister vapors in the intake manifold.

EVAP Running Loss System
Uses an EVAP Purge Monitor to detect all leaks greater than 0.040 inch by performing a vacuum check of the complete EVAP system. The monitor relies upon the CV solenoid to seal the entire EVAP system from atmosphere and the VMV to pull engine vacuum on the fuel tank. Then with the system sealed and vacuum maintained, the monitor uses the FTP sensor to observe the rate at which the vacuum is lost during a period of system vacuum bleed-up.

System Failure
If a target vacuum is not sensed by the FTP sensor within a given time, then a leak or flow fault exists. If the target vacuum is reached, then both solenoids are closed in order to hold the vacuum for a calibrated period of time. If the vacuum bleeds up above a fault threshold within that period of time, the EVAP Monitor test fails. Diagnostic Trouble Codes (DTC's) are set after three unsuccessful attempts to hold vacuum. The Malfunction Indicator Light (MIL) is activated after one of the above tests fail on two consecutive drive cycles.

DTC's associated with with a minor or gross EVAP leak are DTC's PO442 and PO455. The DTC associated with the EVAP system unable to bleed up fuel tank vacuum is DTC P1450.

CONSTRUCTION
Consists of a fuel tank, fuel fill cap, fuel vapor valve, carbon canister, EVAP Canister Purge solenoid. Vapor Management Valve, Purge Flow Sensor, fuel vapor hoses and crankcase vent tube assembly.