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1B-2 Aux. Emission Control Devices:
Purge Valve Leak Test
If the EVAP purge valve does not seal properly, fuel
vapors could enter the engine at an undesired time,
causing driveability concerns. The control module tests
for this by commanding the EVAP purge valve OFF,
closed; and vent valve ON, closed; sealing the system,
and monitoring the FTP for an increase in vacuum. If the
control module detects that EVAP system vacuum
increases above a calibrated value, DTC P0496 sets.
EVAP System Components
The EVAP system consists of the following components:
EVAP Canister
The canister is filled with carbon pellets used to absorb
and store fuel vapors. Fuel vapor is stored in the canister
until the control module determines that the vapor can
be consumed in the normal combustion process.
EVAP Purge Valve
The EVAP purge valve controls the flow of vapors from
the EVAP system to the intake manifold. This normally
closed valve is pulse width modulated (PWM) by the
control module to precisely control the flow of fuel vapor
to the engine. The valve will also be opened during some
portions of the EVAP testing, allowing engine vacuum to
enter the EVAP system.
EVAP Vent Valve
The EVAP vent valve controls fresh airflow into the
EVAP canister. The valve is normally open. The control
module will command the valve closed during some
EVAP tests, allowing the system to be tested for leaks.
Fuel Tank Pressure Sensor
The FTP sensor measures the difference between the
pressure or vacuum in the fuel tank and outside air
pressure. The control module provides a 5-volt reference
and a ground to the FTP sensor. The FTP sensor
provides a signal voltage back to the control module that
can vary between 0.1-4.9 volts. As FTP increases, FTP
sensor voltage decreases, high pressure = low voltage.
As FTP decreases, FTP voltage increases, low pressure
or vacuum = high voltage.
Crankcase Ventilation System Description
A crankcase ventilation system is used to consume
crankcase vapors created during the combustion
process instead of venting them to the atmosphere.
Fresh air is supplied through a filter to the crankcase, the
crankcase mixes the fresh air with the blow-by gases
and then passed through a positive crankcase
ventilation (PCV) or if iced tube into the intake manifold.
The PCV or if iced tube restricts the flow rate of the blow-
by gases using a 3 mm (0.118 in) (a) orifice located at
the end of the tube. If abnormal operating conditions
arise, the system is designed to allow excessive
amounts of blow-by gases to back flow through the
crankcase vent tube into the throttle body in order to be
consumed by normal combustion.
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Troubleshooting
