General Description - Evaporative Emission (Evap) System; Evap Emission Control System Purpose . 6E1-439; Evap Emission Control System Operation 6E1-439; Enhanced Evaporative Emission Control System - Isuzu 1999 RODEO Workshop Manual

For A/C wiring diagrams and diagnosis for the A/C
electrical system, refer to A/C Clutch Circuit Diagnosis.
GENERAL DESCRIPTION —
EVAPORATIVE EMISSION (EVAP)
SYSTEM
EVAP Emission Control System Purpose
The basic evaporative emission (EVAP) control system
used on all vehicles is the charcoal canister storage
method. Gasoline vapors from the fuel tank flow into the
canister through the inlet labeled "TANK." These vapors
are absorbed into the activated carbon (charcoal) storage
device (canister) in order to hold the vapors when the
vehicle is not operating. The canister is purged by PCM
control when the engine coolant temperature is over 60°C
(140°F), the IAT reading is over 10°C (50°F), and the
engine has been running. Air is drawn canister through
the air inlet grid. The air mixes with the vapor and the
mixture is drawn into the intake manifold.
EVAP Emission Control System Operation
The EVAP canister purge is controlled by a solenoid valve
that allows the manifold vacuum to purge the canister.
The Powertrain control module (PCM) supplies a ground
to energize the solenoid valve (purge on). The EVAP
purge solenoid control is pulse–width modulated (PWM)
(turned on and off several times a second). The duty cycle
(pulse width) is determined by engine operating
conditions including load, throttle position, coolant
temperature and ambient temperature. The duty cycle is
calculated by the PCM. The output is commanded when
the appropriate conditions have been met. These
conditions are:
f The engine is fully warmed up.
f The engine has been running for a specified time.
f The IAT reading is above 10°C (50°F).
f A continuous purge condition with no purge
commanded by the PCM willset a DTC P1441.
Poor idle, stalling and Poor driveability can be caused by:
f A malfunctioning purge solenoid.
f A damaged canister.
f Hoses that are split, cracked, or not connected
properly.
Enhanced Evaporative Emission Control
System
The basic purpose of the Enhanced Evaporative
Emissions control system is the same as other EVAP
systems. A charcoal–filled canister captures and stores
gasoline fumes. When the PCM determines that the time
is right, it opens a purge valve which allows engine
vacuum to draw the fumes into the intake manifold. The
difference between this and other systems is that the
PCM monitors the vacuum and/or pressure in the system
to determine if there is any leakage. If the PCM
determines that the EVAP system is leaking or not
functioning properly, it sets a Diagnostic Trouble Code
(DTC) in the PCM memory.
RODEO X22SE 2.2L ENGINE DRIVEABILITY AND EMISSION
The enhanced EVAP system is required to detect
evaporative fuel system leaks as small as 0.040 in. (1.0
mm) between the fuel filler cap and purge solenoid. The
system can test the evaporative system integrity by
applying a vacuum signal (ported or manifold) to the fuel
tank to create a small vacuum. The PCM then monitors
the ability of the system to maintain the vacuum. If the
vacuum remains for a specified period of time, there are
no evaporative leaks and a PASS report is sent to the
diagnostic executive. If there is a leak, the system either
will not achieve a vacuum, or a vacuum cannot be
maintained. Usually, a failure can only be detected after a
cold start with a trip of sufficient length and driving
conditions to run the needed tests. The enhanced EVAP
system diagnostic will conduct up to eight specific
sub–tests to detect fault conditions. If the diagnostic fails
a sub–test, the PCM will store a Diagnostic Trouble Code
(DTC) to indicate the type of fault detected.
Electrical Components
The electrical components that make up the enhanced
EVAP system are:
Fuel Tank Pressure Sensor – The fuel tank pressure
sensor is a three–wire strain gauge sensor similar to a
common MAP sensor. However, the fuel tank pressure
sensor has very different electrical characteristics due to
its pressure differential design. The sensor measures the
difference between the air pressure (or vacuum) in the
fuel tank and the outside air pressure.
The sensor mounts at the top of the fuel pump assembly.
A three–wire electrical harness connects it to the PCM.
The PCM supplies a five–volt reference voltage and a
ground to the sensor. The sensor will return a voltage
between 0.1 and 4.9 volts. When the air pressure in the
fuel tank is equal to the outside air pressure, such as
when the fuel cap is removed, the output voltage of the
sensor will be 1.3 to 1.7 volts.
When the air pressure in the fuel tank is 4.5 in. H2O (1.25
kPa), the sensor output voltage will be 0.5 +/– 0.2 V.
When there is neither vacuum nor pressure in the fuel
tank, the sensor voltage will be 1.5 V. At –14 in. H2O
(–3.75 kPa), the sensor voltage will be 4.5 +/– 0.2 V.
EVAP Canister Purge Solenoid – Normally closed, the
purge solenoid opens upon the PCM's signal to allow
engine vacuum to purge gasoline fumes from the
canister. Mounted on top of the upper intake manifold
assembly.
EVAP Canister Vent Solenoid – Located next to the
canister, the vent solenoid opens to allow air into the
EVAP system. Fresh air is necessary to completely
remove gasoline fumes from the canister during purge.
The EVAP vent solenoid closes to seal off the evaporative
emissions system for leak testing.
6E1–439