Isuzu D-Max / Isuzu Rodeo (TFR/TFS). Manual - part 202

ENGINE DRIVEABILITY AND EMISSIONS

6E–49

GENERAL DESCRIPTION FOR 
EVAPORATIVE EMISSION SYSTEM

EVAP Emission Control System Purpose

The basic evaporative emission control system used on
the charcoal canister storage method. The method
transfers fuel vapor from the fuel tank to an activated
carbon (charcoal) storage devise to hold the vapors
when the vehicle is not operating.
The canister is located on the rear axle housing by the
frame cross-member.
When the engine is running, the fuel vapor is purged
from the carbon element by intake air flow and
consumed in the normal combustion process.

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 engine control module (ECM) 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 ECM. the output is
commanded when the appropriate conditions have
been met. These conditions are:

• The engine is fully warmed up.

• The engine has been running for a specified time.

• The IAT reading is above 10°C (50°F).

• Purge/Vacuum Hoses. Made of rubber compounds,

these hoses route the gasoline fumes from their
sources to the canister and from the canister to the
intake air flow.

• EVAP Canister. Mounted on a bracket ahead of the

fuel tank, the canister stores fuel vapors until the
ECM determined that engine conditions are right for
them to be removed and burned.

Poor idle, stalling and Poor driveability can be caused
by:

• A malfunctioning purge solenoid.

• A damaged canister.

• Hoses that are split, cracked, or not connected

properly.

System Fault Detection

The EVAP leak detection strategy is based on applying
vacuum to the EVAP system and monitoring vacuum
decay. At an appropriate time, the EVAP purge solenoid
is turned  “ON,” allowing the engine vacuum to draw a
small vacuum on the entire evaporative emission
system.
After the desired vacuum level has been achieved, the
EVAP purge solenoid is turned  “OFF,” sealing the
system. A leak is detected by monitoring for a decrease
in vacuum level over a given time period, all other
variables remaining constant.
If the desired vacuum level cannot be achieved in the
test described above, a large leak or a faulty EVAP
purge control solenoid valve is indicated.
Leaks can be caused by the following conditions:

• Missing or faulty fuel cap

• Disconnected, damaged, pinched, or blocked EVAP

purge line

• Disconnected, damaged, pinched, or blocked fuel

tank vapor line

• Disconnected or faulty EVAP purge control solenoid

valve

• Open ignition feed circuit to the purge solenoid

(1) Purge Solenoid Valve
(2) From Canistor to Purge Solenoid
(3) From Purge Solenoid to Intake

(1) Canistor
(2) Air Separator

1

3

2

1

2

6E–50

ENGINE DRIVEABILITY AND EMISSIONS

• Damaged EVAP canister

• Leaking fuel sender assembly O-ring

• Leaking fuel tank or fuel filler neck
The ECM supplies a ground to energize the purge
control solenoid valve (purge  “ON” ). The EVAP purge
control is 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 ECM and the output
is commanded when the appropriate conditions have
been met.
The system checks for conditions that cause the EVAP
system to purge continuously by commanding the EVAP
purge solenoid  “OFF”, EVAP purge solenoid duty ratio
“0%”. If fuel tank vacuum level increases during the test,
a continuous purge flow condition is indicated. This can
be caused by the following conditions:

• EVAP purge solenoid leaking

• EVAP purge and engine vacuum lines switched at the

EVAP purge control solenoid valve

• EVAP purge control solenoid valve driver circuit

grounded

ENGINE DRIVEABILITY AND EMISSIONS

6E–51

POSITIVE CRANKCASE 

VENTILATION (PCV) SYSTEM

Crankcase Ventilation System Purpose

The crankcase ventilation system is used to consume
crankcase vapors in the combustion process instead of
venting them to the atmosphere. Fresh air from the
throttle body is supplied to the crankcase and mixed
with blow-by gases. This mixture is then passed through
the positive crankcase ventilation (PCV) port into the
intake manifold.
While the engine is running, exhaust gases and small
amounts of the fuel/air mixture escape past the piston
rings and enter the crankcase. these gases are mixed
with clean air entering through a tube from the air intake
duct.

During normal, part-throttle operation, the system is
designed to allow crankcase gases to flow through the
PCV hose into the intake manifold to be consumed by
normal combustion.
A plugged positive crankcase ventilation port or PCV
hose may cause the following conditions:

• Rough idle.

• Stalling or slow idle speed.

• Oil leaks.

• Sludge in the engine.
A leaking PCV hose would cause:

• Rough idle.

• Stalling.

• High idle speed.

6E–52

ENGINE DRIVEABILITY AND EMISSIONS

A/C CLUTCH DIAGNOSIS

A/C Clutch Circuit Operation

A 12-volt signal is supplied to the A/C request input of
the ECM when the A/C is selected through the A/C
control switch.
The A/C compressor clutch relay is controlled through
the ECM. This allows the ECM to modify the idle air
control position prior to the A/C clutch engagement for
better idle quality. If the engine operating conditions are
within their specified calibrated acceptable ranges, the
ECM will enable the A/C compressor relay. This is done
by providing a ground path for the A/C relay coil within
the ECM. When the A/C compressor relay is enabled,
battery voltage is supplied to the compressor relay is
enabled, battery voltage is supplied to the compressor
clutch coil.
The ECM will enable the A/C compressor clutch
whenever the engine is running and the A/C has been
requested. The ECM will not enable the A/C
compressor clutch if any of the following conditions are
met:

• The engine speed is greater than 6000 RPM.

• The ECT is greater than 122°C (251°F).

• The throttle is more than 95% open.

A/C Clutch Circuit Purpose

The A/C compressor operation is controlled by the
engine control module (ECM) for the following reasons:

• It improves idle quality during compressor clutch

engagement.

• It improves wide open throttle (WOT) performance.

• It provides A/C compressor protection from operation

with incorrect refrigerant pressures.

The A/C electrical system consists of the following
components:

• The A/C control switch.

• The A/C refrigerant pressure switches.

• The A/C compressor clutch.

• The A/C compressor clutch relay.

• The ECM.

A/C Request Signal

This signal tells the ECM when the A/C mode is
selected at the A/C control switch. The ECM uses this
input to adjust the idle speed before turning on the A/C
clutch. The A/C compressor will be inoperative if this
signal is not available to the ECM.
Refer to A/C Clutch Circuit Diagnosis for A/C wiring
diagrams and diagnosis for the A/C electrical system.