Isuzu KB P190. Manual - part 557

6E–58

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–59

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–60

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.

ENGINE DRIVEABILITY AND EMISSIONS

6E–61

ISUZU STRATEGY BASED DIAGNOSTICS

Overview

As a retail service technician, you are part of the ISUZU
service team. The team goal is FIX IT RIGHT THE
FIRST TIME for the satisfaction of every customer. You
are a very important member of the team as you
diagnose and repair customer vehicles.

You have maximum efficiency in diagnosis when you
have an effective, organized plan for your work.
Strategy Based Diagnostics (refer to Figure 1) provides
you with guidance as you create and follow a plan of
action for each specific diagnostic situation.

STRATEGY BASED DIAGNOSTICS CHART