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Isuzu Trooper (1998-2002 year). Manual - part 416

6E–34

6VE1 3.5 ENGINE DRIVEABILITY AND EMISSIONS

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6E–35

6VE1 3.5L ENGINE DRIVEABILITY AND EMISSIONS

Diagnosis

Strategy-Based Diagnostics

The strategy-based diagnostic is a uniform approach to
repair all Electrical/Electronic (E/E) systems. The
diagnostic flow can always be used to resolve an E/E
system problem and is a starting point when repairs are
necessary. The following steps will instruct the technician
how to proceed with a diagnosis:

1. Verify the customer complaint.

To verify the customer complaint, the technician
should know the normal operation of the system.

2. Perform preliminary checks.

Conduct a thorough visual inspection.

Review the service history.

Detect unusual sounds or odors.

Gather diagnostic trouble code information to
achieve an effective repair.

3. Check bulletins and other service information.

This includes videos, newsletters, etc.

4. Refer to service information (manual) system

check(s).

“System checks” contain information on a system
that may not be supported by one or more DTCs.
System checks verify proper operation of the
system. This will lead the technician in an
organized approach to diagnostics.

5. Refer to service diagnostics.

DTC Stored

Follow the designated DTC chart exactly to make an
effective repair.

No DTC

Select the symptom from the symptom tables. Follow the
diagnostic paths or suggestions to complete the repair.
You may refer to the applicable component/system check
in the system checks.

No Matching Symptom

1. Analyze the complaint.
2. Develop a plan for diagnostics.
3. Utilize the wiring diagrams and the theory of

operation.

Combine technician knowledge with efficient use of the
available service information.

Intermittents

Conditions that are not always present are called
intermittents. To resolve intermittents, perform the
following steps:

1. Observe history DTCs, DTC modes, and freeze

frame data.

2. Evaluate the symptoms and the conditions described

by the customer.

3. Use a check sheet or other method to identify the

circuit or electrical system component.

4. Follow the suggestions for intermittent diagnosis

found in the service documentation.

Most Scan Tools, such as the Tech 2, have data-capturing
capabilities that can assist in detecting intermittents.

No Trouble Found

This condition exists when the vehicle is found to operate
normally. The condition described by the customer may
be normal. Verify the customer complaint against another
vehicle that is operating normally. The condition may be
intermittent. Verify the complaint under the conditions
described by the customer before releasing the vehicle.

1. Re-examine the complaint.

When the complaint cannot be successfully found or
isolated, a re-evaluation is necessary. The complaint
should be re-verified and could be intermittent as
defined in

Intermittents section, or could be normal.

2. Repair and verify.

After isolating the cause, the repairs should be made.
Validate for proper operation and verify that the
symptom has been corrected. This may involve road
testing or other methods to verify that the complaint
has been resolved under the following conditions:

Conditions noted by the customer.

If a DTC was diagnosed, verify a repair by
duplicating conditions present when the DTC was
set as noted in the Failure Records or Freeze
Frame data.

Verifying Vehicle Repair

Verification of the vehicle repair will be more
comprehensive for vehicles with OBD system
diagnostics. Following a repair, the technician should
perform the following steps:

IMPORTANT:

Follow the steps below when you verify

repairs on OBD systems. Failure to follow these steps
could result in unnecessary repairs.

1. Review and record the Failure Records and the

Freeze Frame data for the DTC which has been
diagnosed (Freeze Frame data will only be stored for
an A or B type diagnostic and only if the MIL(”Check
Engine” lamp) has been requested).

2. Clear the DTC(S).
3. Operate the vehicle within conditions noted in the

Failure Records and Freeze Frame data.

4. Monitor the DTC status information for the DTC which

has been diagnosed until the diagnostic test
associated with that DTC runs.

6E–36

6VE1 3.5 ENGINE DRIVEABILITY AND EMISSIONS

General Service Information

OBD Serviceablity Issues

With the introduction of OBD diagnostics across the
entire passenger car and light-duty truck market in 1996,
illumination of the MIL (“Check Engine” lamp) due to a
non-vehicle fault could lead to misdiagnosis of the
vehicle, increased warranty expense and customer
dissatisfaction. The following list of non-vehicle faults
does not include every possible fault and may not apply
equally to all product lines.

Non-OEM Parts
All of the OBD diagnostics have been calibrated to run
with OEM parts. Something as simple as a
high-performance exhaust system that affects exhaust
system back pressure could potentially interfere with the
operation of the EGR valve and thereby turn on the MIL
(“Check Engine” lamp). Small leaks in the exhaust
system near the post catalyst oxygen sensor can also
cause the MIL (“Check Engine” lamp) to turn on.
Aftermarket electronics, such as transceivers, stereos,
and anti-theft devices, may radiate EMI into the control
system if they are improperly installed. This may cause a
false sensor reading and turn on the MIL (“Check Engine”
lamp).

Environment
Temporary environmental conditions, such as localized
flooding, will have an effect on the vehicle ignition system.
If the ignition system is rain-soaked, it can temporarily
cause engine misfire and turn on the MIL (“Check Engine”
lamp).

Refueling
A new OBD diagnostic was introduced in 1996 on some
vehicles. This diagnostic checks the integrity of the entire
evaporative emission system. If the vehicle is restarted
after refueling and the fuel cap is not secured correctly,
the on-board diagnostic system will sense this as a
system fault and turn on the MIL (“Check Engine” lamp)
with a DTC P0440.

Vehicle Marshaling
The transportation of new vehicles from the assembly
plant to the dealership can involve as many as 60 key
cycles within 2 to 3 miles of driving. This type of operation
contributes to the fuel fouling of the spark plugs and will
turn on the MIL (“Check Engine” lamp) with a P0300
Misfire DTC.

Poor Vehicle Maintenance
The sensitivity of OBD diagnostics will cause the MIL
(“Check Engine” lamp) to turn on if the vehicle is not
maintained properly. Restricted air filters, fuel filters, and
crankcase deposits due to lack of oil changes or improper
oil viscosity can trigger actual vehicle faults that were not
previously monitored prior to OBD. Poor vehicle
maintenance can’t be classified as a “non-vehicle fault”,
but with the sensitivity of OBD diagnostics, vehicle
maintenance schedules must be more closely followed.

Related System Faults
Many of the OBD system diagnostics will not run if the
PCM detects a fault on a related system or component.
One example would be that if the PCM detected a Misfire
fault, the diagnostics on the catalytic converter would be
suspended until Misfire fault was repaired. If the Misfire
fault was severe enough, the catalytic converter could be
damaged due to overheating and would never set a
Catalyst DTC until the Misfire fault was repaired and the
Catalyst diagnostic was allowed to run to completion. If
this happens, the customer may have to make two trips to
the dealership in order to repair the vehicle.

Visual / Physical Engine Compartment
Inspection

Perform a careful visual and physical engine
compartment inspection when performing any diagnostic
procedure or diagnosing the cause of an emission test
failure. This can often lead to repairing a problem without
further steps. Use the following guidelines when
performing a visual/physical inspection:

Inspect all vacuum hoses for pinches, cuts,
disconnections, and proper routing.

Inspect hoses that are difficult to see behind other
components.

Inspect all wires in the engine compartment for proper
connections, burned or chafed spots, pinched wires,
contact with sharp edges or contact with hot exhaust
manifolds or pipes.

Basic Knowledge of Tools Required

NOTE: Lack of basic knowledge of this powertrain when
performing diagnostic procedures could result in an
incorrect diagnosis or damage to powertrain
components. Do not attempt to diagnose a powertrain
problem without this basic knowledge.

A basic understanding of hand tools is necessary to
effectively use this section of the Service Manual.

6E–37

6VE1 3.5L ENGINE DRIVEABILITY AND EMISSIONS

Serial Data Communications

Class 2 Serial Data Communications

Government regulations require that all vehicle
manufacturers establish a common communication
system. This vehicle utilizes the “Class 2” communication
system. Each bit of information can have one of two
lengths: long or short. This allows vehicle wiring to be
reduced by transmitting and receiving multiple signals
over a single wire. The messages carried on Class 2 data
streams are also prioritized. If two messages attempt to
establish communications on the data line at the same
time, only the message with higher priority will continue.
The device with the lower priority message must wait.
The most significant result of this regulation is that it
provides Scan tool manufacturers with the capability to
access data from any make or model vehicle that is sold.
The data displayed on other Scan tools will appear the
same, with some exceptions. Some Scan tools will only
be able to display certain vehicle parameters as values
that are a coded representation of the true or actual value.
On this vehicle the Scan tool displays the actual values for
vehicle parameters. It will not be necessary to perform
any conversions from coded values to actual values.

On-Board Diagnostic (OBD)

On-Board Diagnostic Tests

A diagnostic test is a series of steps, the result of which is
a pass or fail reported to the diagnostic executive. When
a diagnostic test reports a pass result, the diagnostic
executive records the following data:

The diagnostic test has been completed since the last
ignition cycle.

The diagnostic test has passed during the current
ignition cycle.

The fault identified by the diagnostic test is not
currently active.

When a diagnostic test reports a fail result, the diagnostic
executive records the following data:

The diagnostic test has been completed since the last
ignition cycle.

The fault identified by the diagnostic test is currently
active.

The fault has been active during this ignition cycle.

The operating conditions at the time of the failure.

Remember, a fuel trim DTC may be triggered by a list of
vehicle faults. Make use of all information available (other
DTCs stored, rich or lean condition, etc.) when
diagnosing a fuel trim fault.

Comprehensive Component Monitor
Diagnostic Operation

Comprehensive component monitoring diagnostics are
required to monitor emissions-related input and output
powertrain components.

Input Components:
Input components are monitored for circuit continuity and
out-of-range values. This includes rationality checking.
Rationality checking refers to indicating a fault when the
signal from a sensor does not seem reasonable, i.e.
Throttle Position (TP) sensor that indicates high throttle
position at low engine loads or MAP voltage. Input
components may include, but are not limited to the
following sensors:

Vehicle Speed Sensor (VSS)

Crankshaft Position (CKP) sensor

Throttle Position (TP) sensor

Engine Coolant Temperature (ECT) sensor

Manifold Absolute Pressure (MAP) sensor

Mass Air Flow (MAF) sensor

In addition to the circuit continuity and rationality check,
the ECT sensor is monitored for its ability to achieve a
steady state temperature to enable closed loop fuel
control.

Output Components:
Output components are diagnosed for proper response to
control module commands. Components where
functional monitoring is not feasible will be monitored for
circuit continuity and out-of-range values if applicable.
Output components to be monitored include, but are not
limited to, the following circuits:

Control module controlled EVAP Canister Purge
Valve

Electronic Transmission controls

A/C relays

VSS output

MIL control

Cruise control inhibit

Refer to PCM and Sensors in General Descriptions.

Passive and Active Diagnostic Tests
A passive test is a diagnostic test which simply monitors a
vehicle system or component. Conversely, an active test,
actually takes some sort of action when performing
diagnostic functions, often in response to a failed passive
test. For example, the EGR diagnostic active test will
force the EGR valve open during closed throttle decel
and/or force the EGR valve closed during a steady state.
Either action should result in a change in manifold
pressure.

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