Isuzu Trooper (1998-2002 year). Manual - part 295

6E–93

ENGINE DRIVEABILITY AND EMISSIONS

Idle Air Control (IAC) System Check

Circuit Description

The powertrain control module (PCM) controls engine
idle speed with the idle air control (IAC) valve. To increase
idle speed, the PCM retracts the IAC valve pintle away
from its seat, allowing more air to bypass the throttle bore.
To decrease idle speed, it extends the IAC valve pintle
towards its seat, reducing by pass air flow. Tech 2 will
read the PCM commands to the IAC valve in counts.
Higher counts indicate more air bypass (higher idle).
Lower counts indicate less air is allowed to bypass (lower
idle).

Diagnostic Aids

A slow, unstable, or fast idle may be caused by a non-IAC
system problem that cannot be overcome by the IAC
valve. Out of control range IAC Tech 2 counts will be
above 60 if idle is too low, and zero counts if idle is too
high. The following checks should be made to repair a
non-IAC system problem:

Vacuum leak (high idle) – If idle is too high, stop the
engine. Fully extend (low) IAC with the Tech 2. Start
the engine. If idle speed is above 800 RPM, locate and
correct the vacuum leak, including the PCV system.
Check for binding of the throttle blade or linkage.

Lean heated oxygen sensor signal (high air/fuel ratio) –
The idle speed may be too high or too low. Engine
speed may vary up and down, and disconnecting the
IAC valve does not help. Diagnostic trouble codes
P0131, P0151, P0171, or P0174 may be set. Tech 2
oxygen (O2) voltage will be less than 100 mV (0.1 V).
Check for low regulated fuel pressure, water in fuel, or
a restricted injector.

Rich heated oxygen sensor signal (low air/fuel ratio) –
The idle speed will be too low. Tech 2 IAC counts will
usually be above 80. The system is obviously rich and
may exhibit black smoke in the exhaust.

Tech 2 O2 voltage will be fixed at about 750 mV (0.75
V). Check for high fuel pressure, or a leaking or
sticking injector. A silicon-contaminated heated
oxygen sensor will show an O2 voltage slow to
respond on Tech 2.

Throttle body – Remove the IAC valve and inspect the
bore for foreign material.

IAC valve electrical connections – IAC valve
connections should be carefully checked for proper
contact.

PCV valve – An incorrect or faulty PCV valve may
result in an incorrect idle speed. Refer to

Diagnosis,

Rough Idle, Stalling. If intermittent poor driveability or
idle symptoms are resolved by disconnecting the IAC,
carefully recheck the connections and valve terminal
resistance, or replace the IAC.

Test Description

Number(s) below refer to the step number(s) on the
Diagnostic Chart.
1. The Tech 2 is used to extend and retract the IAC

valve. Valve movement is verified by an engine
speed change. If no change in engine speed
occurs, the valve can be resettled when removed
from the throttle body.

2. This step checks the quality of the IAC movement in

step 1. Between 700 revolutions per minute (RPM)
and about 1500 RPM, the engine speed should
change smoothly with each flash of the tester light
in both extend and retract. If the IAC valve is
retracted beyond the control range (about 1500
RPM), it may take many flashes to extend the IAC
valve before engine speed will begin to drop. This
is normal on certain engines. Fully extending the
IAC may cause engine stall. This may be normal.

6E–94

ENGINE DRIVEABILITY AND EMISSIONS

Idle Air Control (IAC) System Check

Step

Action

Value(s)

Yes

1. Ignition “OFF.”
2. Connect the Tech 2.
3. Set the parking brake.
4. Block the wheels.
5. Turn the air conditioning “OFF.”
6. Idle the engine in Park (A/T) or Neutral (M/T).
7. Operate the IAC test.
8. The engine speed should decrease and increase as

the IAC is cycled.

Does the RPM change?

—

Go to

Step 2

Go to

Step 3

RPM should change smoothly.

Does the RPM change within the range specified?

700-1500

RPM

—

Go to

Step 3

Check the IAC passages.

Are the IAC passages OK?

—

Go to

Step 4

Go to

Step 5

Clear any obstruction from the IAC passages.

Is the action complete?

—

Verify repair

—

Replace the IAC. Refer to

On-Vehicle Service, Idle Air

Control Valve.

Is the action complete?

—

Verify repair

—

6E–95

ENGINE DRIVEABILITY AND EMISSIONS

Knock Sensor (KS) System Check

(Engine Knock, Poor Performance, or Poor Economy)

D06RW035

Circuit Description

The knock sensor (KS) sends an AC voltage signal to the
powertrain control module (PCM). As the KS detects
engine knock, the signal to the PCM changes in amplitude
and frequency. The PCM retards timing if the engine
speed is over 900 RPM.

Diagnostic Aids

If the KS system checks OK, but detonation is the
complaint, refer to

Diagnosis, Detonation/Spark Knock.

Test Description

The numbers below refer to the step numbers on the
Diagnostic Chart.
9. The change in signal speed depends on how hard

the tapping is done. Normally there is about 1.5 to
10 mV at PCM pin A2 with the engine off. Loud
tapping should be able to make the reading jump to
20-25 mV AC.

6E–96

ENGINE DRIVEABILITY AND EMISSIONS

Knock Sensor (KS) System Check

(Engine Knock, Poor Performance, or Poor Economy)

Step

Action

Value(s)

Yes

Is DTC P0325 or P0327 set?

—

Go to

DTC

P0325 or

DTC P0327

Go to

Step 2

Run the engine at 1500 RPM.

Is there an internal engine knock?

—

Go to

Step 3

Go to

Step 4

Repair the mechanical problem.

Is the action complete?

—

Verify repair

—

1. Install Tech 2.
2. Turn the ignition “ON.”
3. Cycle through the list until “Knock Retard” is

displayed.

Is knock retard at the specified value?

Go to

Step 6

Go to

Step 7

Replace the PCM.

IMPORTANT: The replacement PCM must be
programmed. Refer to

UBS 98model year Immobilizer

Workshop Manual.

Is the action complete?

—

Verify repair

—

1. Start the engine.
2. Monitor the knock retard display on Tech 2 while

changing the throttle setting to place different loads
on the engine.

Is knock retard at the specified value? (Turn the ignition
“OFF.”)

Go to

Step 9

Go to

Step 7

1. At the rear of the engine, behind the rear fuel

injector on the lift side, disconnect the 2-wire knock
sensor harness connector.

2. Attach the positive lead of DVM to B+.
3. On the main harness side of the connector, use the

negative lead of the DVM to probe the connector pin
that is connected to the black wire.

Dose the DVM indicate the specified value?
(Reconnect the knock sensor harness.)

B+

Go to

Step 9

Go to

Step 8

Repair the open black wire ground for the shield which
prevents stray electromagnetic pulses from affecting
the knock signal.

Is the action complete?

—

Verify repair

—

1. Reconnect the wire harness.
2. Set a DVM to AC voltage.
3. With the DVM, backprobe the PCM connector at

A2.

4. Tap the engine lift brackprobe with a socket

extension.

Did the DVM show an increase in AC voltage while
tapping on the lift bracket?

—

System OK

Go to

Step 10

Replace the knock sensor.

Is the action complete?

—

Verify repair

—

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