GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 2000 🢒 Grand Prix 🢒 Engine 🢒 Engine Controls - 3.8L 🢒 Idle Air Control (IAC) System Diagnosis

Object Number: 240243 Size: SF
Inputs
OBD II Symbol Description Notice
Handling ESD Sensitive Parts Notice

Circuit Description

The powertrain control module (PCM) controls the engine idle speed by adjusting the position of the idle air control (IAC) motor pintle. The IAC is a bi-directional motor that is driven by 2 coils. The PCM pulses current to the IAC coils in steps, or counts, in order to extend the IAC pintle into a passage in the throttle body in order to decrease the air flow. This method allows highly accurate control of the idle speed, and quick response to changes in the engine load.

Diagnostic Aids
    • Inspect all wiring and connections for the PCM harness and for the IAC motor.
    • The following conditions may cause an intermittent:
       - Poor connections
       - Rubbed-through wire insulation
       - Broken wire inside the insulation
       - Damaged harness
       - Improper mating
       - Improperly formed or damaged terminals
       - Poor terminal-to-wire connection
       - Broken locks
    • Inspect the following components for a restricted air intake system:
       - A collapsed air intake duct
       - A restricted air filter element
       - Foreign objects blocking the air intake system
       - A vacuum leak--Inspect for a condition that causes a vacuum leak:
   • Disconnected or damaged hoses
   • Leaks at the EGR valve
   • Leaks at the EGR pipe to the intake manifold
   • Leaks at the throttle body
   • A faulty or incorrectly-installed PCV valve
   • Leaks at intake manifold brake booster hose disconnect
    • Inspect the throttle body for the following conditions:
       - Objects blocking the IAC passage or blocking the throttle bore
       - Objects blocking the IAC pintle from fully extending
       - Excessive deposits in the IAC passage and on the IAC pintle
       - Excessive deposits in the throttle bore and on the throttle plate
       - A sticking throttle plate
    • Reviewing the Failure Records vehicle mileage since the diagnostic test last failed may help to determine how often the condition that caused the DTC to be set occurs. This may help to diagnose the condition.

Test Description

The numbers below refer to the step numbers on the diagnostic table:

  1. When you replace PCM is replaced, you must program the new PCM.

Idle Air Control System Check

Step

Action

Value(s)

Yes

No

1

Did you perform the On-Board Diagnostic (OBD) System Check?

--

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

2
  1. Start the engine.
  2. Turn OFF all accessories, including the following:
    3.  - The air-conditioning
     - The rear defroster
     - Other accessory components
  3. Use a scan tool in order to command the RPM up to 1500, down to 650, and then up to 1500 while monitoring the engine speed on the scan tool.

Does the engine speed remain within the specified value of the desired idle during each RPM command?

±50 RPM

System OK

Go to Step 3

3
  1. Disconnect the IAC.
  2. Install a J 37027-A IAC System Monitor.
  3. With the engine running, command RPM up to 1500, down to 650, and then up to 1500 while observing the IAC System Monitor node lights.

Does each node light cycle red and green, but never OFF?

--

Go to Step 5

Go to Step 4

4
  1. Check the following circuits for an open, for a short to voltage, or for a short to ground:
    2. • The IAC A Low
    • The IAC A High
    • The IAC B Low
    • The IAC B High
  2. Repair any condition as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find a problem?

--

Go to Step 12

Go to Step 10

5

Did you notice a low idle symptom or a rich condition?

--

Go to Step 6

Go to Step 7

6
  1. Inspect for the following conditions:
    2. • Throttle body tampering
    • A restricted air intake system
    • A collapsed air intake duct
    • A restricted air filter element
    • Foreign objects blocking the air intake system
    • Blockages of the throttle body from the following conditions:
       - Objects blocking the IAC passage or the throttle bore
       - Objects blocking the IAC pintle from fully extending
       - Excessive deposits in the IAC passage and on the IAC pintle
       - Excessive deposits in the throttle bore and on the throttle plate
       - A sticking throttle plate
  2. Repair any condition as necessary.

Did you find a problem?

--

Go to Step 11

Go to Step 8

7
  1. Inspect for the following conditions:
    2. • Vacuum leaks-- Refer to Emissions Hose Routing Diagram

    EVAP Control System Overview
    Object Number: 200517 Size: LF
    (1)EVAP Vent Valve/Solenoid
    (2)EVAP Vent Valve/Solenoid Ignition Feed Circuit Terminal
    (3)EVAP Vent Valve/Solenoid Control Circuit Terminal
    (4)EVAP Vent Valve/Solenoid Filter
    (5)EVAP Vapor Lines
    (6)Fuel Tank Pressure Sensor
    (7)Fuel Tank Pressure Sensor Ground Circuit Terminal
    (8)Fuel Tank Pressure Sensor Signal Circuit Terminal
    (9)Fuel Tank Pressure Sensor Circuit 5 Volt Reference Circuit Terminal
    (10)Fuel Filler Pipe
    (11)Check Valve (Spitback)
    (12)Modular Fuel Sender Assembly
    (13)Fuel Limiter Vent Valve (FLVV)
    (14)Pressure/Vacuum Relief Valve (Optional)
    (15)EVAP Canister
    (16)EVAP Canister Purge Valve/Solenoid Ignition Feed Circuit Terminal
    (17)EVAP Canister Purge Valve/Solenoid Control Circuit Terminal
    (18)Intake Manifold Vacuum Source
    (19)EVAP Canister Purge Valve/Solenoid
    (20)EVAP Service Port
    .
    • Throttle body tampering
    • Throttle plate or throttle shaft for binding
    • Accelerator and cruise control cables that are misadjusted or binding--Refer to Accelerator Control Cable Replacement .
    • A faulty, missing, or incorrectly installed PCV valve-- Refer to Crankcase Ventilation System Inspection .
  2. Repair any condition as necessary.

Did you find a problem?

--

Go to Step 11

Go to Step 8

8
  1. Check for a poor connection at the IAC harness connector.
  2. Replace the faulty terminals as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find a problem?

--

Go to Step 12

Go to Step 9

9

Replace the IAC valve. Refer to Idle Air Control Valve Replacement .

Is action complete?

--

Go to Step 12

--

10
  1. Check for a poor connection at the PCM harness connector.
  2. If you find a problem, replace the faulty terminals as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find a problem?

--

Go to Step 12

Go to Step 11

11

Important: You must program the replacement PCM.

Replace the PCM. Refer to Powertrain Control Module Replacement/Programming .

Did you complete the replacement?

--

Go to Step 12

--

12
  1. Clear the DTCs.
  2. Start the engine.
  3. Turn OFF all accessories, including the air-conditioning and the rear defroster
  4. Using a scan tool, command RPM up to 1500, down to 650, and then up to 1500 while monitoring Engine Speed on the scan tool.

Does the engine speed remain within the specified value of the desired idle for each RPM command?

± 50 RPM

System OK

Go to Step 3