Idle Speed Inspection with Scan Tool
Description
The engine idle speed is controlled by the powertrain control module (PCM) through the idle air control (IAC) valve. The IAC valve opens and closes the idle air (bypass) passage in the throttle body according to signals from the PCM. The PCM determines the correct engine idle speed by using information from various sensors and switches. The PCM increases IAC duty (increases the opening in the idle air passage) when engine speed drops below a specified value. The PCM maintains a smooth and stable idle by monitoring certain engine components in order to respond to changes in engine load. The engine idle speed is a factory set calibration and is not adjustable.
Diagnostic Aids
DO NOT attempt to adjust the IAC valve or the throttle valve opening. The engine IAC valve and idle speed are not adjustable. Operate the IAC valve with the scan tool by selecting the appropriate Output Control feature. Command the IAC valve to increase and decrease engine idle speed. Diagnose and replace the faulty component or locate and repair the cause of the idle concern. Check for any of the following conditions if the IAC valve fails to control engine speed when commanded by the scan tool:
| • | Vacuum leaks |
| • | Blockage of the idle air (bypass) passage |
| • | Improper PCV system operation |
| • | Excessive accessory engine loads |
| • | A faulty IAC valve |
| • | Faulty electrical connections in the IAC valve electrical circuit |
Check the A/C Idle-Up circuit if the engine idle speed is a concern when the air conditioning or windshield defroster are running. Refer to Air Conditioning (A/C) Idle Circuit Diagnosis .
Check the Electrical Load Idle-Up Signal circuit if the engine idle speed is a concern when the headlights, blower motor, or rear defogger are on. Refer to Electrical/Accessory Load Idle-Up Signal Diagnosis .
Test Description
The numbers below refer to the step numbers in the Diagnostic Table.
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The Powertrain OBD system check prompts the technician to complete some basic checks and store the Freeze Frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.
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This step checks the engine idle speed with the engine at normal operating temperatures and no load. DO NOT attempt to adjust the IAC valve or the throttle valve opening. The engine IAC valve and idle speed (throttle valve opening) are not adjustable. Locate and replace the faulty component or repair the cause of the idle concern.
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This step checks that the opening in the idle air (bypass) passage of the IAC valve is within the expected percentage (value). A DTC P0505 may set if this parameter stays outside of the specified value.
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This step checks for vacuum leaks or faulty connections in the fuel injection and EVAP control systems.
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This step checks the accelerator control cable for a binding or misadjusted condition.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
Did you perform the Powertrain On-Board Diagnostic System Check? | -- | |||
|
Important: Place the transmission gearshift control lever in the Neutral position and apply the parking brake. Block the drive wheels.
Is the idle speed within the specified value? | 700-800 RPM | |||
Observe the IAC Duty Cycle parameter on the scan tool. Is the IAC Duty Cycle parameter within the specified value? | 8%-25% (8%-35% at high altitude-above 6,560 ft) | Go to Diagnostic Aids | ||
Was a repair necessary? | -- | |||
Was an accelerator control cable adjustment necessary? | -- | |||
6 |
Is the idle speed within the specified value? | 825-925 RPM | ||
7 | Re-evaluate the customer concern. Operate the vehicle at the speed and the conditions that duplicate the concern. Is the engine's performance at idle speed within acceptable limits? | -- | System OK |
Idle Speed Inspection w/o Scan Tool
The engine idle speed is controlled by the powertrain control module (PCM) through the idle air control (IAC) valve. The IAC valve opens and closes the idle air passage in the throttle body according to signals from the PCM. The PCM uses various information from sensors and from switches in order to determine the correct engine idle speed. The PCM increases the opening in the idle air passage when the engine speed drops below a specified value. The PCM maintains a smooth idle by monitoring certain engine components in order to respond to changes in engine load. The engine idle speed is a factory set calibration and is not adjustable.
- Perform all of the following checks and operations:
- Start the engine.
- Run the engine to normal operating temperatures.
- Connect a tachometer to the vehicle.
- Access the duty check data link connector (8) that is located next to the PCM (7) behind the glove box.
- Connect a fused jumper wire to the terminal 2 diagnostic switch and the terminal 4 ground of the connector.
- Connect a DMM to the duty check terminal 6 and to the ground terminal 4 of the connector.
- Select the DC voltage and the duty cycle (Hz) or dwell settings on the DMM.
- Monitor the readings on the tachometer and on the DMM while turning ON the A/C.
- Compare the observed engine speed and the observed IAC duty cycle with the values specified in the table.
| • | Check the accelerator cable for free movement and for correct free-play. Refer to Accelerator Cable Adjustment . |
| • | Inspect the lead wires and the hoses of the sequential multiport fuel injection (SFI) and of the EVAP control systems for secure connections. |
| • | Turn off all of the accessories. |
| • | Verify that the air cleaner is installed correctly. |
| • | Check the air induction system for restrictions. |
| • | Check the vacuum hoses for correct routing. Refer to the Emission Label. |
| • | Check for and repair any vacuum leaks as necessary. |
Important: Before starting the engine, place the automatic transaxle selector in Park, or place the manual transaxle control lever in neutral. Set the parking brake, and block the drive wheels.
| • | If the indicated idle speed is not within the specified value, refer to Rough, Unstable, or Incorrect Idle and Stalling . |
| • | If the idle speed does not increase to the specified value when the A/C turns ON, refer to Air Conditioning (A/C) Idle Circuit Diagnosis . |
| • | Check the IAC valve and the IAC circuit for faults if the indicated IAC duty cycle is not within the specified value. Repair as necessary. |
A/C OFF | A/C ON | |
|---|---|---|
Engine Speed | 700-800 RPM | 825-925 RPM |
IAC Duty Cycle | 8%-25% *8%-35% | -- |
* High Altitude Only -- above 2 000 meters (6,560 feet) | ||
