The engine idle speed is controlled by the idle air control (IAC) valve. The IAC valve is on the throttle body. The IAC valve pintle moves in and out of an idle air passage bore to control air flow around the throttle plate. The valve consists of a movable pintle, driven by a gear attached to a two phase bi-polar permanent magnet electric motor called a stepper motor. The stepper motor is capable of highly accurate rotation, or of movement, called steps. The stepper motor has two separate windings that are called coils. Each coil is fed by two circuits from the powertrain control module (PCM). When the PCM changes polarity of a coil, the stepper motor moves one step. The PCM uses a predetermined number of counts to determine the IAC pintle position. Observe IAC counts with a scan tool. The IAC counts will increment up or down as the PCM attempts to change the IAC valve pintle position. An IAC Reset will occur when the ignition key is turned OFF. First, the PCM will seat the IAC pintle in the idle air passage bore. Second, the PCM will retract the pintle a predetermined number of counts to allow for efficient engine start-up. If the engine idle speed is out of range for a calibrated period of time, an idle speed diagnostic trouble code (DTC) may set.
Inspect for the following conditions:
• | A skewed high throttle position (TP) sensor |
• | Restricted air intake system |
• | Objects blocking the IAC passage or throttle bore. |
• | The correct positive crankcase ventilation (PCV) valve, properly installed and proper operation of the PCV valve. |
• | Proper operation and installation of all air intake components. |
• | Proper installation and operation of the mass airflow (MAF) sensor, if equipped. |
• | A tampered with or damaged throttle stop screw. |
• | A tampered with or damaged throttle plate, throttle shaft, throttle linkage, or cruise control linkage, if equipped. |
• | Excessive deposits in the IAC passage or on the IAC pintle. |
• | Excessive deposits in the throttle bore or on the throttle plate. |
• | Vacuum leaks |
• | Excessive load on engine i.e. transmission, power steering, alternator, etc. |
• | A high or unstable idle condition could be caused by a non-IAC system problem that can not be overcome by the IAC valve. Refer to Symptoms - Engine Controls . |
• | A low or unstable idle condition could be caused by a non-IAC system problem that can not be overcome by the IAC valve. Refer to Symptoms - Engine Controls . |
• | If the problem is determined to be intermittent, refer to Intermittent Conditions . |
The numbers below refer to the step numbers on the diagnostic table.
This test will determine the ability of the PCM and IAC valve circuits to control the IAC valve.
This test will determine the ability of the PCM to provide the IAC valve circuits with a ground. On a normally operating system, the test lamp should not flash while the IAC counts are incrementing.
Step | Action | Yes | No | ||||||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics | |||||||||||||||||||||||||
1 | Did you perform the Diagnostic System Check-Engine Controls? | Go to Step 2 | |||||||||||||||||||||||
2 |
Important: Ensure engine speed stabilizes with each commanded RPM change to determine if engine speed stays within 100 RPM less than commanded engine speed for a manual transmission. Or within 60 RPM less than commanded engine speed for an automatic transmission. Or 60 RPM above commanded engine speed.
Did the engine speed stay within 100 RPM less than commanded engine speed for a manual transmission. Or within 60 RPM less than commanded engine speed for an automatic transmission. Or within 60 RPM above commanded engine speed during the above test? | Go to Diagnostic Aids | Go to Step 3 | ||||||||||||||||||||||
3 |
Did the engine speed steadily decrease to near 600 RPM and steadily increase to near 1,700 RPM when the IAC valve was commanded in and out? | Go to Step 5 | Go to Step 4 | ||||||||||||||||||||||
4 | Did you observe an excessively high idle condition that could not be controlled with the IAC motor driver during the last test? | Go to Step 11 | Go to Step 12 | ||||||||||||||||||||||
Did the test lamp remain ON, never flashing, while the IAC counts where incrementing at any of the IAC valve circuits during the above test? | Go to Step 10 | Go to Step 6 | |||||||||||||||||||||||
6 | Did the test lamp remain OFF, never flashing, while the IAC counts where incrementing at any of the IAC valve circuits during the above test? | Go to Step 9 | Go to Step 7 | ||||||||||||||||||||||
Did the test lamp stay illuminated, never flashing, while the IAC counts where incrementing during the above test? | Go to Step 8 | Go to Step 17 | |||||||||||||||||||||||
8 | Inspect for a poor connection at the IAC valve harness connector. Refer to Testing for Intermittent Conditions and Poor Connections and to Connector Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 18 | Go to Step 13 | ||||||||||||||||||||||
9 |
Did you find and correct the condition? | Go to Step 18 | Go to Step 13 | ||||||||||||||||||||||
10 |
Did you find and correct the condition? | Go to Step 18 | Go to Step 13 | ||||||||||||||||||||||
11 |
Did you find and correct the condition? | Go to Step 18 | Go to Step 15 | ||||||||||||||||||||||
12 |
Did you find and correct the condition? | Go to Step 18 | Go to Step 15 | ||||||||||||||||||||||
13 | Inspect for a poor connection at the PCM harness connectors. Refer to Testing for Intermittent Conditions and Poor Connections and to Connector Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 18 | Go to Step 14 | ||||||||||||||||||||||
14 |
Did you find and correct the condition? | Go to Step 18 | Go to Step 17 | ||||||||||||||||||||||
15 | Inspect for a poor connection at the IAC valve harness connector. Refer to Testing for Intermittent Conditions and Poor Connections and to Connector Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 18 | Go to Step 16 | ||||||||||||||||||||||
16 | Replace the IAC valve. Refer to Idle Air Control Valve Replacement . Did you complete the replacement? | Go to Step 18 | -- | ||||||||||||||||||||||
17 | Replace the PCM. Refer to Powertrain Control Module Replacement . Did you complete the replacement? | Go to Step 18 | -- | ||||||||||||||||||||||
18 |
Did you verify the repair? | Go to Step 19 | Go to Step 2 | ||||||||||||||||||||||
19 | With a scan tool, observe the stored information, in Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | System OK |