The idle air control (IAC) valve is located in the throttle body of the SFI system. It consists of a movable pintle, driven by a gear attached to an electric motor called a stepper motor. The IAC valve motor is a two phase bi-polar permanent magnet stepper motor that is capable of highly accurate rotation, or movement, every time the polarity of a winding is changed. This change in polarity can be seen when observing a test lamp connected between ground or battery positive voltage and an IAC valve circuit while the PCM is attempting to change engine RPM (the test lamp will flash on or off each time the polarity is changed). The PCM does not use a physical sensor to determine IAC pintle position, but uses a predicted number of counts, one count represents one change in polarity which equals one step of the stepper motor. The PCM counts the steps it has commanded to determine IAC pintle position. The PCM uses the IAC valve to control engine idle speed. It does this by changing the pintle position In the idle air passage of the throttle body. This varies the air flow around the throttle plate when the throttle is closed. To determine the desired position of the IAC pintle at idle or during deceleration, the PCM refers to the following inputs: engine RPM, battery voltage, air temperature, engine coolant temperature, throttle position sensor angle, engine load, and vehicle speed. When the ignition key is turned OFF, after an ignition cycle, the PCM will first seat the IAC pintle in the air bypass bore and then retract it a predetermined amount of counts to allow the proper amount of air to bypass the throttle plate for engine start-up. This procedure is known as an IAC Reset.
Inspect for the following conditions:
• | A skewed high TP sensor |
• | Restricted air intake system |
• | Objects blocking the IAC passage or throttle bore. |
• | The correct 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 air flow 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 . |
• | 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 | ||||||||||||||||||||||
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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 or within 150 RPM above commanded engine speed.
Did the engine speed stabilize within 100 RPM less than commanded engine speed or within 150 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 1,800 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 16 | |||||||||||||||||||||||
8 | Inspect for a poor connection at the IAC valve harness connector. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 17 | Go to Diagnostic Aids | ||||||||||||||||||||||
9 |
Did you find and correct the condition? | Go to Step 17 | Go to Step 13 | ||||||||||||||||||||||
10 |
Did you find and correct the condition? | Go to Step 17 | Go to Step 13 | ||||||||||||||||||||||
11 |
Did you find and correct the condition? | Go to Step 17 | Go to Step 14 | ||||||||||||||||||||||
12 |
Did you find and correct the condition? | Go to Step 17 | Go to Step 14 | ||||||||||||||||||||||
13 | Inspect for a poor connection at the PCM harness connectors. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 17 | Go to Step 16 | ||||||||||||||||||||||
14 | Inspect for a poor connection at the IAC valve harness connector. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems. Did you find and correct the condition? | Go to Step 17 | Go to Step 15 | ||||||||||||||||||||||
15 | Replace the IAC valve. Refer to Idle Air Control Valve Replacement . Did you complete the replacement? | Go to Step 17 | -- | ||||||||||||||||||||||
16 | Replace the PCM. Refer to Powertrain Control Module Replacement . Did you complete the replacement? | Go to Step 17 | -- | ||||||||||||||||||||||
17 |
Did you verify the repair? | Go to Step 18 | Go to Step 2 | ||||||||||||||||||||||
18 | With a scan tool observe the Stored information, Capture info. Does the scan tool display any DTCs that you have not diagnosed? | System OK |