The idle air control (IAC) valve is located in the throttle body of both sequential multiport fuel injection (SFI) and the multiport fuel injection (MFI) systems. The IAC valve 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 B+ and an IAC valve circuit while the powertrain control module (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 that have been commanded to determine IAC pintle position. The PCM uses the IAC valve in order to control engine idle speed. The PCM 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 (TP) 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 the IAC pintle 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.
• | DTCs P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0171, P0172, P0201, P0202, P0203, P0204, P0205, P0206, P0300 P0401, P0403, P0404, P0405, P0443, P1121, P1404, or P1441 are not set. |
• | The engine run time is more than 2 minutes. |
• | The engine coolant temperature (ECT) is greater than 70°C (158°F). |
• | The intake air temperature (IAT) greater than -18°C (0.4°F) |
• | The barometric pressure (BARO) greater than 70 kPa (10.1 psi). |
• | The ignition voltage between 9-18 volts. |
• | The vehicle speed sensor (VSS) is less than 4.8 km/h (3 mph). |
• | The TPS is less than 1.3 percent. |
• | The above conditions are present for 10 seconds. |
The actual engine speed is 100 RPM less than the desired engine speed.
The diagnostic must fail 4 consecutive times per key cycle.
• | The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails. |
• | The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records. |
• | The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail. |
• | A current DTC, Last Test Failed, clears when the diagnostic runs and passes. |
• | A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic. |
• | Clear the MIL and the DTC with a scan tool. |
Inspect for the following conditions:
• | A restricted air intake system--Check for roper operation and installation of all air intake components i.e. collapsed, clogged, or loose air intake ducts, clogged air filter, proper operation of the mass air flow (MAF) sensor, if equipped, etc. |
• | A tampered with or damaged throttle stop screw |
• | A tampered with or damaged throttle plate, throttle shaft, or throttle linkage |
• | Objects blocking the IAC passage or throttle bore |
• | Excessive deposits in the IAC passage or on the IAC pintle |
• | Excessive deposits in the throttle bore or on the throttle plate |
• | Vacuum leaks |
• | A low or unstable idle condition could be caused by a non-IAC system problem that cannot be overcome by the IAC valve. Refer to Symptoms - Engine Controls . |
• | If the problem is 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 Engine Controller and IAC valve circuits to control the IAC valve.
This test will determine the ability of the PCM to provide the IAC circuits with a ground. On a normal 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 that the engine speed stabilizes with each commanded RPM change, in order the to determine if engine speed stays within 100 RPM of the commanded RPM.
Does the engine speed stabilize within 100 RPM of the commanded RPM during the above test? | Go to Step 3 | Go to Step 4 | ||||||||||||||
3 |
Does the DTC fail this ignition? | Go to Step 4 | Go to Diagnostic Aids | ||||||||||||||
4 |
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 10 | ||||||||||||||
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 9 | 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 8 | Go to Step 7 | ||||||||||||||
did the test lamp stay illuminated (never flashing) while the IAC counts where incrementing during the above test? | Go to Diagnostic Aids | Go to Step 14 | |||||||||||||||
8 |
Did you find and correct the condition? | Go to Step 15 | Go to Step 11 | ||||||||||||||
9 |
Did you find and correct the condition? | Go to Step 15 | Go to Step 11 | ||||||||||||||
10 | Visually/Physically inspect for the following:
Repair any of the above conditions as necessary. Refer to the appropriate repair procedure. Did you find and correct the condition? | Go to Step 15 | Go to Step 12 | ||||||||||||||
11 | 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 15 | Go to Step 14 | ||||||||||||||
12 | 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 15 | Go to Step 13 | ||||||||||||||
13 | Replace the IAC valve. Refer to Idle Air Control Valve Replacement . Did you complete the replacement? | Go to Step 15 | -- | ||||||||||||||
14 | Replace the PCM. Refer to Powertrain Control Module Replacement . Did you complete the replacement? | Go to Step 15 | -- | ||||||||||||||
15 |
Does the DTC run and pass? | Go to Step 16 | Go to Step 2 | ||||||||||||||
16 | With a scan tool, observe the stored information, Capture info. Does the scan tool display any DTCs that you have not diagnosed? | System OK |