Idle air control valve
Idle air control valve refers to the idle-control valve used by the Pontiac Fiero ECM.
The idle air control valve is often abbreviated as IAC. It allows the ECM to control idle speed by adjusting the amount of air that bypasses the throttle plate.
Overview
The IAC valve is part of the throttle-body and idle-control system.
When the throttle is closed, the engine still needs air to idle. The ECM controls this bypass air through the IAC valve. By moving the IAC valve in or out, the ECM can raise or lower idle speed.
The IAC valve is involved in cold idle, warm idle, deceleration behavior, idle recovery, and idle stability.
Location
Image wanted: Photo or diagram showing the idle air control valve location on the Fiero throttle body. Ideally include both the 2.8L V6 and 2.5L four-cylinder locations if different.
The idle air control valve is mounted on the throttle body.
On the 2.8L V6, the IAC valve is mounted in the throttle body assembly and controls an idle air passage around the throttle plate.
On the 2.5L four-cylinder, the IAC valve is also mounted on the throttle body, though the exact appearance and access may vary by year and engine configuration.
The IAC valve should not be confused with the Throttle position sensor. The TPS reports throttle position to the ECM, while the IAC valve is an ECM-controlled output that adjusts idle air flow.
Function
The ECM uses the IAC valve to control idle speed.
The IAC valve can affect:
- Cold idle speed
- Warm idle speed
- Idle recovery after throttle release
- Idle behavior when coming to a stop
- Idle compensation for engine load
- Deceleration behavior
- Stalling prevention
Related trouble codes
Idle-control-related diagnostic trouble codes include:
- Code 35 - Idle speed error
Other codes that can affect idle include:
- Code 21 - Throttle position sensor circuit, high voltage
- Code 22 - Throttle position sensor circuit, low voltage
- Code 33 - MAP sensor circuit, high voltage
- Code 34 - MAP sensor circuit, low voltage
Code 35
Code 35 indicates that the ECM cannot maintain the expected idle speed.
Possible causes include:
- Vacuum leak
- Dirty throttle body
- Faulty idle air control valve
- IAC wiring problem
- Poor IAC connector contact
- Incorrect idle adjustment
- Sticking throttle linkage
- Throttle plate not closing correctly
- Misadjusted throttle position sensor
- EGR leak
- Poor engine grounds
- ECM control problem
Symptoms
Possible symptoms of an IAC or idle-control problem include:
- High idle
- Low idle
- Surging idle
- Rough idle
- Stalling
- Engine stalls when coming to a stop
- Idle speed does not settle after throttle release
- Idle speed changes unexpectedly
- Check engine light
- Code 35
Diagnosis
Basic diagnostic checks include:
- Check for stored diagnostic trouble codes.
- Inspect the IAC connector.
- Inspect IAC wiring for damage.
- Check for Vacuum leaks.
- Inspect the throttle body for dirt or carbon buildup.
- Check that the throttle linkage moves freely.
- Check that the throttle plate returns fully.
- Check the Throttle position sensor signal if applicable.
- Check for EGR leakage.
- Check grounds and ECM connections.
- Check IAC operation if diagnostic equipment or service procedures are available.
Throttle body relationship
The IAC valve works through an idle air passage in the throttle body. Dirt, carbon buildup, incorrect adjustment, or throttle-plate problems can affect idle speed even if the IAC valve itself is not faulty.
Related topics:
Vacuum leaks
A vacuum leak can cause high, unstable, or uncontrollable idle speed. In that situation, the ECM may not be able to bring idle speed down even if the IAC valve is working correctly.
Related topics:
ALDL data
Live ECM data through ALDL diagnostics can be useful when diagnosing idle-control problems.
IAC counts, throttle position, coolant temperature, MAP readings, and engine rpm can help show whether the ECM is trying to correct the idle and whether another problem is preventing it from doing so.
Notes
An idle problem does not automatically mean the idle air control valve is bad.
Vacuum leaks, throttle-body issues, TPS problems, EGR leaks, wiring faults, poor grounds, and engine mechanical problems can all cause idle-control symptoms.
Do not adjust throttle-body stop screws or minimum-air settings unless following a proper service procedure. Incorrect adjustment can make idle problems worse.