Circuit Description
The ECM sends voltage pulses to the IAC motor winding causing the motor shaft and valve to move IN and OUT a given distance for each pulse (called counts) received. This movement controls air flow around the throttle plate, which in turn controls the engine idle speed.
Diagnostic Aids
An unstable idle may be a system problem that cannot be overcome by the IAC. The scan counts will read above 80 counts if too low and will read 0 counts if too high.
| • | If the IAC valve pintle position counts are low or zero, check for vacuum leaks at vacuum fittings, tees and hoses, the throttle body, and the intake manifold. A bottomed (zero count) IAC valve pintle (may indicate a minimum idle speed) above the specification. Refer to Minimum Idle Speed Check . |
| • | If IAC valve pintle position counts are high, look for carbon build-up in the IAC valve air inlet passage or evidence of tampering with stop screw. Also check for low engine power or excessive accessory loads. |
| • | A system too lean (high tailpipe air/fuel ratio) -- Engine speed may vary up and down and disconnecting the IAC may not stabilize engine speed. If a scan and/or a voltmeter reads an oxygen sensor output less than 300 mV (0.3 volt), check for a low regulated fuel pressure or water in the fuel. A DTC 44 (lean O2S) may be set. A lean tailpipe exhaust with an oxygen sensor output fixed above 800 mV (0.8 volt) could be a contaminated sensor, usually silicon. This may set a DTC 45 (rich O2S) even with lean tailpipe exhaust. |
| • | A system too rich (low tailpipe air/fuel ratio) -- System obviously rich and may exhibit black exhaust smoke. scan tool and/or voltmeter will read an oxygen sensor signal fixed above 800 mV (0.8 volt). |
| • | Check for the following items: |
| - | An injector leaking or sticking |
| - | A high fuel pressure |
| - | A vacuum leak in MAP hose |
| - | A normal or low O2S |
| - | Inspect for air being pumped into exhaust manifold in front of O2S (or exhaust leak). |
| • | A throttle body -- Remove IAC and inspect the bore for foreign material or evidence of the IAC valve dragging the bore |
| • | A/C compressor or relay failure -- Refer to A/C Diagnosis. in HVAC. |
| • | Refer to Rough, Unstable, or Incorrect Idle and Stalling . |
IAC Valve Reset Procedure
| • | Turn the ignition OFF for 10 seconds. |
| • | Start and run the engine for 5 seconds. |
| • | Turn the ignition OFF for 10 seconds. |
Test Description
The numbers below refer to the step numbers on the Diagnostic Table.
-
The IAC tester extends and retracts the IAC valve. The valve movement is verified by an engine speed change. If no change engine speed occurs, the valve can be retested when removed from the throttle body.
-
This step checks the quality of the IAC movement in Step 2. Between 700 RPM and about 1500 RPM, the engine speed should change smoothly with each flash of the tester lamp in both extend and retract. If the IAC valve retracts beyond the control range (about 1500 RPM), it may take many flashes in the extend position before the engine speed will begin to drop. This is normal on certain engines, fully extending the IAC may cause engine stall. This may be normal.
-
Steps 2 and 3 verified the proper IAC valve operation while this step checks the IAC circuits. Each lamp on the node lamp should flash red and green while the IAC valve is cycled. While the sequence of color is not important if either lamp is OFF or does not flash red and green, check the circuits for faults, beginning with poor terminal contacts.
A plugged fuel filter may cause a restricted fuel delivery, or a no start condition.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||
Does the RPM change? | -- | |||
Does the RPM change smoothly with each flash of the IAC driver within the specified range? | 700-1500 RPM | |||
Do the lamps flash red and green but never OFF? | -- | |||
5 | Using the other connector on the IAC driver pigtail, check the resistance across the IAC coils. Are the readings within the specified value between terminal A to B and C to D? | 40-80 ohms | ||
6 | Check the resistance between IAC terminals B to C and A to D. Are the resistances at the specified value? | ∞ | Refer to Diagnositc Aids | |
7 | Check for faulty connector terminal contacts. If you find a problem, repair as necessary. Did you find a problem? | -- | ||
8 | Check for open circuits including connections. If you find a problem, repair as necessary. Did you find a problem? | -- | ||
9 | Check for circuits shorted to ground or voltage. If you find a problem, repair as necessary. Did you find a problem? | -- | ||
10 | Check the ECM for faulty connections. If you find a problem, repair as necessary. Did you find a problem found? | -- | ||
11 | Check IAC passages for blockages or restrictions. If you find a problem, repair as necessary. Did you find a problem? | -- | ||
12 | Replace the IAC valve. Is the action complete? | -- | -- | |
13 | Replace the ECM. Refer to Engine Control Module Replacement . Is the action complete? | -- | -- | |
14 | Operate the vehicle within the conditions under which the original symptom was observed. Does the system now operate properly? | -- | System OK | Refer to Diagnostic Aids |
