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Handling No Trouble Code Problems
Your car has a problem. So, naturally, the first thing you do is check for Diagnostic Trouble Codes to see what the problem is. But there are no codes. So now what do you do?

Check Idle Control Valve (IAC) Motor Operation

PURPOSE:
The purpose of the Idle Control Valve (IAC) valve assembly is to control engine idle speed, while preventing stalls due to changes in engine load. The IAC valve, mounted in the throttle body, controls bypass air around the throttle valve.

OPERATION:
By moving a conical valve known as a pintle, "IN" toward the seat (to decrease air flow) or "OUT" away from the seat (to increase air flow), a controlled amount of air moves around the throttle plate. If engine speed is too low, more air is bypassed around the throttle valve to increase RPM. If engine speed is too high, less air is bypassed around the throttle valve to decrease RPM. The PCM moves the IAC valve in small steps called counts which can he measured using a scan tool connected to the Data Link Connector (DLC).

Handling No Trouble Code Problems

During idle, the proper position of the IAC valve is calculated by the Powertrain Control Module (PCM) based on battery voltage, coolant temperature, engine load, and engine RPM. If the RPM drops below specification and the throttle valve is closed, the PCM senses a near stall condition and calculates a new valve position to prevent stalling.

If the IAC valve is disconnected and reconnected while the engine is running, the resulting idle RPM may be wrong and resetting of the IAC valve will be required.

The IAC valve will reset when the ignition is turned "ON" then "OFF." When servicing the IAC valve, it should only be disconnected or connected with the ignition "OFF."

If the PCM is without battery power for any reason, the programmed position of the IAC valve pintle is lost, and replaced with a default value. This causes the engine idle to be unstable for a period of approximately seven to 30 minutes.

The IAC valve affects only the idle characteristics of the vehicle. If it is fully retracted, too much air will be allowed into the manifold and idle speed will be high. If it is stuck closed, too little air will be allowed in the manifold, and idle speed will be too low. If it is stuck part way open, the idle may be rough, and will not respond to engine load changes.

Handling No Trouble Code Problems
IAC Valve (Typical)

  1. Remove IAC Valve.
  2. Inspect IAC Valve operation.
  • Connect the positive (+) lead from the battery to terminal +B and negative (-) lead to terminal Na, and check that the valve is closed.
  • If operation is not as specified, replace the IAC valve.

Handling No Trouble Code Problems

  • Connect the positive (+) lead from the battery to terminal +B and negative (-) lead to terminal Nb, and check that the valve is open.
  • If operation is not as specified, replace the IAC valve.

Handling No Trouble Code Problems

As always, check the service manual for the proper procedures and specifications for your particular vehicle.

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Additional information provided courtesy of ALLDATA

© 2003 Vincent T. Ciulla


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