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Tech Tips Idle Air Control

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Tech Tips Idle Air Control



Datum01.04.2019
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Tech Tips - Idle Air Control

Some of the following information was originally posted to the VetteNet Listserver and is reprinted here with the author's permission.



Question: >Could someone explain the purpose of the IAC and how it works? I have an '86 L98 with some idling problems. I replaced the IAC last night. Now when I start the car the idle is rock solid until it heats up and then the idle runs at 1200 rpm and fluctuates. Before I replaced the IAC, my idle would run high right from a cold start and would drop to 800 only once the car was hot and at a full stop.

Answers:

The IAC is, basically, a valve controlled by the ECM in order to control the speed of the engine at idle. The IAC pintle is moved in and out by an integral stepper motor to any one of 256 positions. When serial data is viewed via a scanner, the IAC position at idle is typically around 20.

As the IAC pintle extends (less air entering the motor), RPM will drop. As the IAC pintle retracts (more air entering the motor), RPM will increase.

This allows the ECM to maintain the correct idle speed regardless of load on the engine (ie a/c turned "on").

Based on your description, it sounds as though the pintle on the new IAC valve was improperly set. Within your Service Manual you should be able to find a procedure for "setting" the pintle prior to installing the IAC into the throttle body. I believe the measurement is 1 1/8" from the pintle tip to the IAC gasket seating surface on the IAC body. *Please* double check this via your Service Manual. There should also be a procedure for resetting the IAC after setting the minimum idle air.

Besides resetting the IAC, I would recommend you check for vacuum leaks, verify the TPS is set correctly, check between the MAF and throttle body for "false air" leaks, and check the PCV valve for proper operation.

Good Luck,
Jim "Tail Lights First" Mason
'96 Grand Sport #007

Any tweaking of the throttle stop screw will give the ECM fits. The ECM wants to be in control of the idle and is not happy when it can't control it. If the normal learn limits of the ECM are exceeded, they will be reset to nominal, causing an erratic idle. Doing this assumes a working knowledge of your engine, and basic hand tools and an accurate tach are needed (a scan tool is ideal).

The IAC and associated passages need to be clean to work right. Remove the IAC. If the car has significant mileage, it may be dirty. You can clean it using carb cleaner and a small brass brush. At this time also clean the IAC passages in the throttle body. Once clean, install the IAC back in the throttle body and reconnect the IAC wires.

For the ECM to properly control idle, the throttle stop screw must be set for "minimum air". This is a process that sets the idle with the IAC fully extended. To fully extend the IAC, jumper ALDL pins A and B together, and turn the key on, but do not start the car. With the key on, not running, and in diags mode, the ECM will keep trying to fully extend the IAC. After 10 seconds or so, pull the IAC connector off the IAC *before* doing anything else. This will capture the IAC fully extended. Now pull out the jumper in the ALDL, and start the car. Typically the "minimum air" idle speed is in the 400 to 500 RPM range, so you may need the manual for the spec for your model year and engine. Set the idle to spec (if you have no spec, try 450 RPM) using the throttle stop screw. (The engine should be fully warm to do this.) Now shut the engine off and reconnect the IAC wires. The ECM does not 'know' where the IAC present position is, so pull the ECM fuse for 20-30 seconds. (This will cause a complete ECM reset of all learned parameters, including the learned IAC ones.) Then reinstall the ECM fuse. Turn the key on, wait 10 seconds or so, and turn the key back off. This will now reset the IAC to a known key-off "park" position. Now start the car. The engine should idle properly under control of the ECM. There are some learned values, such as an IAC offset for A/C, etc. that need to be learned, but this will happen under normal driving conditions. But if the idle is now still out of control, you have other problems.

From F-BODY.ORG

Question: >Thanks for the reply. Since I don't have a service manual, could someone let me know how to properly set the IAC before install. I had no idea it needed to be set. In order to modify the length of the pintle, do I push it in or out by hand?

Answer:

On a new IAC, you can adjust the pintle length by **gently** pushing in the pintle. The Service Manual says NOT to do this with an old one or risk stripping the gear.

To reset the IAC.....and this is from memory so someone needs to verify or provide the correct information......Slightly depress the accelerator pedal and hold. Start the engine briefly (just for a second) and then turn the key off. Wait 10 seconds and then start the engine normally.

Good Luck,


Jim "Tail Lights First" Mason
'96 Grand Sport #007

Question: >Another question, I've pulled the wire connector off the IAC w/ the motor running and I don't get any codes/service engine light. Is this normal?

Answer:

Yes, that's normal. The IAC is not monitored by the ECM for faults.



Good Luck,
Jim "Tail Lights First" Mason
'96 Grand Sport #007

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