Setting Base Idle


Corvette 1989 - 350 TPI Engine



Setting Base Idle Speed (using 89 GM Helms Manual)


If this has not been done on your car before, a pressed-in plug will cover the adjusting screw. It’s located on the driver’s side of the throttle body. Remove this plug by using a small punch. Impact the plug (lightly) on the edge of its face and this will allow its removal.

1. Start the car and let the engine warm up to its operating temperature. (It is suggested that you make sure the timing of the engine is set to specification prior to doing this procedure.) During this step, the engine should be in closed loop operation and all electrical accessories INCLUDING COOLING FANS should be off. Shut off the engine after it is warmed up.

2. Turn ignition switch to ON, but do not start engine. Adjust the Throttle Position Sensor to .54 +/- .08 volts using a digital meter. There are 3 wires stacked vertically on the TPS. You will need to be able to measure the voltage between the two top wires. You can either buy a special harness connector that breaks these wires out (from Mid America costing $12), or gently pierce the insulation of the wires with the pointy prongs on your voltmeter. You can also stick a paper clip into each of the two top locations of the connector and clamp onto the paper clips to measure the voltage. The adjustment is accomplished using a Torx driver and loosening the two screws that hold the TPS to the throttle body. Then rotate the sensor either clockwise or counter clockwise (with the ignition on and engine not running) to obtain the .54 volt specification. Retighten the two TPS Torx screws to “lock in” the adjustment.

3. With the IAC connected and the ignition “OFF,” insert an Auto Zone tool (free upon request from Auto Zone) into the ALDL diagnostic connector terminals “A” to “B.” (These are the upper right two holes in the ALDL connector.) This grounds the ECM and places it into the diagnostic mode.

4. Turn the ignition to the “ON” position without starting the engine. Wait 30 seconds before doing step 5. Doing this forces the IAC pintle to the closed position (closes air passage) within the throttle body.

5. With the ignition still in the “ON” position, disconnect the IAC connector at the IAC valve. This connector has four wires and is located on the passenger side of the throttle body. This action prevents the ECM from controlling the idle speed using the IAC valve.

6. Turn ignition switch off!

7. Remove the Auto Zone tool from the diagnostic connector.

8. Disconnect the EST timing connector located just behind the driver’s side valve cover. This is necessary to prevent the ECM from attempting to adjust the timing as you adjust the idle speed. This will be a brown wire with a white stripe and easily separates using a small screwdriver to raise the locking tab. Failure to do this step will allow the ECM to control the idle and you will not be able to properly adjust the base idle.

9. Start the engine. The idle RPM will be very low, and you may have to coax the engine using the gas pedal or your hand and the throttle cable to keep it running. I find it useful to insert a small screwdriver between the end of the idle adjusting screw and the throttle body to keep the car running at this low idle point. HOWEVER, REMOVE THE SMALL SCREWDRIVER BEFORE MAKING THE FINAL IDLE ADJUSTMENT!!!

10. If your car is an automatic, set the parking brake and put the transmission in park. If your car is a manual, leave it in neutral.

11. Adjust the idle speed screw to obtain 425 +/-25 engine rpm. This will require a Torx tool to adjust the idle screw in the throttle body. As you adjust the idle screw to obtain the desire idle speed, you are changing the TPS voltage setting. The TPS will require re-adjustment in step 12. Make sure the cooling fans are not running while you make this adjustment.

12. Shut off the engine, re-connect the IAC and the EST timing connectors.

13.Turn on the ignition switch and do not start the engine. Reset the TPS to .54 +/-.08 volts as stated in step 2. Turn off engine.

14.Reset the IAC pintle position by:
a.Depress accelerator pedal slightly.
b.Start and run engine for about 5 seconds.
c.Turn off engine for 10 seconds.
d.Start engine and check for proper idle operation.

15. The idle speed of the engine should be in the 600 – 700 range per the ECM programming with all connections re-established and engine at operating temp.

You are now in adjustment on base idle speed and TPS output. Start the engine. It
may take a few minutes for the car to “catch on” or learn its new settings.

Revised by Bob Lamb 10/03 from a tech article written by Lars Grimsrud 10/01. These changes were required to align the Grimsrud article to the Helms GM manual for an 89 Corvette.