Idle Speed: Adjustments

NOTE: The curb and fast idle speeds are controlled by the Powertrain Control Module (PCM) and Idle Air Control (IAC) valve assembly. The IAC valve is not adjustable. A large increase or decrease in closed plate airflow from the calibrated level will not allow effective idle speed control.
The throttle body incorporates an orifice in the throttle plate controlling idle airflow as part of the sludge tolerant design.

Throttle Body Assembly:

Throttle bodies with sludge tolerant design are identified with a yellow/black decal. This decal warns that the throttle plate stop screw must not be adjusted counterclockwise (backed off), as this will not reduce the engine speed but may cause the throttle plate to stick in the bore. Backing out the screw may be required if the throttle body has been previously serviced (a plug in the throttle plate orifice may be present), or the plate stop screw tampered with (TP Self-Test output out of range). The decal also warns that the throttle body must NOT be cleaned inside the bore, as this cleaning will impair the sensitive coating. The sludge accumulation will not affect the idle air flow. The cleaning procedure for the IAC valve still applies.

REMEMBER

A change in idle airflow can occur not only at the throttle body but other areas as well. You should enter the idle speed procedure only after other possible causes in the following areas are eliminated:
- Battery and charging system integrity
- Contamination within the Idle Air Control (IAC) component
- Lack of fuel system control (excessively rich or lean)
- Throttle sticking or binding
- Engine not reaching operating temperature
- Incorrect ignition timing
- Incorrect or clogged PCV system
- Exhaust system, EGR
- Vacuum leaks (intake manifold, vacuum hoses, vacuum reservoirs, power brake booster, etc.)

VERIFY

- Transmission is in Park (A/T), or Neutral (M/T)
- Parking brake is applied (automatic brake release disconnected where applicable)
- Wheels blocked
- Engine at operating temperature
- Proper fuel pressure and no exhaust smoke
- Correct and clean PCV system
- Heater, A/C, and other accessories Off
- Ignition timing is set to specification
- EEC-IV diagnostics have been performed and vehicle malfunction indicated diagnostic trouble codes have been resolved.

PROCEDURE - STAR TESTER OR EQUIVALENT SCANNER REQUIRED

STEP 1
Activate engine running Self-Test.

STEP 2
After diagnostic trouble code 11 or 111 has been displayed, unlatch and within 4 seconds latch the STI button.

STEP 3
A single pulse code indicates the entry mode, then observe the Self-Test Output (STO) of the STAR tester in Step 4. If adjustment is required in Step 4, ensure all other causes have been addressed and corrected before continuing.

STEP 4
a. A constant tone, solid light or "STO LO" readout means base idle rpm is within range. To exit test, unlatch STI button, then wait 4 seconds for reinitialization (after 10 minutes it will exit by itself).
b. Beeping tone, flashing light, or "STO LO" readout at (8 Hz) indicates Throttle Position (TP) sensor is out of range due to over adjustment; adjustment may be required.
c. Beeping tone, flashing light, or "STO LO" readout at (4 Hz) indicates base idle rpm is too fast, adjustment is required, go to Step 6.
d. Beeping tone, flashing light, or "STO LO" readout at (1 Hz) indicates base idle is too low, adjustment is required, go to Step 5.

STEP 5
RPM TOO LOW
Do not clean the throttle body, Check the plate orifice plug. If there is no plug turn screw clockwise until conditions in Step 4a are satisfied. If there is a plug from previous service remove plug then adjust screw in either direction as required. Screw must be in contact with the lever pad after adjustment.

STEP 6
RPM TOO HIGH
a. Turn Off the engine.
b. Temporarily block the throttle plate orifice with tape. If the orifice already has a plug from previous service, proceed to Step (d).
c. Restart the engine, and check idle speed, using a Self-Test.
NOTE: On vehicles with Mass Air Flow Sensors, reconnect the air intake duct hose before checking idle rpm. If the engine stalls, crack open the plate stop screw, but DO NOT over adjust.
d. If rpm continues to be fast, Run Key On Engine Off (KOEO) Self-Test.
^ If TP output code is within range, remove tape from throttle body, and go to diagnostic routines for other possible causes.
^ If TP output is out of range, adjust screw for proper TP code. Lever must be in contact with the screw after adjustment is completed.
^ If rpm is still fast, terminate this procedure, and go to diagnostic routines for other possible causes.
e. If rpm drops to the the value in Step 7 or lower, or the engine stalls, turn the engine Off, disconnect the air cleaner hose, and remove tape from throttle plate.
f. Install the proper color coded orifice plug, according to the throttle plate orifice size.
g. Reconnect the air cleaner hose, and Start the engine. Check the idle rpm using a tachometer. Turn the plate stop screw clockwise to the nominal rpm +/- 25 rpm shown in Step 7.

STEP 7
Run the Key On Engine Off (KOEO) Self-Test for proper TP output code.

STEP 8
Verify the throttle is not stuck or sticking in the bore, and the linkage does not prevent the throttle from closing.

STEP 9
On Automatic Overdrive (AOD) transmission applications check TV pressure adjustment.