Idle Speed: Adjustments

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.

Throttle Body Assembly Decal Location:

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
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 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
- Vacuum leaks (intake manifold, vacuum hoses, vacuum reservoirs, power brake booster, etc.)
- Exhaust system, EGR.

VERIFY
- Transmission is in Park (A/T), or Neutral (M/T)
- Parking brake is applied (automatic brake release disconnected where applicable)
- Wheels blocked
- Heater, A/C, and other accessories Off
- Engine at operating temperature
- Proper fuel pressure and no exhaust smoke
- Correct and clean PCV system
- 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 (DTC) 11 or 111 has been displayed, unlatch and within 4 seconds latch the Self-Test Input (STI) button.

STEP 3
A single pulse code indicates the entry mode, then observe the Self-Test Output (STO) of the scan tool 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 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.

Throttle Plate Orifice Plug:

STEP 5 - Rpm Too Low
DO NOT clean the throttle body. Check for throttle plate orifice plug.
- No plug found, turn throttle stop screw clockwise until conditions in Step 4a are satisfied.
- Plug is present from previous service, remove and adjust throttle stop screw in either direction as required.
NOTE: Throttle stop screw must be in contact with the lever pad after adjustment.

STEP 6 - Rpm Too High
a. Turn "Off" engine.
b. Block orifice on the throttle plate temporarily with tape. If the orifice already has a plug from previous service, go to Step (d), otherwise proceed with next step.
c REStart engine and check idle speed using Self-Test. If engine stalls, crack open throttle plate with the throttle plate stop screw.
NOTE: Mass Airflow models require the air intake hose to be reattached first.
d. If rpm continues to be fast, perform Key On Engine Off (KOEO) self-test for proper Throttle Position (TP) sensor DTC.
- If TP sensor output code is within range, remove tape and Go To Diagnostic Routines for other causes.
- If TP sensor is out of range, adjust screw for proper TP sensor code (lever pad must be in contact with throttle stop screw after adjustment).
- If rpm is still fast, terminate this procedure and Go To Diagnostic Routines for other causes.
e. If rpm drops to or below desired level indicated by Self-Test Output tone, turn engine "Off", disconnect air cleaner hose and remove tape.
f. Install orifice plug with proper color code depending on throttle plate orifice size (Refer to PROCEDURES FOR THROTTLE PLATE ORIFICE PLUG INSTALLATION, below).
g. Reconnect air cleaner hose. "Start" engine, and turn throttle plate stop screw clockwise (DO NOT turn it counterclockwise as this may cause the throttle to stick at idle) until the conditions in Step 4a are satisfied.

STEP 7
Run the KOEO Self-Test for proper TP sensor DTC.

STEP 8
Verify throttle plate is not sticking in bore, and linkage is not preventing the throttle from closing properly.

STEP 9
Automatic Overdrive Transmission (AOD) equipped vehicles, check Throttle Valve (TV) pressure adjustment.


THROTTLE PLATE ORIFICE PLUG INSTALLATION
1. Remove air inlet tube(s) from throttle body.

2. Select proper color plug by using the Go/No-Go gauge pegs, included with the service kit.

3. Start with largest diameter gauge peg and attempt to insert it through throttle plate orifice.

- If gauge peg goes through orifice, use corresponding colored plug.

- If gauge peg does not go through, proceed with next smaller gauge peg for Go/No-Go Test.

NOTE: It is important that the largest Go/No-Go combination is used in determining the proper plug size.

4. If the smallest gauge peg does not go through the orifice, use reamer bit and handle included with service kit, to enlarge throttle plate orifice. Wipe bearing grease on both sides of the plate orifice and on reamer bit to hold the brass chips. After reaming, wipe throttle plate clean, then return to Step 3 to determine proper orifice plug size.

5. Use the installation tool from service kit, push plug into orifice until it bottoms out in throttle plate.

6. Open and snap shut the throttle several times to verify proper plug retention.

7. Reconnect air inlet tube(s).

8. Reset idle rpm per engine requirement, using throttle plate stop screw.