Idle Speed: Adjustments

NOTE: The curb and fast idle speeds are controlled by the EEC-IV processor and the Idle Air Bypass Valve assembly. The Idle Air Bypass Valve assembly is not adjustable. A large increase or decrease in closed plate airflow from the calibrated level will not allow this device to control the speed.
To reflect the expanded application of the Self-Test idle speed options, the idle speed setting procedure has been revised considerably.

Throttle Body Description:

As part of the sludge tolerant design, the throttle body incorporates an orifice in the throttle plate to control idle airflow. Throttle bodies with sludge tolerant design are identified with a Yellow/Black decal, warning that the throttle plate stop screw must not be adjusted counterclockwise (backed off). 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 (TPS 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 Idle Air Bypass 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.
- Contamination within the idle speed control device
- 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.

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 service output codes have been resolved.


PROCEDURE - STAR TESTER OR EQUIVALENT SCANNER REQUIRED

STEP 1
Activate engine running Self-Test.

STEP 2
After service 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 (8hz) 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 (4Hz) indicates base idle rpm is too fast, adjustment is required, go to STEP 6.
d. Beeping tone, flashing light, or "STO LO" readout at (1Hz) indicates base idle is too low, adjustment is required, go to STEP 5.

Idle Air Trim Screw:

Air Trim Screw Channel Cover:

STEP 5
DO NOT clean the throttle body turn the air trim screw counterclockwise until conditions in Step 4(a) are satisfied. Process completed.

STEP 6
DO NOT clean the throttle body, turn the air trim screw clockwise until conditions in step 4(a) are satisfied.

THROTTLE BODY ORIFICE PLUG INSTALLATION

1. Remove air inlet tube(s) from throttle body.
2. Select the proper color plug by using the Go/No-Go gauge pegs, included with the service kit.
3. Starting with the largest diameter gauge peg, attempt to insert it through the throttle plate orifice.
4. If gauge peg goes through the orifice, use the corresponding colored plug. If gauge peg does not go through, proceed with the 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.
5. If the smallest gauge peg does not go through the orifice, use the reamer bit and handle, included with the service kit, to enlarge the plate orifice. Wipe bearing grease on both sides of the plate orifice and on the reamer bit to hold the brass chips. After reaming, wipe plate clean, then return to Step 3 to determine the proper plug size.
6. Using the installation tool from the service kit, push the plug into the orifice until it bottoms out at the throttle plate.
7. Open and snap closed the throttle several times to verify proper plug retention.
8. Reconnect the air inlet tube(s).
9. Reset idle rpm per engine requirement, using the throttle plate stop screw.