Idle Speed: Adjustments
NOTE: 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
- 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.
IDLE SPEED CHECK PROCEDURE
STEP 1
Engine "Off", connect a tachometer according to manufacturer's instructions. Disconnect negative (-) terminal of the battery for 5 minutes, then reconnect.
STEP 2
Start engine and stabilize for two minutes, then goose (snap) the throttle and let it return to idle, lightly depress and release the accelerator and let engine idle.
STEP 3
If the engine does not idle properly, Shut "Off" engine and unplug SPOUT line connector.
STEP 4
Start engine, verify that ignition timing is at base +/- 2° BTDC, adjust if necessary. Shut "Off" engine.
STEP 5
Remove PCV hose from intake manifold and plug it. Remove CANP hose from intake manifold and connect Rotunda T89P-9600-AH or equivalent between PCV and CANP ports.
STEP 6
Disconnect IAC solenoid.
STEP 7
Start engine, rpm at idle for 30 seconds.
STEP 8
Place transmission in Park (A/T), or Neutral (M/T).
STEP 9
Using a tachometer, verify idle rpm is 800 +/- 30.
RPM TOO LOW OR TOO HIGH
Turn throttle return screw to 800 +/- 25 rpm.
STEP 10
Shut engine "Off" and disconnect battery for ten minutes minimum.
STEP 11
Reconnect SPOUT connector.
STEP 12
Remove Rotunda tool. Unplug PCV hose. Reconnect CANP and PCV hoses to intake manifold.
STEP 13
Engine "Off", reconnect IAC solenoid, verify throttle plate is not stuck in the bore at idle position and linkage is not preventing throttle from closing.
STEP 14
Start engine and stabilize for two minutes, then goose (snap) throttle and return to idle. Lightly press and release accelerator and let engine idle.
THROTTLE PLATE ORIFICE PLUG INSTALLATION
Throttle Body Assembly:
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 the 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.