Emission Control Systems: Adjustments

NOTE: The curb and fast idle speeds are controlled by the Engine Control Assembly (ECA) and the Idle Speed Control Bypass Air Valve (ISC-BPA). The ISC-BPA valve 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.
The idle speed setting procedure has been revised considerably to reflect the expanded application of the Self-Test idle speed options and because the throttle body incorporates an orifice in the throttle plate to control idle airflow as part of the sludge tolerant design.

Service Decal Location (Typical Throttle Body):

Throttle bodies with sludge tolerant design are clearly 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 (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 ISC-BPA 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 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.


STEP 1
With the engine OFF, connect a tachometer, according to manufacturer's instructions, disconnect the negative (-) terminal of the battery for 5 minutes, then reconnect it.
STEP 2
START the engine, stabilize for two minutes, goose the engine, then let it return to idle. Lightly depress and release the accelerator, then let the engine idle.
STEP 3
If the vehicle does not idle smoothly, proceed with this procedure.
STEP 4
STOP the engine, disconnect Idle Speed Control Air Bypass Solenoid.
STEP 5
START and RUN the engine at 2500 rpm for 30 seconds.
STEP 6
Place the transmission in NEUTRAL (M/T), or PARK (A/T).
STEP 7
Using a tachometer, verify idle rpm is 725 +0/-125 rpm.
RPM TOO LOW
^ Turn the plate stop screw to reach an idle speed of 725 +/- 25 rpm (during engine break-in, reduce the rpm by 100).
RPM T00 HIGH
^ Turn the plate stop screw to reach an idle speed of 725 +/- 25 rpm.
STEP 9
Turn engine OFF, and repeat Steps 5, 6, and 7.
STEP 10
Run the KOEO Self-Test for proper TPS code output.
STEP 11
Turn engine OFF, and disconnect the battery for at least 5 minutes.
STEP 12
Reconnect the Idle Speed Control Air Bypass Solenoid. Verify that the throttle plate is not stuck in the bore, and linkage is not preventing throttle from closing.
STEP 13
START the engine and let idle for two minutes. Goose the accelerator and let the engine return to idle. Lightly depress and release the accelerator, then let engine idle.
If idle problem still exists, refer to Powertrain Management/Computers and Control Systems/Testing and Inspection/Procedures.