Idle Speed: Adjustments

*** Updated by TSB #91176 August 21, 1991

CAUTION: Adjustments to the base idle RPM are not recommended because they are not effective repairs for driveability concerns. Improvements in the calibration make such adjustments unnecessary. However, if service is required, follow the procedures listed below.

NOTE: The curb and fast idle speeds are controlled by the EEC-IV processor and the Air Bypass Valve Assembly. The 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.
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 Idle Air Bypass Valve (IABP) 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.

STEP 1
With the engine OFF, connect a tachometer according to manufacturers instructions. Disconnect the negative (-) terminal of the battery for 5 minutes, then reconnect it.
STEP 2
START the engine and allow to stabilize for two minutes. Goose the accelerator, and let the engine return to idle. Lightly depress and release the accelerator, then let the engine idle.
NOTE: If electric cooling fan comes ON, wait until it turns itself OFF.
STEP 3
If the engine does not idle properly, proceed with this procedure.
STEP 4
Disconnect Idle Speed Control Air Bypass valve.
STEP 5
Without touching the throttle, START the engine then accelerate and RUN engine at 2000 rpm for 60 seconds.
STEP 6
Place the transmission in PARK (A/T), or NEUTRAL (M/T).
STEP 7
Using a tachometer, verify idle speed is 600 +/- 150 rpm.

RPM TOO LOW

Typical Throttle Return Screw Location:

Turn the plate stop screw to the value shown in Step 7 +/- 25 rpm (during engine break-in reduce the rpm by 100).

RPM TOO HIGH
Turn the plate stop screw to the value shown in Step 7 +/- 25 rpm.
STEP 8
With the engine OFF, reconnect the Idle Speed Control Air Bypass valve. Verify the throttle is not stuck or sticking in the bore, and the throttle linkage does not prevent the throttle from closing.
STEP 9
START the engine, and allow the idle to stabilize 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, check for other possible causes.