Emission Control Systems: Adjustments
*** UPDATED BY TSB #9111B4, DATED 6/14/91NOTE: The curb and fast idle speeds are controlled by the Electronic Control Assembly (ECA) and the Idle Speed Control Air Bypass (ISC-BPA) valve assembly. The ISC-BPA 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.
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:
^ 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.)
^ Contamination within the idle speed control device
^ Lack of fuel system control (excessively rich or lean)
^ Throttle sticking or binding
VERIFY
^ Engine at operating temperature
^ Ignition timing is set to specification
^ EEC-IV diagnostics have been performed and vehicle malfunction indicated service output codes have been resolved.
^ Proper fuel pressure and no exhaust smoke
^ Correct and clean PCV system
^ Heater, A/C, and other electrical accessories OFF
^ Transmission is in NEUTRAL
^ Parking brake is applied (automatic brake release disconnected where applicable)
^ Wheels blocked
PROCEDURE
STEP 1
Engine OFF, disconnect the negative terminal of the battery for five minutes, then reconnect it.
STEP 2
START engine and stabilize for two minutes, then goose engine and return to idle, lightly depress and release the accelerator and let engine idle. Does engine idle properly?
NOTE: If electric fan comes on, wait until it turns itself off.
STEP 3
If engine does not idle properly proceed to next step.
STEP 4
Unplug SPOUT line and verify ignition timing is 10° +/- 2 ° BTDC.
STEP 5
Remove PCV hose from intake manifold and plug it. Remove canister purge hose from intake manifold and connect Rotunda Tool T89P-9600-AH or equivalent between PCV and canister purge ports.
STEP 6
Disconnect the ISC-BPA.
STEP 7
Without touching the throttle, START and RUN the engine at idle.
STEP 8
Place transmission in NEUTRAL.
Typical Throttle Return Screw Location:
STEP 9
Check idle rpm range using a tachometer. Idle range is 800 +/- 30 rpm. If the idle is too low or too high, turn the plate stop screw to obtain 800 +/- 25 rpm.
STEP 10
Turn engine OFF and disconnect battery for 5 minutes minimum.
STEP 11
Reconnect the in-line SPOUT connector.
STEP 12
Remove Rotunda Tool T89P-9600-AH from the PCV and canister purge ports. Reconnect the PCV and canister purge hoses.
STEP 13
Reconnect the ISC-BPA valve. Verify the throttle is not sticking to the bore and the linkage is not preventing the throttle from closing.
STEP 14
If the vehicle still exhibits an idle problem, check for other possible causes.