Air/Fuel Mixture: Adjustments
AIR/FUEL MIXTURE CHECKING / ADJUSTING PROCEDUREPRE-CHECK REQUIREMENTS
1. Engine temperature at 185°F (85°C) minimum (operating temp.)
2. Throttle valve in idle position
3. Closed throttle position switch operating properly
4. All electrical consumers turned OFF
5. A/C turned OFF
6. Fuel pressure gauge NOT connected
7. Radiator fan MUST NOT run while taking measurements
8. Crankcase breather disconnected and plugged
9. NO diagnostic trouble codes stored in ECM fault memory
AIR/FUEL MIXTURE CHECKING PROCEDURE
V6 Crankcase Vent Hose:
1. Disconnect hoses from valve covers and close off both hose ends with suitable plugs as shown. To remove hoses push oval outer ring together at locations shown (arrows).
2. Connect CO tester to right CO tap tube (right side exhaust manifold near Heated Oxygen Sensor) using adaptor 1363/3 and connecting hose 6006-0019 or equivalent.
3. START engine and let idle, CO content should be:
^ Checking value 0.3 to 1.2%
4. Remove CO test probe from right CO tap tube, reinstall tube cap.
5. Connect CO tester to left CO tap tube (left side exhaust manifold near Heated Oxygen Sensor) using adaptor 1363/3 and connecting hose 6006-0019 or equivalent.
6. START engine and let idle, CO content should be:
^ Checking value 0.3 to 1.2%
7. Right side CO reading should be the same as Left side CO reading (only a slight difference is allowed).
8. If CO Readings are NOT EQUAL
^ Check BOTH Heated Oxygen Sensors for proper operation repair or replace as needed, refer to DIAGNOSIS, TESTING AND INSPECTION PROCEDURES for procedure.
^ Ensure that all spark plugs are functioning properly repair or replace as needed.
NOTE: Failure of one spark plug increases CO content by approximately 1.5%.
^ Check and repair any vacuum leaks.
^ After repairs recall ECM fault memory, clear any Diagnostic Trouble Codes (refer to COMPUTERS AND CONTROL SYSTEMS for procedure). Repeat entire MIXTURE CHECKING PROCEDURE above.
9. If CO Readings are EQUAL but OUT OF SPECIFICATION
^ CO adjustment is needed, perform MIXTURE ADJUSTMENT PROCEDURE below.
10. If CO Readings are EQUAL and WITHIN SPECIFICATION
^ NO adjustment needed, turn engine OFF.
^ Remove CO test probe from CO tap tube, reinstall tube cap.
^ Remove plugs from crankcase vent hoses and reinstall onto valve covers.
AIR/FUEL MIXTURE ADJUSTMENT PROCEDURE
CO Fuel Trim Adjusting Screw:
1. Locate CO Fuel Trim Adjustment Screw in Mass Air Flow Sensor (arrow).
2. Remove cap and pry out plastic plug.
3. START engine and let idle.
4. Turn CO Fuel Trim Adjustment Screw to obtain:
^ Adjusting value 0.7%
NOTE: CO value will not change immediately after turning CO Fuel Trim Adjustment Screw, a slight time delay may be noticed.
5. Turn engine OFF:
^ Remove CO test probe from CO tap tube, reinstall tube cap.
^ Install new plug on CO Fuel Trim Adjustment Screw and press on new cap.
6. Remove plugs from crankcase vent hoses and reinstall onto valve covers.
7. IF CO specification still NOT obtained:
^ Recall ECM fault memory and perform Output Diagnostic Test Mode, then retest.