Air/Fuel Mixture: Adjustments
AIR/FUEL MIXTURE CHECKING / ADJUSTING PROCEDURENOTE: Air/fuel mixture is not adjustable on this vehicle but can be checked to verify the system operation.
PRE-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 connected
9. NO diagnostic trouble codes stored in ECM fault memory
AIR/FUEL MIXTURE CHECKING PROCEDURE
1. 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.
2. START engine and let idle, CO content should be:
^ Checking value 0.3 to 1.2%
3. Remove CO test probe from right CO tap tube, reinstall tube cap.
4. 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.
5. START engine and let idle, CO content should be:
^ Checking value 0.3 to 1.2%
6. Right side CO reading should be the same as Left side CO reading (only a slight difference is allowed).
7. 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.
8. If CO Readings are EQUAL but OUT OF SPECIFICATION
^ Remove CO test probe from CO tap tube, reinstall tube cap.
^ Perform ENGINE BASIC ADJUSTMENT PROCEDURE below.
9. 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.
ENGINE BASIC ADJUSTMENT PROCEDURE
The MPI system is "adaptive" this means all engine adjustments are "learned" over a period of time. To return CO to proper levels the "learned" CO adjustment stored in the MPI ECM must be erased and then "relearned". To erase these "learned" settings, battery power to the MPI ECM must be disconnected. The ECM will then return to ENGINE BASIC ADJUSTMENT levels. When the engine is restarted the learning process begins again.
The ECM battery power circuit is not fused, therefore the battery or ECM must be disconnected. Disconnecting the battery is easy, but not recommended as this will also erase memory for things such as radio, seat position, mirror position, climate control, transmission control, phone numbers, etc. The Engine Basic Adjustment Procedure below will describe how to erase MPI ECM memory and initiate the "learning" procedure.
ERASE ECM MEMORY
1. Turn ignition switch OFF.
Relay And Connector Station Locations:
2. Locate the Electronic Control Box in passengers side footwell area. Access can be made by folding back the carpet and removing the cover, which is attached with Phillips screws.
Engine Control Module And Transmission Control Module Locations In The Central Electronics Box:
3. The MPI Engine Control Module (ECM) and automatic Transmission Control Module (TCM) are both located in the Electronic Control Box.
MPI Engine Control Module Connector I.D.:
4. The MPI control module has four separate connectors with a total of 64 pin terminals. The four connectors are identified in the image. Battery power is supplied to the ECM via connector B.
MPI ECM Connector Lock:
5. The individual connectors are attached to the control module with a locking device. Loosen and remove connector B by squeezing the connector shell while pulling it from the control module.
6. Leave connector B unplugged for at least 15 seconds, then reconnect to ECM.
7. Reinstall ECM into electronic control box, reinstall electronic control box to footwell.
INITIATE LEARNING PROCEDURE
When the ECM or battery is disconnected, all learned values in the ECM are erased. Shortly thereafter rough idling can occur. In which case let the engine idle for a few minutes, this allows the learning procedure to switch ON, stabilize idle and switch OFF. Throttle MUST be in idle position for learning procedure to activate.
1. START engine and allow to idle for several minutes, do not rev engine.
2. Test drive vehicle for at least 5 miles.
3. Recall ECM fault memory and check for codes.
4. Turn engine OFF.