ENG. SURGE, KNOCK, ROUGH IDLE, STALL, HARD START. GMC Truck Sonoma Pickup

REVISION: 06/23/98 DIVS INVL 1 5 7

THIS BULLETIN IS BEING REVISED TO UPDATE THE INITIAL SERVICE CALIBRATION INFORMATION THAT MAY SET A FALSE DTC P0401 AND TO ADD NEW CD ROM DISK INFORMATION. PLEASE DISCARD CORPORATE BULLETIN NUMBER 666507 (SECTION 6E - ENGINE FUEL & EMISSION). ______________________________________________________________________

SUBJECT: ENGINE SURGE, KNOCK, ROUGH IDLE, STALL, HARD START, DTC P1406 (FLASH CALIBRATION)

MODELS: 1995 CHEVROLET AND GMC S/T MODELS WITH VCMX-A CONTROLLED 4.3L ENGINE (VIN CODE W - RPO L35)

CONDITION:

SOME OWNERS WITH A VCMX-A EQUIPPED 4.3L ENGINE MAY EXPERIENCE LINEAR EGR VALVE STICKING. THIS MAY CAUSE DTC 1406 TO BE SET WHICH MAY BE ACCOMPANIED BY POOR DRIVEABILITY CONDITION(S) SUCH AS ENGINE SURGE, KNOCK ON MODERATE ACCELERATION, ENGINE STALL, ROUGH IDLE AND/OR HARD STARTING. VEHICLES THAT HAVE BEEN PREVIOUSLY UPDATED PER THE ORIGINAL BULLETIN MAY SET A FALSE DTC P0401. THE NEW ENGINE CALIBRATIONS LISTED BELOW SHOULD ELIMINATE THIS FALSE CODE. THE VCMX-A CONTROLLER IS LOCATED UNDER THE HOOD ON TOP OF THE COOLANT BOTTLE AND CAN BE IDENTIFIED BY ITS COOLING FINS.

CAUSE:

THE LINEAR EGR VALVE MAY BE STICKING BECAUSE CARBON DEPOSITS ON THE VALVE PREVENT THE VALVE FROM RESPONDING TO VCM SYSTEM COMMANDS.

CORRECTION:

FOLLOW THE STRATEGY BASED DIAGNOSTIC STEPS FOR THE REPORTED SYMPTOMS. RECORD STORED DTC'S. FLASH THE NEW CALIBRATION FROM THE TABLE LIST BELOW FOLLOWING YOUR TECHLINE TERMINAL USER INSTRUCTIONS, AND THEN CLEAR ALL DTC'S.

AFTER FLASHING THE NEW CALIBRATION, PERFORM THE EGR VALVE TESTS LISTED BELOW.

EGR DEBRIS PURGE AND FUNCTION CHECK

1. APPLY THE PARKING BRAKE. PLACE THE TRANSMISSION IN PARK, OR NEUTRAL FOR MANUAL TRANSMISSIONS. RUN THE ENGINE AT 1500 RPM WHILE USING THE TECH 1 IN ORDER TO COMMAND THE EGR VALVE TO THE 100% POSITION, AND THEN TO THE 0% POSITION WHILE MONITORING THE ACTUAL AND COMMANDED EGR POSITION. THIS PROCEDURE IS INTENDED TO CLEAR FOREIGN MATERIAL FROM THE EGR VALVE.

2. IF THE EGR VALVE'S DESIRED OR COMMANDED POSITION AGREES WITH THE ACTUAL POSITION, THE EGR VALVE IS FUNCTIONING PROPERLY. IF THE EGR VALVE'S DESIRED OR COMMANDED POSITION DOES NOT AGREE WITH THE ACTUAL POSITION, REPEAT STEP 1.

3. IF THE SECOND ATTEMPT TO CLEAR FOREIGN MATERIAL FROM THE EGR VALVE IS NOT SUCCESSFUL, FOLLOW THE PROCEDURES IN THE DRIVEABILITY AND EMISSIONS SERVICE MANUAL FOR DIAGNOSIS OF DTC P1406 AND/OR ANY OTHER STORED DTC'S.

4. AFTER THE DRIVEABILITY CONCERNS HAVE BEEN DIAGNOSED AND CORRECTED, PERFORM EGR ROAD TEST.

EGR ROAD TEST

1. DURING THE EGR ROAD TEST, WITH THE NEW FLASH CALIBRATION INSTALLED, YOU MAY NOTICE INCREASED EGR ACTIVITY DURING LIGHT THROTTLE MANEUVERS. THIS IS NORMAL BECAUSE THE PROGRAMMED EGR VALVE STROKING IS DESIGNED TO CLEAN THE VALVE OF DEBRIS. DURING THIS STROKING, THE ACTUAL EGR VALVE POSITION WILL MOMENTARILY EXCEED THE DESIRED POSITION.

2. VERIFY THAT THE OWNER'S REPORTED DRIVEABILITY CONCERNS ARE CORRECTED AND THE EGR VALVE IS FUNCTIONING NORMALLY. IF SO, RETURN THE VEHICLE TO THE OWNER. IF NOT, REFER TO THE APPROPRIATE SERVICE MANUAL DIAGNOSIS PROCEDURE.

IMPORTANT: THE CALIBRATIONS LISTED BELOW ARE FLASH CALIBRATIONS AND NOT AVAILABLE FROM GMSPO. SERVICE FLASH CALIBRATIONS ARE AVAILABLE STARTING WITH THE CD ROM LISTED BELOW. ALL ENGINE CALIBRATIONS LISTED BELOW ARE ASSOCIATED WITH OPERATING SYSTEM (PH LEVEL SOFTWARE) P/N 16250645.

NEW TECHLINE FUEL/EMISSION TRANSMISSION CALIBRATION CD

LEADED MANUAL 16251370 1997 DISC 14

LEADED AUTOMATIC 16251371 1997 DISC 14

UNLEADED/CAN., MANUAL 16251372 1997 DISC 14 EUROPE, JAPAN

UNLEADED/FED. MANUAL 16251375 1997 DISC 14 CALIFORNIA

UNLEADED/CALIF. AUTOMATIC 9360009 1998 DISC 10

UNLEADED/CAN., AUTOMATIC 9360010 1998 DISC 10 EUROPE, JAPAN

UNLEADED/FED. AUTOMATIC 9360011 1998 DISC 10

WARRANTY INFORMATION:

FOR VEHICLES REPAIRED UNDER WARRANTY, USE:

LABOR OPERATION DESCRIPTION LABOR TIME J6355 REPROGRAMMING USE PUBLISHED LABOR OPERATION TIME

FIGURES: 0 ATTACHMENTS: 0

GENERAL MOTORS BULLETINS ARE INTENDED FOR USE BY PROFESSIONAL TECHNICIANS, NOT A "DO-IT-YOURSELFER". THEY ARE WRITTEN TO INFORM THOSE TECHNICIANS OF CONDITIONS THAT MAY OCCUR ON SOME VEHICLES, OR TO PROVIDE INFORMATION THAT COULD ASSIST IN THE PROPER SERVICE OF A VEHICLE. PROPERLY TRAINED TECHNICIANS HAVE THE EQUIPMENT, TOOLS, SAFETY INSTRUCTIONS AND KNOW-HOW TO DO A JOB PROPERLY AND SAFELY. IF A CONDITION IS DESCRIBED, DO NOT ASSUME THAT THE BULLETIN APPLIES TO YOUR VEHICLE, OR THAT YOUR VEHICLE WILL HAVE THAT CONDITION. SEE A GENERAL MOTORS DEALER SERVICING YOUR BRAND OF GENERAL MOTORS VEHICLE FOR INFORMATION ON WHETHER YOUR VEHICLE MAY BENEFIT FROM THE INFORMATION.