INFO ON ROUGH RUNNING OR MISS AT IDLE ON COLD START CAUSED. Buick Century 1996

THIS IS A MONTHLY SERVICE NEWSLETTER ARTICLE. THIS BULLETIN IS NOT INTENDED FOR PUBLICATION.

SUBJECT: INFORMATION ON ROUGH RUNNING OR MISS AT IDLE ON COLD START CAUSED BY CAM SENSOR SIGNAL

MODELS: 1995-97 BUICK SKYLARK 1995-97 BUICK CENTURY WITH 3.1L ENGINE (VIN M - RPO L82)

POOR OPEN-LOOP ENGINE OPERATION ON A COLD START. ENGINE WILL APPEAR TO RUN RICH, STALL AND/OR HAVE LOW POWER. PERFORMANCE WILL RESUME TO NORMAL ONCE CLOSED LOOP OPERATION IS REALIZED (HO2S BECOMES ACTIVE).

THE CAM SENSOR SIGNAL MAY BE WEAK OR OTHERWISE INCORRECT. IF THE PCM DOES NOT RECEIVE A CORRECT CAM SIGNAL ON A START BELOW APPROX. 120 DEG. F, THE ENGINE WILL PULSE THE INJECTORS ONCE EVERY THREE CRANKSHAFT SENSOR PULSES, NOT SEQUENTIALLY (EVERY SIX CRANKSHAFT SENSOR PULSES). ONCE CLOSED LOOP OPERATION BEGINS, THE PCM WILL REVERT TO SEQUENTIAL OPERATION REGARDLESS OF CAM SENSOR INPUT. THE CAM SENSOR SIGNAL NEEDS TO BE MISSING TO SET A CAM SENSOR CODE SO NO CODE WILL BE PRESENT IF THE SIGNAL IS WEAK OR INCORRECT. (SCAN DATA WILL NOT AID IN DIAGNOSTICS).

IF SCOPE DIAGNOSTICS ARE AVAILABLE, VERIFY THE INJECTORS ARE BEING FIRED ONCE EVERY THREE CRANKSHAFT SENSOR PULSES. WITHOUT A SCOPE, A WARM ENGINE WILL NOT HAVE A SYMPTOM. THE SYMPTOM CAN BE DUPLICATED ON A WARM ENGINE BY CLEARING ANY CODES PRESENT AND, ON A RUNNING ENGINE, DISCONNECTING THE CAM SENSOR SIGNAL WIRE. THIS WILL DUPLICATE THE COLD RUNNING CONDITION FOR APPROX. 15 SECONDS THEN REVERT TO SEQUENTIAL INJECTION AND CORRECT ITS RUNNING. YOU NEED TO VERIFY THAT THE CAM SENSOR IS FULLY BOTTOMED IN ITS BORE. IF THE ENGINE HAS BEEN REPLACED, SERVICED OR THE INTAKE AND CAM SENSOR REMOVED, IT'S POSSIBLE TO HAVE EXCESS RTV FROM THE INTAKE MANIFOLD WALL AREA PREVENT THE CAM SENSOR MOUNTING FLANGE FROM SEATING FULLY. GENERALLY, SENSOR REPLACEMENT IS NOT NECESSARY.

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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.