INFO. ON ON-BOARD DIAGNOSTICS (OBDII). Chevrolet S10 Pickup

SUBJECT: INFORMATION ON ON-BOARD DIAGNOSTICS (OBD II) SERVICEABILITY ISSUES

MODELS: 1996 PASSENGER CARS AND LIGHT DUTY TRUCKS WITH OBD II

BASED ON THE KNOWLEDGE GAINED FROM A SELECT GROUP OF VEHICLES EQUIPPED WITH OBD II EMISSION MONITORING SYSTEMS, SOLD IN THE 1994 AND 1995 MODEL YEARS, THE GM POWERTRAIN GROUP HAS COMPILED A LIST OF "NON VEHICLE INDUCED FAULTS" THAT COULD AFFECT THE PERFORMANCE OF THE OBD II SYSTEM.

WITH THE INTRODUCTION OF OBD II DIAGNOSTICS ACROSS THE ENTIRE GENERAL MOTORS PASSENGER CAR AND LIGHT DUTY TRUCK FLEET IN 1996, TURNING ON THE MIL (MALFUNCTION INDICATOR LAMP) DUE TO A NON-VEHICLE FAULT COULD LEAD TO MISDIAGNOSIS OF THE VEHICLE, INCREASED WARRANTY EXPENSE AND CUSTOMER DISSATISFACTION. THE FOLLOWING LIST OF NON-VEHICLE FAULTS IS INTENDED TO ASSIST DEALER TECHNICIANS IN PROPERLY DIAGNOSING OBD II VEHICLES. THE LIST IS NOT ALL INCLUSIVE AND MAY NOT APPLY EQUALLY TO ALL PRODUCT LINES.

FUEL CONCERNS FUEL QUALITY HAS LONG BEEN A MAJOR CONCERN OF THE ENTIRE AUTOMOTIVE INDUSTRY. ITS POTENTIAL FOR TURNING ON THE MIL IS HEIGHTENED WITH OBD II SYSTEMS.

IMPROPER VAPOR PRESSURE OF THE FUEL CAN CREATE PROBLEMS IN THE FUEL SYSTEM ESPECIALLY DURING THE SPRING AND FALL MONTHS WHEN SEVERE AMBIENT TEMPERATURE SWINGS OCCUR. A LOW REID VAPOR PRESSURE FUEL USED DURING COLD AMBIENT TEMPERATURES WILL AFFECT THE STARTING ABILITY AND COLD DRIVEABILITY OF THE VEHICLE AND MAY CAUSE A MISFIRE DTC (DIAGNOSTIC TROUBLE CODE). A HIGH REID VAPOR PRESSURE FUEL USED DURING WARM AMBIENT TEMPERATURES COULD CAUSE A FUEL TRIM DTC DUE TO EXCESSIVE CANISTER LOADING. HIGH VAPOR PRESSURES GENERATED IN THE FUEL TANK CAN CAUSE AN EVAPORATIVE EMISSION DTC AS WELL.

USING FUEL WITH THE WRONG OCTANE RATING FOR AN OBD II EQUIPPED VEHICLE MAY INDUCE DRIVEABILITY CONCERNS. MANY OF THE MAJOR FUEL COMPANIES ADVERTISE THAT USING "PREMIUM" FUELS WILL IMPROVE THE PERFORMANCE OF YOUR VEHICLE. PREMIUM FUELS HAVE HIGHER OCTANE RATINGS BUT SOME HAVE LESS MID-RANGE VOLATILITY WHICH IS REQUIRED TO TURN FUEL INTO A COMBUSTIBLE VAPOR AT LOW AMBIENT TEMPERATURES. THIS MAY AFFECT THE STARTING ABILITY AND COLD DRIVEABILITY OF THE ENGINE POSSIBLY TRIGGERING A MISFIRE DTC.

SOME FUELS CONTAIN A MANGANESE OCTANE ENHANCER KNOWN AS MMT WHICH CAN AFFECT VARIOUS COMPONENTS OF THE VEHICLE'S EMISSION CONTROL SYSTEM. THESE EFFECTS MAY RESULT IN MISFIRE OR OXYGEN SENSOR DTC'S. MMT MAY ALSO DECREASE THE ABILITY OF THE CATALYST DIAGNOSTIC TO DETECT ACTUAL FAILURES.

LOW FUEL LEVELS COULD CAUSE FUEL STARVATION, LEAN ENGINE OPERATION, AND EVENTUALLY A MISFIRE DTC.

REFUELING A NEW OBD II DIAGNOSTIC WHICH CHECKS THE INTEGRITY OF THE ENTIRE EVAPORATIVE EMISSION SYSTEM IS BEING INTRODUCED ON SOME 1996 GENERAL MOTORS VEHICLES. IF THE FUEL CAP IS NOT PROPERLY SECURED AFTER REFUELING, THE DIAGNOSTIC SYSTEM WILL DETECT THE LEAKAGE OF FUEL VAPORS INTO THE ATMOSPHERE AND SET A DTC FOR THE EVAPORATIVE EMISSION SYSTEM. TO PROPERLY INSTALL THE FUEL CAP, TURN IT TO THE RIGHT (CLOCKWISE) UNTIL YOU HEAR AT LEAST THREE CLICKS.

NON-OEM PARTS ALL OF THE OBD II DIAGNOSTICS HAVE BEEN CALIBRATED WITH ORIGINAL EQUIPMENT MANUFACTURER (OEM) PARTS. MODIFICATIONS MADE TO THE VEHICLE'S EMISSION CONTROL SYSTEM OR INSTALLATION OF PARTS THAT DO NOT MATCH THE CALIBRATED VALUES OF THE OEM PARTS COULD POTENTIALLY SET DTC'S. FOR EXAMPLE, INSTALLING AN AFTERMARKET HIGH PERFORMANCE EXHAUST SYSTEM THAT AFFECTS ENGINE BACK PRESSURE COULD POTENTIALLY INTERFERE WITH THE OPERATION OF THE EGR VALVE AND SET A DTC. ALSO, AFTERMARKET ELECTRONICS SUCH AS CELL PHONES, STEREOS, ANTI-THEFT DEVICES IF IMPROPERLY INSTALLED MAY RADIATE ELECTRO-MAGNETIC INTERFERENCE INTO THE POWERTRAIN CONTROL SYSTEM CAUSING FALSE SENSOR READINGS WHICH MAY CAUSE DTC'S TO SET.

ENVIRONMENT EXTREME ENVIRONMENTAL CONDITIONS SUCH AS FLOODING CAN HAVE AN EFFECT ON THE VEHICLE'S IGNITION SYSTEM. IF THE IGNITION SYSTEM BECOMES WET, IT CAN TEMPORARILY CAUSE AN ENGINE MISFIRE DTC.

VEHICLE HANDLING REPEATED COLD STARTS WITHOUT COMPLETE ENGINE WARM-UP CAN CAUSE CARBON FOULING OF THE SPARK PLUGS AND SET A MISFIRE DTC. THIS CAN OCCUR DURING THE TRANSPORTATION OF NEW VEHICLES FROM THE ASSEMBLY PLANTS AND REPEATED STARTS AND STOPS AT THE DEALERSHIP. THIS TYPE OF "BRIEF" VEHICLE OPERATION CONTRIBUTES TO THE CARBON FOULING OF THE SPARK PLUGS AND MAY SET A MISFIRE DTC.

SEVERE VIBRATION THE MISFIRE DIAGNOSTIC MEASURES SMALL CHANGES IN THE ROTATIONAL SPEED OF THE CRANKSHAFT. SEVERE DRIVELINE VIBRATIONS IN THE VEHICLE CAN SIMULATE THE SAME EFFECT ON CRANKSHAFT SPEED AS AN ENGINE MISFIRE AND THEREFORE MAY SET A MISFIRE DTC. EXAMPLES OF THIS WOULD BE AN EXCESSIVE AMOUNT OF MUD CAKED ON THE WHEELS OR A BENT PROPELLER SHAFT.

POOR VEHICLE MAINTENANCE THE SENSITIVITY OF OBD II DIAGNOSTICS WILL CAUSE THE DTC'S TO SET IF THE VEHICLE IS NOT MAINTAINED ACCORDING TO THE PUBLISHED GM MAINTENANCE SCHEDULES. RESTRICTED AIR FILTERS AND FUEL FILTERS, AND CRANKCASE DEPOSITS (DUE TO LACK OF OIL CHANGES) CAN TRIGGER ACTUAL VEHICLE FAULTS THAT WERE NOT MONITORED PRIOR TO OBD II. WITH THE SENSITIVITY OF OBD II DIAGNOSTICS, IT IS ESPECIALLY IMPORTANT THAT GM RECOMMENDED MAINTENANCE SCHEDULES BE STRICTLY FOLLOWED.

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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.