BASE IDLE ADJUSTMENT REVISED DIAGNOSTICS

VEHICLES AFFECTED ALL 1988 A, J AND W MODELS WITH A 60 DEGREE V-6 ----------------- 2.8L ENGINE (VIN CODE W) AND 1988 "L" MODELS WITH A VIN PRODUCTION SEQUENCE NUMBER AFTER 500000

The following service information revises the recommended diagnostic procedure (reference sheet 6E3-B-7 of the Service Manual) for ROUGH, UNSTABLE OR INCORRECT IDLE AND STALLING AT IDLE for 1988 vehicles equipped with a 2.8L V-6 MPFI Generation II engine (VIN Code W).

A BASE IDLE RPM ADJUSTMENT IS NOT REQUIRED AND SHOULD NOT BE PERFORMED ON 1988 VEHICLES.

The following information provides an explanation for the above revision.

All 60 degree V-6 engine calibrations for 1988 production and 1987 service include a feature which stores in Electronic Control Module (ECM) memory the idle air control valve (IACV) position at which the engine idles. Each vehicle will have a unique value, depending on the throttle body base air rate setting, deposit buildup, etc. The IACV position information is continuously updated, during vehicle operation and stored in the ECM "keep alive" memory when the engine is turned off.

NOTICE: IF THE ECM IS WITHOUT BATTERY POWER FOR ANY REASON, THEN THE IACV POSITION INFORMATION FOR A STABLE SATISFACTORY ENGINE IDLE IS "LOST" AND IS REPLACED WITH A DEFAULT" VALUE.

To correct the IACV position information due to an absence of battery power and improve the engine idle, perform the following procedure:

1. Restore the battery power and place air conditioning controls (if equipped) in the OFF POSITION.

2. Start the engine and shift the transmission to "drive" for automatics or "neutral" for manuals.

3. Allow the engine to run approximately five minutes until the normal operating temperature is attained.

4. After attaining operating temperature, allow the engine to idle for an additional two minutes.

This procedure will allow the ECM memory to be updated with the "correct" IACV position, for the vehicle, and provide a stable idle RPM.

DRIVEABILITY AND EMISSIONS - FUEL INJECTION (PORT) 6E3-B-7

ROUGH, UNSTABLE, OR INCORRECT IDLE, STALLING

DEFINITION: The engine runs unevenly at idle. If bad enough, the car may shake. Also, the idle may vary in rpm (called "hunting"). Either condition may be bad enough to cause stalling. Engine idles at incorrect speed.

o Perform careful visual check as described at start of Section "B". o Clean Injectors. o CHECK: - Throttle linkage for sticking or binding. - TPS for sticking or binding, be sure output is stable at idle and adjustment specification is correct. - IAC system. See Code 35 chart. - Generator output voltage. Repair if less than 9 or more than 16 volts. - P/N switch circuit. See CHART C-lA, or use "Scan" Tool, and be sure tool indicates vehicle is in drive with gear selector in drive(125C), or overdrive (440-T4). - Injector balance. See CHART C-2A. - PCV valve for proper operation by placing finger over inlet hole in valve end several times. Valve should snap back. If not, replace valve, See Section "C13". - Evaporative Emission Control System. CHART C-3. - Power Steering Pressure switch input. The state of the switch should only change when wheels are turned up against the stops. See CHART C-1E. - ECM ground circuits. - EGR valve: There should be no EGR at idle. o Monitoring block learn values may help identify the cause of the problem. If the system is running lean (block learn greater than 138) refer to Diagnostic Aids on facing page of Code 44. If the system is running rich (block learn values less than 118) refer to Diagnostic Aids on facing page of Code 45. o Run a cylinder compression check. See Section "6". o Check for fuel in pressure regulator hose. If present, replace regulator assembly. o Check ignition system; wires and plugs. o Check for loose or damaged MAF duct between sensor and throttle body. o Disconnect MAF sensor and if condition is corrected, replace sensor. "Scan" tool should read about 4-8 grams per second at idle. o If problem exists with A/C "ON", check A/C system operation CHART C-10.

EXCESSIVE EXHAUST EMISSIONS (ODORS)

o If test shows higher than normal CO and HC, (also has excessive odors): o Check items which cause car to run RICH. o Make sure engine is at normal operating temperature. o CHECK: - Fuel pressure. See CHART A-7. - Canister for fuel loading, See CHART C-3. - Injector balance. See CHART C-2A. - PCV valve for being plugged, stuck, or blocked PCV hose. - Spark plugs. - Check for lead contamination of catalytic converter (look for removal fuel filter neck restrictor). o If the system is running rich (block learn less than 118), refer to Diagnostic Aids on facing page of Code 45.

o IF TEST SHOWS EXCESSIVE NOx: o Check items which cause car to run LEAN, or to run too hot. - EGR valve for not opening or plugged passages. See CHART C-7. - Vacuum leaks. - Coolant system and coolant fan for proper operation. See CHART C- 12. - Remove carbon with top engine cleaner. Follow instructions on can.

o If the system is running lean (block learn greater than 138), refer to Diagnostic Aids on facing page Code 44.

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.