SERVICE MANUAL REVISION FOR CODE 48 CHART

MODELS AFFECTED: 1988 ELECTRA AND LESABRE EQUIPPED WITH 3800 (VIN C) ENGINE

This revised diagnosis chart replaces the Code 48 chart currently in the 1988 New Product Information Service Manual.

CODE 48 MISFIRE DIAGNOSIS 3800 (VIN C) (PORT)

If multiple codes are set, go to the lowest code first. Repairing for a Code 13,44, or 45 may correct Code 48.

Test Description:

Code 48 will set if the following:

* TPS is between .48 and 1.02 volts. * Rpm is between 1250 and 2100. * Mph is between 50 and 60. * 02 cross counts greater than 21. * All of the above for 3 seconds. ---

02 Sensor Test:

Code 48 could be set if the 02 sensor is degraded and cannot travel over the full rich to lean voltage range. This narrowed range could allow 02 cross counts to be above the value necessary to set the code.

* WITH "SCAN" TOOL INSTALLED, VERIFY ENGINE IS AT NORMAL OPERATiNG TEMPERATURE AND IN "CLOSED LOOP".

* ENGINE IDLING IN PARK. * SELECT 02 SENSOR POSITION ON "SCAN". * RAPIDLY FLASH THE THROTTLE FROM IDLE TO NEAR WIDE OPEN THROTTLE AND BACK WHILE OBSERVING 02 VOLTAGE. * REPEAT IF N ECESSARY TO CONFIRM VOLTAGE RANGE, AND "CLOSED LOOP".

VOLTAGE EXCEEDS VOLTAGE REMAINS WITHIN 250-750 mV RANGE. 250-750 mV RANGE. ----------------- ---------------------- 02 SENSOR OK, REPLACE 02 SENSOR. SEE "DIAGNOSTIC AIDS".

Diagnostic Aids:

1. Ignition system checks: Remove each spark plug and inspect (fouled, cracked, worn)

Fouled -- check ignition wires (hi resistance, damage, poor connections, grounds) check coil and module operation (see Section "6E 3" CHART C-4) check basic engine problem (see 3 below)

Cracked or worn -- replace as necessary

2. Fuel system checks: Restricted fuel system ( injectors, fuel pump, lines, and filter)

Injectors -- perform injector balance test (see Section "6E3" CHART C-2) verify each injector circuit with tool J-34730-2 or equivalent

Fuel Pump -- verify proper; fuel pressure and fuel quality

Lines and Filter -- verify no restrictions in lines or filter

3. Basic Engine Checks: Unless spark plug(s) inspection identifies a specific cylinder(s), road test vehicle under test conditions to reverify Code 48 prior to engine disassembly. Basic engine (valves, compression, camshaft, lifters)

Compression -- check rings, pistons, valves

Valves -- check for burned, weak springs, broken parts, worn or loose guide

Camshaft -- check for worn or broken

Lifters -- check for worn, broken

For additional items see Section "B" under "Rough Unstable Idle", 10-7-87 "Hard Start", or "Hesitation, Sag, or Stumble".

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.