POWER LOSS, STALL OR EXTENDED CRANK WHEN HOT-NEW FUEL PUMP

SOME 1985-1988 MEDIUM DUTY VEHICLES EQUIPPED WITH GASOLINE ENGINES MAY EXPERIENCE AN INTERMITTENT LOSS OF ENGINE POWER DUE TO FUEL LINE VAPOR LOCK WHEN OPERATED DURING HIGH AMBIENT TEMPERATURES AND/OR HEAVY LOAD CONDITIONS.

LISTED BELOW ARE SOME OF THE CONDITIONS EXPERIENCED WHEN VAPOR LOCK OCCURS:

1. LOSS OF ENGINE POWER WHEN HOT, BUT OPERATES PROPERLY AFTER SITTING SEVERAL HOURS OR OVERNIGHT. 2. ENGINE STALLING WHEN HOT UPON RESTART. 3. LONG STARTER CRANK TIMES WHEN ATTEMPTING TO START A HOT ENGINE.

VAPOR LOCK OCCURS WHEN FUEL IN THE LINE VAPORIZES. BECAUSE THE ENGINE MECHANICAL FUEL PUMP IS ACTUALLY A LIQUID SUCTION PUMP, WHEN THIS CONDITION OCCURS THE FUEL PUMP EFFICIENCY IS GREATLY REDUCED OR THE PUMP BECOMES INOPERATIVE. ONE WAY TO ELIMINATE THE EFFECTS OF VAPOR LOCK IS TO PUSH THE FUEL TO THE ENGINE MECHANICAL FUEL PUMP WITH AN ELECTRIC PUMP.

SERVICE HAS RELEASED AN IN-TANK ELECTRIC FUEL PUMP KIT WHICH INCORPORATES AN ELECTRIC PUMP, FUEL GAGE SENDER, WIRING HARNESS AND AN ELECTRIC PUMP CONTROL MODULE. THIS KIT WILL IMPROVE THE PERFORMANCE OF THE ENGINE WHEN OPERATED IN HIGH AMBIENT TEMPERATURES OR UNDER HEAVY LOAD CONDITIONS.

LISTED BELOW ARE THE REMOVAL AND INSTALLATION INSTRUCTIONS FOR THE ELECTRIC FUEL PUMP KIT:

SERVICE PROCEDURE - REMOVAL

1. DISCONNECT NEGATIVE BATTERY CABLE. 2. DRAIN FUEL TANK. REFER TO THE MEDIUM DUTY SERVICE MANUAL 6C FOR INSTRUCTIONS AND PRECAUTIONS. NOTE: USE CAUTION WHEN DRAINING FUEL TANK. 3. INSTALL A JACK UNDER THE FUEL TANK FOR SUPPORT. REMOVE AND RETAIN THE NUTS FROM THE TANK SUPPORT STRAP STUD BOLTS. 4. DISCONNECT THE WIRING HARNESS AT THE FUEL TANK SENDING UNIT. 5. SLIDE THE FUEL TANK OUT BOARD OF VEHICLE AND DISCONNECT THE FUEL LINES FROM THE FUEL SENDER. 6. REMOVE AND DISCARD THE FUEL TANK SENDING UNIT AND GASKET. SAVE FASTENERS. NOTE: THE EXISTING MECHANICAL FUEL PUMP IS TO BE LEFT ON THE ENGINE.

SERVICE PROCEDURE - INSTALLATION

1. INSTALL NEW GASKET (4) AND FUEL SENDER/PUMP ASSEMBLY (2) (SEE FIGURE 1). 2. FUEL PUMP HARNESS WILL BE ROUTED THROUGH THE COWL PANEL (SEE FIGURE 2). THERE ARE TWO WAYS, PREFERRED OR OPTIONAL, TO ROUTE THE HARNESS INTO THE ENGINE COMPARTMENT DEPENDING IF THE TRUCK IS BUILT WITH REGULAR PRODUCTION OR SPECIAL EQUIPMENT.

PREFERRED INSTALLATION

3A) REMOVE THE EXISTING GROMMET AND RETRAINING RING FROM THE COWL PANEL WHERE THE INSTRUMENT PANEL HARNESS IS ROUTED. IF THERE IS ROOM, ROUTE THE FUEL PUMP HARNESS (1) THROUGH THIS HOLE. RESEAL THE COWL TO PREVENT WATER FROM ENTERING THE CAB (SEE FIGURE 2).

OPTIONAL INSTALLATION

3B) IF THERE IS NOT ENOUGH ROOM FOR THE FUEL PUMP HARNESS (1) TO ROUTE WITH THE INSTRUMENT PANEL HARNESS AND OTHER HARNESSES, A NEW HOLE MUST BE CUT IN THE COWL PANEL. THIS HOLE IS TO THE RIGHT OF THE FUSE BLOCK JUST BELOW WHERE THE I.P. HARNESS IS ROUTED IN THE ENGINE COMPARTMENT. AFTER LIFTING THE FLOOR MAT INSIDE THE CAB, HOLD THE RETAINING RING (7) AS A TEMPLATE BELOW EXISTING HARNESS HOLE AND CENTER PUNCH THE NEW HARNESS HOLE. DRILL OUT THIS HOLE, USING A 40MM HOLE SAW IN THE COWL COVER PANEL FROM INSIDE THE ENGINE COMPARTMENT.

NOTE: CARE SHOULD BE USED WHEN DRILLING THIS HOLE SO THAT OTHER VEHICLE COMPONENTS OR HARNESS ARE NOT DAMAGED.

USING RETAINING RING (7) AS A TEMPLATE, DRILL TWO 4MM HOLES FOR THE ATTACHING SCREWS (5). ROUTE THE HARNESS THROUGH THIS HOLE AND INSTALL RETAINING RING GROMMET (6). THEN PLACE SEALANT AROUND THIS AREA TO PREVENT WATER FROM ENTERING THE CAB (SEE FIGURE 2). 4. INSTALL FUEL PUMP MODULE (2) ON FUSE BLOCK (SEE FIGURE 3). THEN CONNECT IT TO THE FUEL PUMP HARNESS (1). 5. CONNECT THE FUEL HARNESS TO POSTS 2,6, AND 7 OF THE UPPER FUNCTION BLOCK IN THE ENGINE COMPARTMENT AND TO THE ENGINE SPEED CONTROL BOX IN THE PASSENGER COMPARTMENT (SEE FIGURES 2 AND 4). 6. CONNECT THE FUEL PUMP WIRE ASSEMBLY (3) TO THE FUEL PUMP HARNESS (SEE FIGURE 2). ROUTE THE HARNESS TO THE FUEL TANK SENDER/PUMP USING TIE STRAP (2) TO SECURE THE HARNESS IN PLACE (SEE FIGURES 1 AND 5). 7. RECONNECT THE FUEL LINES TO THE NEW SENDER/PUMP ASSEMBLY. 8. REPOSITION FUEL TANK. 9. REINSTALL THE NUTS ON THE FUEL TANK SUPPORT STRAP STUD BOLTS. 10. REFILL FUEL TANK. 11. RECONNECT NEGATIVE BATTERY CABLE.

PARTS INFORMATION

PART NUMBER DESCRIPTION QUANTITY ----------- ----------- --------

15622083 KIT - INTANK FUEL PUMP/SENDER 1

FOR REPAIRS TO VEHICLES UNDER WARRANTY, USE LABOR OP. T7505, LABOR TIME 1.9 HOURS.

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.