LOSS OF PWR. DUE TO VAPOR LOCK (INSTALL NEW VAPOR LOCK KIT)
SUBJECT: LOSS OF POWER DUE TO VAPOR LOCK
MODELS: 1985-88 C5, 6, 7 WITH 5.7L/6.0L/7.0L GASOLINE ENGINE (VIN CODES A/B/E) AND DUAL 35/50 GALLON FUEL TANKS (INQC/NE6)
Some 1985-88 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 under heavy load conditions.
Listed below are some of the conditions experienced when vapor lock occurs:
1. Loss of engine power hot, but operates properly after sitting several hours or overnight.
2. Engine will not continue to run when hot.
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 may be greatly reduced or the pump may become 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 electric pumps, fuel gage senders, wiring harnesses, fuel selector valve, fuel selector switch, and an electric pump control module. This kit will increase the performance of the engine when operated in high ambient tempera- tures or under heavy load conditions.
IMPORTANT: Review the parts information section of this truck service bulletin carefully to insure the proper parts to make this change are ordered.
Service Procedure
A. Installation of Fuel Tank Sender/Pump ---------------------------------------- 1. Disconnect negative battery cable.
2. Drain fuel tanks. Refer to Medium Duty Service Manual, Section 6C for instructions and precautions.
3. Install a jack under the fuel tank for support. Remove and retain the nuts from the tank support strap stud bolts.
IMPORTANT: Refer to Service Manual, Section 6C for instructions.
4. Disconnect the sender wiring at the fuel tank sending units (Figure 1).
5. Slide the fuel tanks outboard from the vehicle and disconnect the fuel lines from the fuel sender.
6. Remove and discard the fuel tank sending units and gaskets. Save the nuts, washers, and mounting ring.
7. Clean gasket mounting area thoroughly.
8. Follow instructions supplied for assembly of fuel pumps to sending units and then install new gaskets (2) and fuel sender/pump assemblies (1) from kit (Figures 1 and 2).
9. Reinstall nuts, washers and mounting ring. Torque to 10-15 Nm.
IMPORTANT: The existing fuel pump is to be left on the engine.
B. Installation of Fuel Selector Valve Switch and Harness --------------------------------------------------------- 1. Disconnect fuel selector valve switch harness from selector switch on the dash and from fuse block. In order to gain access to harness and switch, loosen instrument panel bezel and remove dash panel padding. Disconnect switch harness from selector valve harness in engine compartment (Figure 3).
2. Remove screws holding selector valve harness grommet and retainer in cowl. Remove harness and discard.
3. Remove existing fuel tank selector valve switch from dash and discard. Install new selector valve switch (2) from kit in same mounting hole. Check switch for free action. If switch jams, straighten sheet metal on dash at bottom of mounting hole.
4. Install new selector valve switch harness (6) (Figure 4) from kit in same position as old switch harness.
IMPORTANT: There are two ways, preferred or optional, to route I.P harness into engine compartment depending if the truck is built with regular production or special equipment.
Preferred Installation - (Figure 4, View D)
(4a)Remove the existing grommet and retaining ring from the cowl panel where the instrument harness is routed. If there is room, route the fuel selector switch harness (6) in same position as old harness. Attach grommet and retainer (3) and place sealant around this area to prevent water from entering.
IMPORTANT: Proceed to step five (5) if this preferred installation was used.
Optional Installation (Figure 4, View C)
(4b)If there is not enough room for the fuel selector harness (6) to route with the instrument panel harness and the other harnesses, a new hole must be drilled 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 (2) 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.
NOTICE: Care should be used when drilling this hole so that other vehicle components or harnesses are not damaged.
Using the retaining ring (2) as a template, drill two 40mm holes for the attaching screws (4). Route the harness through this hole and install retaining ring grommet (2). Then place sealant around this area to prevent water from entering the cab. Attach retaining ring with screws (4) and washers (5).
5. Install fuel pump module (3) (Figure 3) on fuse block. (May be necessary to cut away floor pad when installing fuel pump module.) Connect module to fuel selector switch harness (1) and connect pink wire to fuse block. Install 10 amp fuse in fuse block. Tie strap switch harness to I.P. harness (Figure 3).
6. Route the fuel selector switch harness ring terminals to the upper electrical terminal block posts (Figure 4) as follows: Pink wire to post 2, dark blue wire to post 6, and purple wire to post 7.
7. Remove existing terminal block post nuts and install tooth lock washers (7) over existing terminals. Install new harness ring terminals (as instructed above) and reinstall post nuts.
8. Route black ground wire to dash panel. Attach with existing I.P. harness mounting screw and torque to 1.8-3.4 Nm. Clean paint from area to insure a good electrical ground. (Figure 4, Views C and D).
C. Fuel Tank Selector Valve Installation ----------------------------------------- 1. Remove existing fuel tank selector valve harness from engine compartment cowl and engine harness. Also disconnect at selector valve, senders, and engine harness connector. Discard harness (Figure 2).
2. Remove and discard existing fuel selector valve (Figure 2, View A). Carefully remove vent tube and elbow from old valve and install on new valve. It may be necessary to apply epoxy or other adhesive to hold elbow on valve port. Install new fuel selector valve (3) in same location with existing fasteners.
3. Install new fuel tank selector valve harness (1) and attach using the same mounting holes and clips as old harness.
4. Connect fuel tank selector valve harness (1) to selector valve switch harness (6) in engine compartment (Figure 4). Also connect harness to selector valve at RH fuel tank, engine harness connector at RH fuel tank, and to senders and pumps at RH and LH fuel tanks. Tie strap new harnesses as shown (Figure 2).
D. Reinstallation of Fuel Tank ------------------------------ 1. Reconnect the fuel lines to the new fuel sender/pump assembles (1) (Figure 1).
2. Reinstall nuts on fuel tank support straps stud bolts. Torque to 40-57 Nm.
E. Final Check -------------- 1. Reinstall tank drain plug.
2. Reconnect negative battery cable.
3. Refill fuel tanks with fuel and operate PUMP.
4. Check fuel lines and drain plug for leakage.
IMPORTANT: To check if in-tank pumps are operating, listen for buzzing sound in each tank when the ignition key is turned to the "run" position.
Parts Information
Part Number Description Quantity ----------- ----------- -------- 15628426 Vapor kit for (N-E6) Dual 50 gallon or (INQC) Dual 35 gallon 1
25094473 Sender ASM (INQC) for 35 gallon Fuel Tank 2
25093940 Sender ASM (NE6) for 50 gallon Fuel Tank 2
25115955 Fuel Pump ASM 2
IMPORTANT: When ordering sender ASM, verify type of fuel tank.
Parts are currently available from GMSPO.
For repairs to vehicles under warranty use:
Labor 0p: T7690 Labor Time: 5.0 Hrs.
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.
