REDUCED ENGINE PERFORMANCE FUEL VAPOR LOCK

VEHICLES AFFECTED: 'P' Chassis motor Homes with 7.4L (454 CID) Engines

Reduced engine performance caused by fuel vapour lock is primarily a hot weather condition. This condition may also occur in mild or cool weather if the motor home is being operated at higher altitudes which allows the fuel to vapourize at lower temperatures. The Reid Vapour Pressure level of the fuel has a direct effect on the point where vapour lock can occur. Some brands of gasoline may be more prone to vapour lock than others.

This condition can be repaired by making the following changes. It is not necessary to install additional return line style filters or bypass the mechanical fuel pump.

PARTS REQUIREMENTS

1 - Electrical fuel pump with adjustable regulator (pump must be able to pump 25 GPH, 2 psi minimum, 3 psi maximum at wide open throttle)

25 feet of 1/2 inch steel fuel line or as required to connect engine fuel pump to gas tank of vehicle

1 - Fuel filter with 3/8 inch inlet and outlet nipples or fittings (largest sizes are best) GM P/N 854619, type CF61P or equivalent

Rubber hoses and clamps as required

It may be necessary to obtain some of the above listed parts on a local basis.

ELECTRICAL

Wire as required by fuel pump manufacturer.

GM P/N ------ 1 - 8905447 Electrical Connector 2 - 6294068 Terminals

One of the following engine oil pressure switches as required by engine space requirements and fitting thread size.

GM P/N Connection ------ ---------- 10002798 1/8 - 27 Dryseal (NPTF) 14034354 1/4 - 18 Dryseal (NPTF)

INSTALLATION

Mount the electrical fuel pump on the chassis in the vicinity of the fuel tank following the pump manufacturer's instructions. Be sure that the pump will not be directly exposed to foreign objects that may be thrown off the tires or to high heat radiation such as directly behind a muffler or tailpipe. The preferred location is behind the fuel tank but no higher than the tank. It may also be necessary to use some kind of acoustical insulator for mounting the pump to eliminate pump noise from the interior of the motor home.

Locate the fuel tank outlet line and disconnect it from the existing fuel line.

Connect the tank outlet line to the electrical fuel pump inlet and install an inline fuel filter between the two.

Mount the fuel pressure regulator supplied with the pump next to the fuel pump outlet. It should not be further than six inches from the pump outlet. Connect regulator to pump with 1/2 inch hose and clamps.

Route 1/2 inch steel fuel line from fuel pressure regulator outlet to the engine fuel pump inlet. The preferred location is outside the frame ralls and as far as possible from any heat source. If the fuel line cannot be routed outside the frame rail and passes within four inches of a heat source, exhaust pipe, muffler, etc., then it should have some sort of a shield or insulation around it as a heat insulator. Care should be taken that the heat shield does not touch the heat source or the fuel line and presents no sharp corners or projections to the fuel line. It may be necessary to fabricate the required hose connection fittings to accommodate the 1/2 inch fuel line to the smaller diameter fitting on the regulator and fuel pump.

Remove existing engine fuel pump hose and connect the new line. Be sure that connections are leak proof and that all new line is properly supported to the chassis.

Install the new engine oil pressure switch and connector. Connect electric fuel pump B + wire to engine oil pressure switch, and oil pressure switch to a fuse box location capable of providing 12 volts only when key is in the run position and with a 15 amp protection limit. Be sure that proper ground is provided for the fuel pump ground wire. The frame is usually a good place for the ground connection, but be sure to remove any paint or foreign material from under the connection. Be sure all wiring is secured and not allowed to chafe.

Temporarily install a fuel pressure gauge between the engine fuel pump and the 1/2 inch fuel line. Start the engine and observe the pressure gauge. If the gauge reads over 5 lbs. of pressure, stop the engine and adjust the fuel pressure regulator to obtain no more than 5 lbs. The electric fuel pump must not be operating while adjusting the pressure regulator. Remove gauge and reconnect line to pump.

This system cannot be used with dual tank systems.

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.