pump. Before testing a fuel pump, check for the following:
Restricted fuel filters
Smashed or kinked fuel line or hoses
Air leak into the vacuum side of pump or line
Carburetor or injection system problems
Ignition system problems
Low engine compression
To measure fuel pump pressure, connect a pressure gauge to the output line of the fuel pump. Start and idle the engine at the rpm specified by the manufacturer with a mechanical fuel pump. With an electric fuel pump, you may only need to activate the pump motor. Compare your pressure reading to the manufacturer's specifications. Fuel pressure for a carburetor type system is approximately 4 to 6 psi. A gasoline injection system will usually have a high-pressure output, varying from 15 to 40 psi. If fuel pump pressure is NOT within specifications. check the pump volume, the lines, and the filters before replacing the pump.
Fuel pump volume is the amount of fuel the pump can deliver in a specific amount of time. It is measured by allowing the fuel to pour into a graduated (marked) container for a certain amount of time (normally 30 seconds). Route an output line from the fuel pump to a measuring container. For safety, a valve or clip should be installed to control fuel flow into the container. With the engine idling at a set speed, allow the fuel to pour into the container for the prescribed amount of time. Close off the clip or the valve and compare volume output to the specifications. Output should be a minimum of 1 pint in 30 seconds for carburetor systems. Fuel injection systems have a slightly higher output from the supply pump. Always refer to the service manual specifications for the particular fuel pump and vehicle. If the fuel pump fails both pressure and volume test, then check the fuel pump vacuum.
A vacuum test will eliminate possible problems in the fuel lines, the hoses, the filters, and the pickup screen in the tank. For example, a clogged fuel pickup screen could make the fuel pump fail the volume test. To measure vacuum, connect a vacuum gauge to the inlet side of the pump, leaving the fuel hose from the volume test in the graduated container. Open the control valve on the hose and start the engine and allow it to run on the fuel in the carburetor, or connect voltage, to an electric pump. Compare your reading with the manufacturer's specifications. Normally, fuel pump vacuum should be about 7 to 10 in/hg. A good reading indicates a good fuel pump. If the pump failed the pressure or volume test but passed the vacuum test, the fuel supply lines and filter may be at fault.
Fuel lines and hoses carry fuel from the tank to the engine. The main fuel line allows the fuel pump to draw fuel out of the tank. The fuel is pulled through this line to the pump and then to the carburetor, or metering section of the injection system.
Fuel lines are normally made of double wall steel tubing. For fire safety, a fuel line must be able to withstand the constant and severe vibration produced by the engine and road surface. Lines are placed away from exhaust pipes, mufflers, and manifolds, so that excessive heat will not cause vapor lock. They are attached to the frame, the engine, and other units, so the effects of vibration will be minimized.
Fuel hoses, made of synthetic rubber, are used where severe movement occurs between parts. A flexible hose can absorb movement without breaking. Hose clamps are required to secure fuel hoses to the fuel lines or to metal fittings.
Faulty fuel lines and hoses are a common source of fuel leaks. Fuel hoses can become hard and brittle after being exposed to the engine heat and the elements. Engine oil can soften and swell them. Always inspect hoses closely and replace any in poor condition. Metal fuel lines rarely cause problems; however, they should be replaced if they become smashed, kinked, rusted, or leaking. Remember these rules when working with fuel lines and hoses:
Place a rag around the fuel line fitting during removal. This action will keep fuel from spraying on you or on a hot engine. Use a flare nut or tubing wrench on fuel line fittings.
Use only approved double wall steel tubing for fuel lines. NEVER use copper or plastic tubing.
Make smooth bends when forming a new fuel line. Use a bending spring or bending tool.
Form double lap flares on the ends of fuel lines. A single lap flare is NOT approved for fuel lines.
Reinstall fuel line hold-down clamps and brackets. If not properly supported, the fuel line can vibrate and fail.Continue Reading