injection of the correct amount of fuel at exactly the right time directly into the combustion chamber.

Efficient engine operation demands that the fuel system be maintained in first-class condition at all times. Use only clean water-free fuel. Good maintenance of the fuel filtering system and reasonable care in handling the fuel are the key to a trouble-free fuel system.

Servicing the fuel system is not a difficult task. However, because of the close tolerances of the various fuel system components, mechanics should practice cleanliness and strictly adhere to service instructions.

In this section, troubleshooting, testing, disassembly, cleaning out, inspection, and reassembly of the fuel pump and fuel injector are discussed. Before you work on these components, refer to the manufacturer's maintenance and service manuals.

Troubleshooting

When a piece of equipment is brought into the shop for maintenance and service, the hard card or Equipment Repair Order (ERO) may show a fuel system problem. You can pinpoint the problem by troubleshooting the fuel system until you find the trouble.

Check the fuel lines for improper or faulty connections. If any leaks occur, tighten the connection only enough to stop the leak. Also, check the filter cover bolt for tightness. If the fuel pump fails to function satisfactorily, first check the level of the fuel tank; then make sure the fuel supply valve is open. Check for a broken pump drive shaft or drive coupling by inserting the end of a wire through one of the pump flange drain holes; then crank the engine and note if the wire vibrates. Vibration will be felt if the pump shaft is turning.

The result of most fuel pump failures is that either no fuel or an insufficient amount of fuel is delivered to the fuel injectors. This lack of fuel will show up if the engine runs unevenly, vibrates too much, stalls at idling speeds, or loses power.

The most common failure of a fuel pump is a sticking relief valve. The relief valve, due to its close fit in the valve bore, may stick in a full-open position because a small amount of grit or foreign material lodges between the relief valve and its bore or seat. The fuel oil circulates within the pump rather than being forced through the fuel system. If the fuel pump is not functioning properly, remove the fuel pump from the engine. hen remove the relief valve plug, spring, and pin, and check the movement of the valve within the valve bore. If the valve sticks, recondition it by using fine emery cloth to remove any scuff marks. Clean the valve bore and the valve components. Then lubricate the valve and check it for free movement throughout the entire length of its travel. If its operation is satisfactory, reassemble the valve in the pump. If not, replace it.

After the relief valve has been checked and the fuel pump reinstalled on the engine, start the engine and check the fuel flow at some point between the restricted fitting in the fuel return manifold and the fuel tank.

If, after making the above checks, there is still a lack of power, uneven running, excessive vibration, or stalling at idle, you should suspect a faulty injector in one or more cylinders. Start the engine and run it at part load until it reaches normal operating temperature. Remove the valve rocker cover(s) and let the engine run at idle speed. Hold the injector follower down with a screwdriver, which prevents operation of the injector. If the cylinder has been misfiring, there will be no noticeable difference in the sound or operation of the engine. If the cylinder has been firing properly, there will be a noticeable difference in the sound and operation when the follower is held down. If that cylinder is firing properly, repeat the procedure on the other cylinders until the faulty one has been located.

At this point you can remove the fuel injector for additional testing, provided that the injector operating mechanism of the faulty cylinder is functioning satisfactorily.

TESTING

The General Motors injector tester gives you a means to determine the condition of the injector to avoid unnecessary overhauling. An injector that passes all of the tests outlined below may be considered to be satisfactory for service without disassembly (except for the visual check of the plunger). If an injector fails to pass one or more of the tests, it is unsatisfactory. Be sure to identify each injector and record the pressure drops and fuel output during the tests. Also remember, all tests must be performed before the injector is disassembled.

INJECTOR CONTROL RACK AND PLUNGER MOVEMENT TEST. - To perform this test, lock the injector in a test stand.

CAUTION

Keep your hands away from the tip of the injector while depressing the plunger. High- pressure fuel spray that penetrates the skin will cause blood poisoning.