32 SENR9977 Testing and Adjusting Section Testing and Adjusting Section 2. Install a suitable fuel flow tube with a visual sight gauge in the fuel return line. When possible, install the sight gauge in a straight section of the fuel line that is at least 304.8 mm (12 inches) long. Do not install the sight gauge near the following devices that create turbulence: Fuel System Elbows i02242712 Fuel System - Inspect Relief valves Check valves Observe the fuel flow during engine cranking. Look for air bubbles in the fuel. If there is no fuel that is present in the sight gauge, prime the fuel system. Refer to Testing and Adjusting, “Fuel System - Prime” for more information.
If the engine starts, check for air in the fuel at varying engine speeds. When possible, operate the engine under the conditions which have been suspect. A number of the components that send fuel to the engine can cause low fuel pressure. This can decrease engine performance. 1. Check the fuel level in the fuel tank. Ensure that the vent in the fuel cap is not filled with dirt. 2. Check all fuel lines for fuel leakage. The fuel lines must be free from restrictions and faulty bends. Verify that the fuel return line is not collapsed. 3. Inspect the fuel filter for excess contamination. If necessary, install a new fuel filter. Determine the source of the contamination.
Make the necessary repairs. 4. Service the primary fuel filter (if equipped). 5. Remove any air that may be in the fuel system. Refer to Testing and Adjusting, “Fuel System - Prime”. i01854200 Air in Fuel - Test g00578151 Illustration 39 This procedure checks for air in the fuel system. This procedure also assists in finding the source of the air. (1) A steady stream of small bubbles with a diameter of approximately 1.60 mm (0.063 inch) is an acceptable amount of air in the fuel. (2) Bubbles with a diameter of approximately 6.35 mm (0.250 inch) are also acceptable if there is two seconds to three seconds intervals between bubbles. (3) Excessive air bubbles in the fuel are not acceptable. 1.
Examine the fuel system for leaks. Ensure that the fuel line fittings are properly tightened. Check the fuel level in the fuel tank. Air can enter the fuel system on the suction side between the fuel transfer pump and the fuel tank. 3. If excessive air is seen in the sight gauge in the fuel return line, install a second sight gauge at the inlet to the fuel transfer pump. If a second sight gauge is not available, move the sight gauge from the fuel return line and install the sight gauge at the inlet to the fuel transfer pump. Observe the fuel flow during engine cranking. Look for air bubbles in the fuel. If the engine starts, check for air in the fuel at varying engine speeds.
Work carefully around an engine that is running. Engine parts that are hot, or parts that are moving, can cause personal injury.