GUIDE 4N3668 - Caterpillar

This procedure checks for air in the fuel. This procedure also assists in finding the source of the air.
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.
Install a 2P-8278 Tube As (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 inch) long. Do not install the sight gauge near the following devices that create turbulence:
Elbows
Relief valves
Check valvesObserve the fuel flow during engine cranking. Look for air bubbles in the fuel. If there is no fuel 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 of air in the fuel.
Illustration 1 g01096678
2P-8278 Tube As (SIGHT GAUGE)
(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 2 to 3 seconds intervals between bubbles.
(3) Excessive air bubbles in the fuel are not acceptable.
If excessive air is seen in the sight gauge in the fuel return line, install a second sight gauge at the fuel transfer pump inlet. If a second sight gauge is not available, move the sight gauge from the fuel return line to the fuel transfer pump inlet. 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. If excessive air is not seen at the inlet to the fuel transfer pump, the air is entering the system after the fuel transfer pump. Proceed to Step 6.If excessive air is seen at the inlet to the fuel transfer pump, air is entering through the suction side of the fuel system. Proceed to Step 4.
To avoid personal injury, always wear eye and face protection when using pressurized air.
To avoid damage, do not use more than 55 kPa (8 psi) to pressurize the fuel tank.
Pressurize the fuel tank to 35 kPa (5 psi). Do not use more than 55 kPa (8 psi) in order to avoid damage to the fuel tank. Check for leaks in the fuel lines between the fuel tank and the fuel transfer pump. Repair any leaks that are found.
If the source of the air is not found, disconnect the supply line from the fuel tank. Then, connect an external fuel supply to the fuel transfer pump inlet. If connecting an external fuel supply corrects the problem, repair the fuel tank or the stand pipe in the fuel tank.
If the