This article describes how to test the air in the fuel of a Caterpillar 325C excavator

The air in fuel test procedure is used to check the air in the fuel. This procedure can also 

help find the source of the air.  

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(1) Check the fuel system for leaks. Make sure the fuel line connector is tightened well.

  Check the fuel level in the fuel tank. Air can enter the fuel system from the suction side 

between the fuel transfer pump and the fuel tank.  

(2) Install a 2P-8278 fuel flow pipe (observation instrument) in the fuel return pipe. If 

possible, install a sight gauge in a straight section of fuel pipe that is at least 304.8mm long.

  NOTE: Do not install scopes in right-angle elbows, safety valves, and check valves. As 

shown in Figure 3-1-41, observe the fuel flow when the engine is started. Look for air 

bubbles in the fuel. If there is no oil in the sight gauge, refill the fuel system. If the engine 

starts, check the air in the fuel at various engine speeds. If possible, run the engine with the 

assumption that there is air in the fuel. 

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(3) If too many bubbles are seen in the sight gauge, install a second sight gauge at the inlet 

of the fuel transfer pump. If a second sight gauge is not available, remove the sight gauge 

from the fuel return pipe and install it at the inlet of the fuel transfer pump. Observe fuel 

flow when the engine is started. Look for air bubbles in the fuel. If the engine is started, 

check the air in the fuel at various engine speeds. If at the inlet of the fuel transfer pump If 

excess air is not seen, the air will be sucked into the system after passing through the fuel 

transfer pump, proceed to step (6); if too much air is seen at the inlet of the fuel transfer 

pump, the air will enter through the suction side of the fuel system In the oil transfer pump.

  NOTE: To avoid personal injury, always wear a protective mask when working with 

compressed air. To avoid tank damage, do not pressurize the fuel tank beyond 55kPa.

(4) Pressurize the fuel tank to 35kPa. To avoid damage to the fuel tank, do not apply more 

than 55kPa pressure to the fuel tank. Check the fuel line between the fuel tank and fuel 

transfer pump for leaks. If there is an oil leak, repair it. Check the fuel pressure and make 

sure the fuel transfer pump is working properly. 

 (5) If the source of air is not found, disconnect the fuel tank supply line and connect the 

external fuel supply to the inlet of the fuel transfer pump. If this solves the problem, repair 

the fuel tank or the standpipe in the fuel tank. 

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 (6) If the injector sleeve is damaged or damaged, combustion gases will leak into the fuel 

system. If the O-ring on the injector sleeve is broken, missing, or damaged, combustion 

gases can also leak into the fuel system.

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