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6.7L Cummins Injectors and Fuel Lines – Air in Fuel – General Information

The replacement of fuel supply lines, fuel filters, fuel pump, high-pressure fuel lines, and injectors will let air
into the fuel system. Air in the system can make the engine hard to start, run rough, misfire, produce low power, or can cause excessive smoke and a fuel knock.
A few bubbles exiting the line during the test is expected.
A foamy appearance is indication of a leak that allows air to enter, a severe inlet restriction that causes cavitation,
or a system that is not yet primed. If fuel inlet restriction (Refer to Procedure 006-020 in Section 6.) is not
excessive, the source of air entry must be isolated to one of the following:
• Suction fuel lines
• OEM fuel lines
• Suction-side fuel filter assemblies
• Stand-pipe(s) in the fuel tank(s).
NOTE: Front gear train engines have a lift pump located in the fuel tank. The fuel line from the tank will be pressurized.