3116 and 3126 Truck Engines Air In Fuel

Probable Cause:

Install a 4P8278 sight glass in the fuel return line and operate the engine to check for gas bubbles, an indication of air in the fuel system. With air in the fuel system the engine will normally be difficult to start, run rough and release a large amount of white smoke. If air is in the system, it will generally get in on the suction side of the fuel transfer pump. Check for leakage at the connections between the fuel tank and the fuel transfer pump. If leaks are found, tighten the connections or replace the lines.

NOTE: The fuel priming pump (if equipped) may be used to remove the air from the fuel filter and fuel gallery (in the cylinder head), and fill the fuel system with fuel from the fuel tank before the engine is started.

If there are no visual leaks, remove the fuel supply line from the tank and connect it to an outside fuel supply. If this corrects the problem, the suction line (standpipe) inside the fuel tank has a leak.

If this does not correct the problem, check for combustion gas leakage at the base of the fuel injectors. Remove each injector and examine the base of the sleeve and the injector for evidence of nicks or carbon tracking. A magnifying glass and flashlight or shoplight may be needed. The lower sealing surfaces of the nozzle and sleeve should be smooth and without defect. If any defects are noted, the sleeve can be reamed to restore the surface. Refer to Special Instruction “Using the 122-7093 Sleeve Replacement Tool Group”, SEHS9120. Check the injector tip seal (O-ring) and upper seal (O-ring) and replace if found defective.

NOTE: Refer to Disassembly And Assembly, SENR6553, for the proper procedure for unit injector removal and installation.

The temperature of an exhaust manifold port can be an indication of a cylinder that has air being delivered to it. Check the exhaust manifold temperatures and compare the results. A lower than normal cylinder temperature indicates that the cylinder may be receiving air from the unit injector.

The color of the exhaust smoke can also indicate which cylinder has a combustion leak. The cylinder that has air will produce smoke that is gray or white in color.

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