3406 DIESEL TRUCK ENGINE Caterpillar

Misfiring And Running Rough


Recommended Procedure

1. Air in 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 the engine will not start, loosen a fuel injection line nut at the valve cover base. With the governor lever in the shutoff position, operate the fuel priming pump until the flow of fuel from the loosened fuel injection line is free of air. Tighten the fuel line nut. Fasten the priming pump and start the engine. If the engine still does not run smooth or releases a large amount of white smoke, loosen the fuel line nuts one at a time at the valve cover base until the fuel that comes out is free of air. Tighten the fuel line nuts. If the air can not be remove in this way, put 5 psi (35 kPa) of air pressure to the fuel tank.


Do not use more than 8 psi (55 kPa) of air pressure in the fuel tank or damage to the tank may result.

Check for leaks at the connections between the fuel tank and the fuel transfer pump. If leaks are found, tighten the connections or replace the lines. 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.

2. Valve Adjustment Not Correct ... Check and make necessary adjustments as per Testing and Adjusting section of this Service Manual. Intake valve clearance is .015 in. (0.38 mm) and exhaust valve clearance is .030 in. (0.76 mm). Also check for a bent or broken push rod.
3. Fuel Injection Timing Not Correct ... Check and make necessary adjustments as per Testing and Adjusting section of this Service Manual.
4. Automatic Timing Advance Does Not Operate Correctly ... Check with engine warm. Use the 1P3500 Timing Light Group. Special Instruction Form No. SMHS6964 gives the test procedure. If the timing light is not available, make rapid "acceleration" (increase in speed) from low idle to high idle. Engine must have smooth acceleration. A timing advance that does not operate correctly can cause delays of the engine acceleration at some rpm before high idle, or possibly cause the engine to run rough and have exhaust noise (backfire) during acceleration. This condition is difficult to find if engine acceleration is slow or at a constant engine rpm.
5. Bad Fuel Nozzle(s) ... Find a bad nozzle by running engine at the rpm where it runs rough. Loosen the fuel line nut at the cylinder head enough to stop fuel supply to that cylinder. Each cylinder must be checked this way. If a cylinder is found where loosening of the nut makes no difference in the rough running, test the nozzle for that cylinder. To test a nozzle, remove the nozzle from the engine and test as per Testing and Adjusting section of this Service Manual.
6. Valve Leakage; Wear or Damage to Pistons and/or Piston Rings; Wear or Damage to Cylinder Walls ... The cylinder head will have to be removed to make a visual inspection of these inside problems.
7. Cylinder Head Gasket Leakage ... Leakage at the gasket of the cylinder head can show as an outside leak or can cause loss of coolant through the radiator overflow. Remove the radiator filler cap and, with the engine running, check for air bubbles in coolant caused by exhaust gases.
8. Engine Camshaft Timing Not Correct ... Engine camshaft timing can be checked by finding the top center position for number one cylinder as per Testing and Adjusting section of this Service Manual.
9. Fuel Leakage from Fuel Injection Line Nut ... Tighten nut to 30 ± 5 lb. ft. (40 ± 7 N·m). Again check for leakage. Be sure to check fuel injection lines inside valve cover base. See Recommended Procedure for FUEL IN CRANKCASE OIL.
10. Fuel Has a High "Cloud Point" ... The fuel "cloud point" is the temperature at which wax begins to form in the fuel. If the atmospheric temperature is lower than the "cloud point" of the fuel, wax will form and plug the filter. Change the filter and drain the tank and the complete fuel system. The replacement fuel must be of a better grade with a lower "cloud point".
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