3306 DIESEL TRUCK ENGINE Caterpillar

Too Much Exhaust Smoke - Black or Gray


Engine Runs Smoothly

Recommended Procedure

1. Engine Used in a Lug Condition ... "Lugging" (when the truck is used in a gear too high for engine rpm to go up as accelerator pedal is pushed farther down, or when the truck is used in a gear where engine rpm goes down with accelerator pedal at maximum travel) the engine causes a reduction in the intake of air with full fuel delivery to the cylinders. Because there is not enough air to burn all the fuel, the fuel that is not used comes out the exhaust as black smoke. To prevent lugging the engine, use a gear where engine can have "acceleration" (increase in speed) under load.
2. Dirty Air Cleaner ... If the air cleaner has a restriction indicator, see if the red piston is in view. If there is no restriction indicator, restriction can be checked with a water manometer or a vacuum gauge (which measures in inches of water). Make a connection to the piping between the air cleaner and the inlet of the turbocharger. Check with the engine running at full load rpm. Maximum restriction is 25 in. (635 mm) of water. If a gauge is not available, visually check the air cleaner element for dirt. If the element is dirty, clean the element or install a new element.
3. Air Inlet Piping Damage or Restriction- ... Make a visual inspection of the air inlet system and check for damage to piping, rags in the inlet piping, or damage to the rain cap or the cap pushed too far on the inlet pipe. If no damage is seen, check inlet retriction with a clean air cleaner element.
4. Exhaust System Restriction ... Make a visual inspection of the exhaust system. Check for damage to piping or for a bad muffler. If no damage is found, you can check the system by checking the back pressure from the exhaust (pressure difference measurement between exhaust outlet and atmosphere). The back pressure must not be more than 40 in. (1016 mm) of water. You can also check by removing the exhaust pipes from the exhaust manifolds. With the exhaust pipes removed, start and load the engine on a chassis dynamometer to see if the problem is corrected.
5. Fuel Injection Timing Not Correct ... Check and make necessary adjustments as per Testing and Adjusting section of this Service Manual.
6. Fuel Setting is Not Correct ... Check and make necessary adjustments as per Testing and Adjusting Section of this Service Manual. See the RACK SETTING INFORMATION for the correct fuel setting.
7. Low Quality Fuel ... Test the engine with fuel according to recommendations by Caterpillar Tractor Co.
8. Valve Adjustment Not Correct or Valve Leakage ... 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 .025 in. (0.64 mm). Valve leakage normally causes the engine to "misfire" (injection not regular) and run rough.
9. Bad Fuel Nozzle(s) ... Bad fuel nozzles will normally cause the engine to "misfire" (injection not regular) and run rough, but can cause too much smoke with engine still running smooth. Remove the fuel nozzles and test as per Testing and Adjusting of this Service Manual.

Engine Runs Rough

10. Misfiring Cylinder(s) ... See Misfiring and Running Rough.
11. Fuel Injection Timing Not Correct ... Check and make necessary adjustments as per Testing and Adjusting section of this Service Manual.
12. 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.
13. 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 cylinder head. 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 cylinder head until the fuel that comes out is free of air. Tighten the fuel line nuts. If the air can not be removed 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 leakage 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.

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