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. To remove the air from the fuel system open the manual bleed valve on the fuel injection pump housing. Operate the priming pump until the flow of fuel from the manual bleed valve is free of air. Close the manual bleed valve and fasten the fuel priming pump. 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 heads, and permit the fuel to come out until it is free of air. Tighten the fuel line nuts. If air is not removed in this way, put 5 psi (35 kPa) [CAUTION do not use more than 8 psi (55 kPa)] of air pressure to the fuel tank and check for leaks at connections between the fuel tank and the fuel transfer pump. If there are no leaks at the connections, 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 adjustment is .015 in. (0.38 mm) and exhaust valve adjustment is .025 in. (0.64 mm).
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 (GMG00501) 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 range 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 - Check with the cylinder leakage tester. If leakage is over specification and leaking is heard at the air inlet to the engine (intake valve leaks), exhaust manifold (exhaust valve leaks), or at the oil filler opening (leaking past piston rings). Special Instruction (GMG00694) gives the test procedure.
7. Cylinder Head Gasket Leakage - Check with the cylinder leakage tester. If leakage is higher than specification, check at the fuel nozzle hole of the next cylinder to see if the leak is between cylinders. Leakage at the gasket of the cylinder head can show as an outside leak or can cause loss of coolant through the radiator overflow.
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. If timing is not correct remove the front cover and check timing marks. If timing marks are correct check for a broken drive dowel in the camshaft drive gear.
9. Fuel Leakage from Nozzle to Fuel Line Adapter - A loose nut will permit fuel leakage through adapter orifice to outside of valve compartment. Tighten nut to 30 ± 5 lb. ft. (40 ± 7 N·m). Again check for leakage.