7009 Mercruiser DIODE RECTIFIER, NEGATIVE

Air Or Water 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 through the head adapter and crank the engine until fuel comes out. Tighten the fuel line nut. Start the engine. If the engine does not run smooth or releases a large amount of white smoke, loosen the fuel line nuts one at a time at the through the head adapters until the fuel that comes out is free of air. Tighten the fuel line nuts. If the air cannot be removed in this way, put 35 kPa (5 psi) of air pressure to the fuel tank.
Do not use more than 55 kPa (8 psi) of air pressure in the fuel tank or damage to the tank may result.
Check for leaks at the connection 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 furl supply. If this corrects the problem, the suction line (standpipe) inside the fuel tank has a leak.Water in the fuel can cause rough running and possible fuel system damage.Valve Adjustment Not Correct
Check and make necessary adjustments as in Testing And Adjusting Section of the Service Manual. Intake valve lash is 0.38 mm (.015 in) and exhaust valve lash is 0.64 mm (.025 in). Also check for a bent or broken push rod.Defective Fuel Nozzle(s)
Find a defective nozzle by running engine at the rpm where it runs rough. Loosen the fuel line nut at the through the head adapter 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 in Testing and Adjusting Section of this Service Manual.Fuel Leakage From Fuel Injection Line Nut
Tighten nut to 40 7 N m (30 5 lb ft). Again check for leakage. Be sure to check fuel injection lines inside valve cover base.Defective Fuel Injection Pump
An injection pump can have a good fuel flow coming from it but cause rough running because of slow timing that is caused by wear on the bottom end of the plunger. See the Testing and Adjusting Section in this Service Manual for the correct specifications and procedure to check plungers and lifters.Fuel pumps which are severely scored from debris can cause rough running but fuel dilution usually occurs before horsepower is affected.Low installation torque on the fuel pump retaining nut can cause misfire, rough running and low power.Fuel Has A High "Cloud Point"
In cold weather operation this condition should be checked first. 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".Fuel Injection Timing Not Correct
Check and make necessary adjustments as in Testing and Adjusting Section of this Service Manual.Automatic Timing Advance Does Not Operate Correctly
Check with engine warm. Use the 8T5300 Engine Timing Indicator Group and 8T5301 Diesel Timing Adapter Group. Special Instruction Form No. SEHS8580 has instructions and calibration procedures for this tool group. If the timing indicator group 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 possible 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.Fuel Return Line Has A Restriction
This condition blocks or slows the fuel flow back to the fuel tank. The result is higher fuel temperatures in the fuel injection pump housing. Also, the removal of air from the fuel is prevented. Make a visual inspection of the fuel lines and fittings for damage and make repairs or replacements as needed.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.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 combustion gases.
Do not loosen the filler cap or pressure cap on a hot engine. Steam or hot coolant can cause severe burns.
Engine Camshaft Timing Not Correct
Engine camshaft timing can be checked by finding the top center position for number one cylinder as in Testing and Adjusting Section of this Service Manual.