12144 Mercruiser COVER, Water Passage

Fuel Flow
(1) Fuel Tank. (2) Fuel return line. (3) Fuel injection nozzle. (4) Fuel injection line. (5) Fuel injection pump. (6) Primary fuel filter. (7) Fuel transfer pump. (8) Secondary fuel filter. (9) Constant bleed valve. (10) Fuel injection pump housing.Fuel is pulled from fuel tank (1) through primary fuel filter (6) by fuel transfer pump (7). From the fuel transfer pump the fuel is pushed through secondary fuel filter (8) and to the fuel manifold in fuel injection pump housing (10). A bypass valve in the fuel transfer pump keeps the fuel pressure in the system at 140 to 280 kPa (20 to 40 psi). Constant bleed valve (9) lets a constant flow of fuel go through fuel return line (2) to fuel tank (1). The constant bleed valve returns approximately 34 liters (9 gal) per hour of fuel and air to the fuel tank. This helps keep the fuel cool and free of air.Fuel injection pump (5) gets fuel from the fuel manifold and pushes fuel at very high pressure through fuel line (4) to fuel injection nozzle (3). The fuel injection nozzle has very small holes in the tip that change the flow of fuel to a very fine spray that gives good fuel combustion in the cylinder.Fuel Injection Pump
The fuel injection pump increases the pressure of the fuel and sends an exact amount of fuel to the fuel injection nozzle. There is one fuel injection pump for each cylinder in the engine.The fuel injection pump is moved by cam (12) of the fuel pump camshaft. When the camshaft turns, the cam raises lifter (11) and pump plunger (7) to the top of the stroke. The pump plunger always makes a full stroke. As the camshaft turns farther, spring (8) returns the pump plunger and lifter to the bottom of the stroke.When the pump plunger is at the bottom of the stroke, fuel transfer pump pressure goes into inlet passage (1), around the pump barrel and to bypass closed port (3). Fuel fills the area above the pump plunger.After the pump plunger begins the up stroke, fuel will be pushed out the bypass closed port until the top of the pump plunger closes the port. As the pump plunger travels farther up, the pressure of the fuel increases. At approximately 690 kPa (100 psi), check valve (2) opens and lets fuel flow into the fuel injection line to the fuel injection nozzle. When the pump plunger travels farther up, scroll (5) uncovers spill port (4). The fuel above the pump plunger goes through slot (6), along the edge of scroll (5) and out spill port (4) back to the fuel manifold. This is the end of the injection stroke. The pump plunger can have more travel up, but no more fuel will be sent to the fuel injection nozzle.
Fuel Injection Pump
(1) Inlet passage. (2) Check valve. (3) Bypass closed port. (4) Spill port. (5) Scroll. (6) Slot. (7) Pump plunger. (8) Spring. (9) Fuel rack. (10) Gear. (11) Lifter. (12) Cam.When the pump plunger travels down and uncovers bypass closed port (3), fuel begins to fill the area above the pump plunger again, and the pump is ready to begin another stroke.The amount of fuel the injection pump sends to the injection nozzle is changed by the rotation of the pump plunger. Gear (10) is attached to the pump plunger and is in mesh with fuel rack (9). The governor moves the fuel rack according to the fuel needs of the engine. When the governor moves the fuel rack, and the fuel rack turns the pump plunger, scroll (5) changes the distance the pump plunger pushes fuel between bypass closed port (3) and spill port (4) opening. The longer the distance from the top of the pump plunger to the point where scroll (5) uncovers spill port (4), the more fuel will be injected.To stop the engine, the pump plunger is rotated so that slot (6) on the pump plunger is in line with spill port (4). The fuel will now go out the spill port and not to the injection nozzle.Operation Of Fuel Injection Nozzles
1W3996 Fuel Nozzle and 7000 Series Nozzle
The fuel injection nozzle goes through the cylinder head into the combustion chamber. The fuel injection pump sends fuel with high pressure to the fuel injection nozzle where the fuel is made into a fine spray for good combustion.
Fuel Injection Nozzle
(1) Carbon dam. (2) Seal. (3) Spring. (4) Passage. (5) Inlet passage. (6) Orifice. (7) Valve. (8) Diameter.Seal (2) goes against the cylinder head and prevents leakage of compression from the cylinder. Carbon dam (1) keeps carbon out of the bore in the cylinder head for the nozzle.Fuel with high pressure from the fuel injection pump goes into inlet passage (5). Fuel then goes into passage (4) to the area below diameter (8) of valve (7). When the pressure of the fuel that pushes against diameter (8) becomes greater than the force of spring (3), valve (7) lifts up. When valve (7) lifts, the tip of the valve comes off the nozzle seat and the fuel will go through the four 0.29 mm (.011 in) orifices (6) into the combustion chamber.The injection of fuel continues until the pressure of fuel against diameter (8) becomes less than the force of spring (3). With less pressure against diameter (8), spring (3) pushes valve (7) against the nozzle seat and stops the flow of the fuel to the combustion chamber.Fuel Transfer Pump
The fuel transfer pump is a single piston pump that is moved by a cam lobe on the camshaft for the fuel injection pump.When the camshaft turns, the cam lobe moves tappet (1) into the pump body. The tappet pushes piston (3) against the force of pumping spring (6). Outlet check valve (2) closes and inlet check valve (4) in the piston opens. Fuel in fuel inlet chamber (5) flows through the inlet check valve to the outlet side of the piston.As the camshaft continues to