Mercury 200 OptiMax DTS : Installation manual : Page 59

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Fuel

Fuel for the engine is stored in a fuel tank. A primer bulb is installed into the

fuel line to allow priming of the fuel system. A crankcase mounted pulse driven

diaphragm fuel pump (electric on V-6 M2 Jet Models) draws fuel through the

fuel line, primer bulb, fuel pump assembly and then pushes the fuel thru a

water separating fuel filter. This filter removes any contaminates and water

before the fuel reaches the vapor separator.

Fuel vapors are bled into the air compressor inlet in the front of the flywheel

cover preventing a vapor lock of the electric fuel pump assembly which is

mounted in the vapor separator. The low pressure electric fuel pump was

added in 1998 to eliminate potential vapor locking of the fuel system. This

pump draws fuel from the main chamber of the vapor separator tank (VST)

and pushes the fuel into the chamber where the high pressure electric fuel

pump is located. This creates a pressure of approximately 6 to 7 psi on the

intake side of the high pressure fuel pump.

Excess fuel in the high pressure chamber returns to the main VST chamber

through a 0.030” hole. The high pressure electric fuel pump is different than

the fuel pump that is utilized on the standard EFI engine (non OptiMax), and is

capable of developing fuel pressures in excess of 90 psi. Fuel inside the rail

must remain pressurized at exactly 10 psi over the air rail pressure or the

ECM (map) calibrations will be incorrect. Fuel from the vapor separator is

supplied to the top of one fuel rail. A fuel line connects the bottom of the first

rail to the opposite fuel rail. Fuel is stored inside the rail until an injector opens.

A fuel pressure regulator controls pressure in the fuel rails, and allows excess

fuel to return into the vapor separator. The fuel regulator not only regulates

fuel pressure but also regulates it at approximately 10 p.s.i. higher than

whatever the air rail pressure is. The fuel regulator diaphragm is held closed

with a spring that requires 10 p.s.i. to force the diaphragm off the diaphragm

seat. The back side of the diaphragm is exposed to air rail pressure. As the air

rail pressure increases, the fuel pressure needed to open the regulator will

equally increase. Example: If there is 50 p.s.i. of air pressure on the air rail

side of the diaphragm, 60 p.s.i. of fuel pressure will be required to open the

regulator. The return fuel line to the vapor separator is water cooled. This

design is used to prevent cold fuel from the fuel tank hitting the hot fuel

returning from the fuel rail and flashing off the light ends (boiling over).

To equalize the pulses developed by the pumps (both air and fuel) a tracker

diaphragm is installed in the starboard rail. The tracker diaphragm is

positioned between the fuel and air passages. The tracker diaphragm is a

rubber diaphragm which expands and retracts depending upon which side of

the diaphragm senses the pressure increase (pulse).

Notes

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