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sending unit flange" or at the "filler neck". The connection should be made below the flapper valve of
the filler neck. The filler neck or sending unit must be removed from the tank to perform this operation.
DANGER!
PROPER INSTALLATION OF THE FUEL RETURN LINE MAY REQUIRE THE COMPLETE REMOVAL OF THE
FUEL TANK. THIS WORK SHOULD BE DONE BY A FUEL TANK SPECIALIST, WHO REGULARLY DOES
THIS WORK AND IS FAMILIAR WITH SAFETY REGULATIONS AND PRECAUTIONS NECESSARY TO DO
THIS WORK. IF A PERSON ATTEMPTS THIS WORK WHO IS NOT FAMILIAR WITH THE SAFETY
REGULATIONS AND PRECAUTIONS, AN EXPLOSION HAZARD MAY RESULT CAUSING SERIOUS INJURY
OR DEATH.
The fuel return line from the fuel pressure regulator must go back to
the fuel tank and enter the fuel tank with a fitting that has an internal
diameter of at least 3/8".
The #6 fitting on the fuel pressure regulator is internally sealed with
an O-ring. This fitting should only be finger tight in the regulator or it
will leak.
Returning fuel should be routed back to the tank away from the fuel
tank pick-up tube. This will allow air bubbles from the returned fuel
to dissipate before they are drawn into the pick-up tube.
There are several options including the following that meet the
above criteria for returning the fuel to the fuel tank.
• Use a fitting already in the fuel tank
• Use the vent fitting. Be very careful not to block the fuel
vapor from escaping the tank.
Figure 4
• Route the return line into the filler neck. This can be done by cutting the filler neck, Inserting a welded "T" fitting, and
securing the neck with multiple clamps.
• Drill and weld a new fitting into the tank
NOTICE:
For best performance of your fuel supply system an in tank pump is recommended. Such an installation
can be achieved by either buying a late model fuel tank or have your tank modified by professional
companies that install an in-tank pump with the required swirl pods and baffles.
DANGER!
MODIFICATIONS TO FUEL TANKS SHOULD BE DONE BY FUEL TANK SPECIALIST, WHO REGULARLY
DOES THIS WORK AND IS FAMILIARM SAFETY REGULATIONS AND PRECAUTIONS NECESSARY TO DO
THIS WORK. IF A PERSON ATTEMPTS THIS WORK WHO IS NOT FAMILIAR WITH THE SAFETY
REGULATIONS AND PRECAUTIONS, AN EXPLOSION HAZARD MAY RESULT CAUSING SERIOUS INJURY
OR DEATH.
10.7 Oxygen Sensor Installation
10.7.1 Oxygen Sensor Function, Theory, and Use
The MPI PRO-JECTION system allows the user to operate their engine in a closed loop fuel management mode using the
oxygen sensor. The oxygen sensor monitors the exhaust gases and generates a voltage output signal that corresponds to the
fuel/air mixture ratio. The range of voltage output signal from the oxygen sensor is 0.0 to 1.0 volts. A lean fuel/air mixture gives
a lower oxygen sensor voltage output signal while a rich fuel/air mixture gives a higher oxygen sensor voltage output signal. The
stoichiometric or chemically perfect fuel/air mixture gives an oxygen sensor output of approximately 0.45 volts. The engine ECU
reads this output from the oxygen sensor and adjusts the fuel delivery to maintain a stoichiometric fuel/air mixture. Most
automobiles, both currently and for the past 15 years, are using oxygen sensors to monitor the engine fuel/air mixture. This is
done primarily to improve engine emissions. It also gives the advantage of maintaining a stoichiometric fuel/air mixture under
varied operating conditions giving generally better engine performance and fuel economy.
Holley includes oxygen sensor feedback capability with the MPI PRO-JECTION system and recommends the user to use the
oxygen sensor. For most user applications, better performance and fuel economy will be obtained by using the oxygen sensor.
In addition, the oxygen sensor can make the tuning process much easier since the user can monitor the ECU functions as it
adjusts the fuel to meet a stoichiometric fuel/air mixture.
The user should read paragraph 4 of section 13.3 on engine configuration related concerns to help them determine whether an
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Troubleshooting
