Fundamentals Multec 3.5 Fuel Injector Application Manual
2-16 Delphi Energy and Chassis Systems
Revision: 11/05-1
• High volatility fuel is being used. This includes high RVP gasoline,
especially moderate to high RVP ethanol/ methanol blends of gasoline.
One method of reducing susceptibility to hot re-start problems is to raise
the fuel system operating pressure above this vaporization pressure.
Other areas that should be investigated to make the fuel system more
robust to premature vaporization are as follows:
• Fuel System Operating Pressure (as mentioned above). While higher
pressure is better for hot re-start performance, it can have negative
impacts on tip leakage, fuel pump noise and fuel pump durability. The
trade-off between these issues should be well understood.
• Impact of vacuum bias at start-up. Biasing the regulator at start-up
will tend to reduce the upstream pressure on the fuel. This can
negatively impact hot re-start performance. While vacuum biasing
should not be eliminated because of this, the impact of it needs to be
understood when establishing the regulation pressure.
• Ability of fuel system to "check" pressure between the pump and
injectors. The fuel system needs to be able to maintain some level of
fuel pressure for extended periods of time. If it cannot, the pressure
may drop low enough that vapor will form in the fuel system during a
hot soak.
• Injector tip temperature. Consideration should be given to the location
of the fuel injector. Cylinder head mounting will typically generate
higher injector tip temperatures than intake manifold mounting.
• Fuel rail design. Recirculating fuel systems typically perform better
than returnless systems under the same conditions. This is due to the
ability of the recirculating system to replace the hot fuel in the rail
with a fresh supply of cooler fuel from the fuel tank.
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