Purpose

The ECU commands 100% duty cycle from the fuel pump via the fuel pump controller for startup, and for a significant period of time post engine start i.e. 30 seconds. For vehicles with stock fuel pumps, this poses no concern, but we wanted to provide an improvement for some vehicles utilizing upgraded fuel pumps that retain use of the factory fuel pump controller.  When the fuel pump output is commanded to 100% duty cycle, and engine demand for fuel is low, the pump may flow more fuel than the return system can support. This causes an unwanted increase in fuel pressure commonly referred to as "over-running the regulator", but the return line itself or other parts of the system may be the limiting factor. The excess fuel pressure causes the engine to run richer than expected, since injector output increases with an increase in differential fuel pressure.

Suggested Adjustment

Start engine, let idle while logging fuel pressure differential or watching a fuel pressure gauge and MAP reading to determine current fuel pressure vs. desired fuel pressure. Be sure to also view the Fuel Pump Duty monitor on the AP, or via ATP. See if fuel pressure is on target while fuel pump duty is 100%, and then check again at 33% duty.

• If fuel pressure stays on target or close to it while fuel pump duty is 100% and 33%, this table does not need adjusted.
• If fuel pressure strays significantly over the appropriate level while fuel pump duty is 100% and 33%, you have a situation that cannot be resolved by adjusting this table. One of the following is likely occurring:
  • The pump is no longer being controlled via the fuel pump controller due to aftermarket wiring changes.
  • The pump flows so much, that even at 33% duty it is outflowing your FPR and/or return hose. Additional fuel return flow is required to get fuel pressure on target.
  • You have an aftermarket adjustable FPR that needs adjusted to achieve the desired differential fuel pressure level.
• If fuel pressure strays significantly over the appropriate level while fuel pump duty is 100%, but is on target when fuel pump duty drops to 33%, you will likely benefit from adjustment of this table. I suggest reducing the value to 5 seconds as a starting point. Then adjust from there to achieve desired results.
  
  • Caution is suggested if considering very low values since flushing the lines quickly on a hot restart does improve engine operation.