2001 Ford Mustang 3.8L V6 (returnless fuel system). I have been chasing a fuel pressure issue for approximately 9 months. The car starts, idles, and drives perfectly when cold. Fuel pressure is normally around 58-60 PSI. After approximately 30-60 minutes of driving, especially under load, uphill, or heavy throttle, fuel pressure begins to gradually fall. It will initially drop from 60 PSI to around 40-30 PSI under throttle, but if I let off the gas it will recover back to 60 PSI. After this happens a few times, the problem gets worse and fuel pressure eventually drops to around 20 PSI, the engine begins misfiring (primarily cylinders 4 and 5), loses power, and the check engine light flashes. Once pressure reaches approximately 20 PSI it stays there and does not recover during driving. If I pull over and cycle the key off and back on, fuel pressure immediately returns to approximately 60 PSI and the car drives normally again for a short period until the problem repeats.
Fuel pressure has been verified with a mechanical gauge connected directly to the Schrader valve and also verified through scan tool data. Both agree that pressure is physically dropping. During failure, I have measured voltage directly at the fuel pump connector and consistently see approximately 13 volts. I have also measured fuel pump current on the pump power circuit and current increases from roughly 2-3 amps during normal operation to approximately 4 amps when the failure occurs. In other words, fuel pressure drops while pump voltage and pump current both increase.
Parts replaced include multiple OEM fuel pump/module assemblies (approximately 4-5 complete assemblies including pump, basket, top hat, wiring, and internal hoses), fuel filter multiple times, fuel rail pressure sensor (FRPS), injectors, FPDM (fuel pump driver module) with a known-good unit from another Mustang, fuel feed line, and the engine itself. The replacement engine came with a different fuel rail, so the rail was effectively changed during the engine swap as well. The problem remained identical before and after the engine swap.
Diagnostics performed include full tank fuel testing (no change), driving with the gas cap removed (no change), tank inspection and cleaning, wiring inspections, multiple fuel pump rewires, direct voltage verification at the pump connector, current measurements on the pump circuit, connector bypass testing, and FPDM replacement. The issue persists regardless of fuel level. The tank was inspected and cleaned and nothing obvious was found. The car does not exhibit the problem when cold and does not exhibit the problem at idle. The issue only appears after the vehicle is fully heat soaked and under real driving load.
At this point I am not looking for guesses such as "replace the fuel pump" because multiple OEM pump/module assemblies have already been installed. I am looking for a failure mode that can explain a physical fuel pressure drop from 60 PSI to approximately 20 PSI under load while pump voltage and pump current both increase during the failure event, and where a key cycle immediately restores normal pressure.
Fuel pressure has been verified with a mechanical gauge connected directly to the Schrader valve and also verified through scan tool data. Both agree that pressure is physically dropping. During failure, I have measured voltage directly at the fuel pump connector and consistently see approximately 13 volts. I have also measured fuel pump current on the pump power circuit and current increases from roughly 2-3 amps during normal operation to approximately 4 amps when the failure occurs. In other words, fuel pressure drops while pump voltage and pump current both increase.
Parts replaced include multiple OEM fuel pump/module assemblies (approximately 4-5 complete assemblies including pump, basket, top hat, wiring, and internal hoses), fuel filter multiple times, fuel rail pressure sensor (FRPS), injectors, FPDM (fuel pump driver module) with a known-good unit from another Mustang, fuel feed line, and the engine itself. The replacement engine came with a different fuel rail, so the rail was effectively changed during the engine swap as well. The problem remained identical before and after the engine swap.
Diagnostics performed include full tank fuel testing (no change), driving with the gas cap removed (no change), tank inspection and cleaning, wiring inspections, multiple fuel pump rewires, direct voltage verification at the pump connector, current measurements on the pump circuit, connector bypass testing, and FPDM replacement. The issue persists regardless of fuel level. The tank was inspected and cleaned and nothing obvious was found. The car does not exhibit the problem when cold and does not exhibit the problem at idle. The issue only appears after the vehicle is fully heat soaked and under real driving load.
At this point I am not looking for guesses such as "replace the fuel pump" because multiple OEM pump/module assemblies have already been installed. I am looking for a failure mode that can explain a physical fuel pressure drop from 60 PSI to approximately 20 PSI under load while pump voltage and pump current both increase during the failure event, and where a key cycle immediately restores normal pressure.