Fuel Static Fuel Pressure Drop

Discussion in 'Fox 5.0 Mustang Tech' started by chuzie, Oct 22, 2013.

  1. Most of our EFI fuel systems prime to 40psi; mine does too. I have noticed some folks' fuel pressure remains at 40psi long after the key is off for hours or even days.

    My system drops to 20psi after sitting for 5 minutes. I thought this may be an indication of a failing fuel pressure regulator so I replaced it yesterday. Unfortunately, the result was the same. It was not until I clamped my fuel line from the HP pump to the fuel rail did the pressure cease to drop.

    My pressures with the engine running are fine and without issue.

    Does this static pressure drop mean anything? Should I care?

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  2. Usually from a bad check valve in the pump if it has one. Not all setups hold fuel pressure. Its really not an issue unless its the regulator or fuel injectors causing the leak down. Very few occasions the S-hose on the stock fuel pump assembly can leak and cause the same issue. By clamping the supply line before the rail, youve eliminated the regulator and injectors as being the cause.
  3. Thx bro

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  4. You actually mean, residual pressure, not static pressure.

    I have a fuel pressure gauge permanently attached to the rail. Also have had different pumps & different FPR's. Some fuel pumps have a check valve, some fuel pressure regulators have a check valve.

    If there is no check valve, the pressure will drop to 0 psi, when fuel pump is shut off.

    Not to worry.
  5. I would understand 40psi or 0psi, but drop half way to 20psi? Just weird.

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  6. Mine did the same with the old fuel pump. The check valves allow a bit of leakage, so you will see a drop in pressure. Large at first, then slower later. What you see is "common".

    It may be feature of standard pumps, but my high pressure, high flow 255 doesn't act like any of them. Pressure is nearly instantaneous after power-on & vacuum compensation is minimal. The same design of pump has increased greatly in output over the years.

    With the old Mallory FPR, it goes to 0-psi immediately, with the Kirban, it does what you see, initial drop, then slow. I've had both FPR's apart, the Kirban has a small spring on the diaphram to close the orifice. The seat is metal & the plunger is metal, not a good seal. Don't know why they go to the trouble, never asked them. It was originally designed for the Buick Grand National, turbo V6.

    Have had this gauge kit for many years, still better than most.
  7. Great information. Thx

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