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My factory service manual states fuel pressure should be 50 psi with the engine running at idle. This is a six cylinder 2008 Subaru 3.0R engine with a return-less fuel line to the injectors.

Connected up a parts store fuel pressure gauge that shows pressure is on spec with the engine idling. However it drops 10 psi to 40psi after the engine is off for three hours. Overnight pressure drops to 24 psi.

What is acceptable fuel rail pressure drop with the engine off please ? Also wondering where the fuel goes to decrease the pressure as there's no gas smell unless it's working it's way back to the tank or thru the injectors.

EDIT: Thanks for the responses. I carefully clamped the incoming fuel pipe from the pump using vise grips with rubber hoses over the jaws to re-run the test. Fuel pressure now holds much higher for longer so guessing that the check valve or in tank fuel pressure regulator is getting tired.

Fuel line clamped for fuel pressure test

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Every manufacturer has it's own rates of residual pressure. A general drop in pressure would be about a pound a minute dropping no more than 10 pounds. Usually a bad fuel pump or leaking lines will drop to 0 in a matter of seconds. Leaking fuel injector(s) will drop closer to 0 a little slower.

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    I would add that leaking injectors make for hard starting and running poorly when first starting due to a cylinder (or multiples) being flooded with fuel. You'd also get a lot of fuel past the rings and into the oil, which you'd be able to smell on the dipstick. Fuel pumps have check valves in them to keep pressure in the lines after shutdown.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Mar 3, 2021 at 14:54
  • The Subaru factory service manuals do not detail rates of residual pressure
    – ajayel
    Commented Mar 30, 2021 at 4:26

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