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Old 08-03-2018, 01:24 PM   #1
Scooby Newbie
Member#: 34226
Join Date: Mar 2003
Default High fuel pressure on startup

Recently I installed a fuel pressure gauge and sensor in my fuel rail.

After the install, I noticed that everytime I started the engine, there would be a loud droning noise coming from the fuel pump. At the same time, the fuel pressure reading will be relatively high at 5 bar.

After about 25-30 seconds, the sound will cease and the fuel pressure reading will drop to 3.5 bar. It generally does not change much thereafter (unless I get into boost)

Video below for reference:

Anyone have any ideas or clues as to what could be causing this? Thanks!
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Old 08-03-2018, 02:35 PM   #2
Scooby Specialist
Member#: 163648
Join Date: Nov 2007
Chapter/Region: MWSOC
Location: Minnesota, mpls
2002 Resurrected WRX


Fuel pump controller runs at 100% on startup and then after a short period of time goes to 33%.
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Old 08-03-2018, 08:48 PM   #3
Scooby Guru
Member#: 32669
Join Date: Feb 2003
Chapter/Region: SCIC
Location: Woodland Hills
2007 2.34LR, EFR7670
LINK G4+ hybrid STi


That's normal. At key on the fuel pump runs at full speed for 3 seconds to build pressure. It's not until the car is started that the FPC controls pump speed from 33%=>66%=>100%
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Old 08-03-2018, 09:23 PM   #4
Scooby Specialist
Member#: 298403
Join Date: Oct 2011
Chapter/Region: W. Canada
Location: Alberta, Canada
2008 WRX Premium
Spark Silver


What was said above as per fuel pump controller is accurate.

What I'm not sure is normal for the STi is your gauge reading. When you first cycle the key on but not starting, your fuel pump should run for a few seconds. Your fuel pressure will come up to the static pressure, which your gauge should read, ~43.5 psi/~3 bar. Then when the fuel pump stops priming, the pressure should drop slightly. When you start it, the gauge should read your static pressure less vacuum, which is usually around negative 5-8 psi/0.35-0.55 bar (aka vacuum). When you drive around and your a 0 vacuum and 0 pressure, the gauge should read static pressure. When under boost, your gauge should read static pressure plus boost (ie, if you only boosted to 7psi/0.5 bar, your fuel pressure gauge should read ~50.5psi/3.5bar). Your regulator, which is (should be) 1:1 rising rate, will adjust fuel pressure based on your boost and vacuum.

Either your sensor or gauge is doing some kind of calibration or something is a miss. I can't explain the noise, unless you have a noisy pump??? Is the boost reference to the fuel pressure regulator cracked or old? I honestly can't explain the noise you're experiencing.

What fuel pressure are you running (ie, stock)? Do you have an adjustable fuel pressure regulator?
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Old 08-05-2018, 05:36 PM   #5
Scooby Specialist
Member#: 198053
Join Date: Dec 2008
Chapter/Region: Tri-State
Location: CT
02 WRX Wagon


Larger pump, stock regulator? Probably overrunning the stock regulator on startup when the FP duty is 100% if that is the case
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