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Old 02-16-2019, 08:40 PM   #1
OUScooby
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'04 WRX 307whp278tq
03WRX wgn, '72 MGB

Default Fuel pressure question.

Hey all, had a quick fuel pressure question. What should the fuel pressure be when testing on the FPR line? I had it hooked up between the FPR and the fuel rail.

Here's the backstory:

Last week when driving on the freeway my 03 WRX began hesitating and missing and then shut off entirely. I towed it back home and after troubleshooting a few things went to test the fuel pressure. I hooked up a fuel pressure tester between the FPR and the fuel rain. It was reading 10-12psi.
I had ready that it should read 40psi.

So I swapped out the FPR with the one from my 04 wrx, and it still reads 10 psi. So now I've ruled out the FPR as the problem. Next change the fuel filter, still getting 10psi when tested at the FPR, so that rules out the fuel filter.

OK so next step fuel pump. Fuel pump is getting power, I can hear it priming. So I order a Walbro 255 and just finished installing it, and I'm still getting only 10psi when tested on the FPR.

Just out of curiosity I tested the fuel pressure between the fuel filter and the fuel rail, there the gauge was reading 45psi.
So if I'm getting 40psi on the fuel feed line is it normal to have only 10psi at the FPR?
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Old 02-16-2019, 11:26 PM   #2
TXwrxWagon
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WRB not so minty fresh

Default

I want to know this too...

I am curious how are you testing the PSI? what hook-up method?

obviously Subaru wasn't gracious enough to give us a schrader test port/valve anywhere like almost every other company since 1987 does... ugh!

I just redid my entire underhood fuel system and would like to know more about what "should be"

~Rob
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Old 02-16-2019, 11:45 PM   #3
OUScooby
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03WRX wgn, '72 MGB

Default

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Originally Posted by TXwrxWagon View Post
I want to know this too...

I am curious how are you testing the PSI? what hook-up method?

obviously Subaru wasn't gracious enough to give us a schrader test port/valve anywhere like almost every other company since 1987 does... ugh!

I just redid my entire underhood fuel system and would like to know more about what "should be"

~Rob
I unhooked the line coming into the FPR from the fuel rail and hooked in a "T" with a line going to a fuel pressure tester. The tester was a loaner tool from auto zone. Then I took all that off and hooked in the "T" between the fuel filter and the fuel rail on the fuel in line.
It wasn't difficult just unscrewing/unclipping clamps and pulling off fuel lines.

What I don't understand is how the fuel pressure can be lower before the FPR on the line out than it is on the line in. Doesn't the FPR, you know, REGULATE the fuel pressure, basically by providing back pressure to the fuel lines and pump?
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Old 02-17-2019, 03:37 PM   #4
TXwrxWagon
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WRB not so minty fresh

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wait.. I'm confused... the OE FPR is mounted to the end of the passenger side metal fuel rail. there are no easily accessible rubber fuel lines after the FPR on the rail until it gets back over to the driver's side.

is the fuel system all stock? I'm not understanding where you could "T" in except just after the fuel filter or jusas its coming back to the fuel dampener(S) on the rails over by the filter on the driver's strut area.

I'm just trying to visualize where you "T'ed" in.. and we are talking the fuel line not the vacuum line correct? not insulting you at all just trying to get a clear picture of where you are...

~Rob
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Old 02-17-2019, 03:50 PM   #5
OUScooby
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03WRX wgn, '72 MGB

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Quote:
Originally Posted by TXwrxWagon View Post
wait.. I'm confused... the OE FPR is mounted to the end of the passenger side metal fuel rail. there are no easily accessible rubber fuel lines after the FPR on the rail until it gets back over to the driver's side.

is the fuel system all stock? I'm not understanding where you could "T" in except just after the fuel filter or jusas its coming back to the fuel dampener(S) on the rails over by the filter on the driver's strut area.

I'm just trying to visualize where you "T'ed" in.. and we are talking the fuel line not the vacuum line correct? not insulting you at all just trying to get a clear picture of where you are...

~Rob
Is the FPR not that black "UFO" shaped thing by the fuel filter?

EDIT: My bad, i was testing pressure on the evap line, not the fuel return line. So I changed out everything for nothing and never had a fuel pressure issue, now I'm back to square on finding out what caused the car to break down.

Last edited by OUScooby; 02-17-2019 at 04:41 PM.
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Old 02-17-2019, 10:38 PM   #6
TXwrxWagon
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2004 WRX Wagon
WRB not so minty fresh

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its ok... you are learning and paying attention. Thats a GREAT thing!

the round black devices are what are call fuel dampeners. They are actually on the fuel returm line. To check fuel ressure on a totally stock system I would follow the fuel line from the filter on the driver's strut tower to the intake maybe 18" away.

There you will see three hard lines coming up to a bracket between the runners of the upper intake manifold.

The one from the filter is the FEED into the fuel rails and it is the top hard line that is 5/16" diam.

The next one I think is also 5/16" and that is the return and it goes to a slightly longer hose that leads to the "ufo(s)" on the return line. The third line is odd sized its not 5/16" but its larger than 1/4" that is the EVAP tube. USually doesn't have clamps on it. Some cars I've seen from 02-05 have 1 or 2 dampeners (ufo's). My '04 had 2.

Test coming off the hardline at the intake and before the ufo's that's post FPR. Or should be.

Also if you said you swapped out the FPR off your '04 wrx but were swapping Dampeners it could still be the FPR. The FPR is over on the passenger side behind the P/S pump and under the turbo inlet. And trapped under the "Bracket of Frustration" that kinda supports the upper cooling expansion tank.

~Rob
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