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How To: Install 04-07 STI Fuel Pressure Regulator (FPR) and Use a New Vacuum Source

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Taking this out of my build thread and posting it here for easier find-ability.


Legacy's run a fairly complex fuel pressure regulator setup, this setup has a lot of cross talk between the feed and return fuel lines. It's believed the stock FPR assembly is the reason for a lot of the infamous stumbles. Newer STI's (2008+) have a similar FPR setup and have been known to stumble a lot more then same year WRX's that have a traditional FPR setup. It also seems like aftermarket fuel pumps really don't like the stock complex FPR assembly.


Parts list:

04-07 STI FPR - 22670AA351 - $100 new, I got mine for $20 used on the forums

5/16" Fuel Injection hose - $8/foot

(OPTIONAL) 4 x 5/16" Universal Connectors - Doorman 800-120 - $8/each (Only needed if you don't want to cut up stock lines)

5/16" Fuel Hose connectors - Can use left over stock.


Now not all four universal connectors are required, you can probably get away with just two since stock hoses wont be long enough.


New FPR Vacuum Source

Reading through the "The famous stumble...fixed?" someone mentioned that Cobb recently wrote an article about relocating the FPR's vacuum line.


08+ STI's, like 05+ LGT's, switched their FPR's vacuum line source to Cylinder 4 from the plenum. Cobb found that this created a lot of oscillation in the fuel pressures causing fueling issues and adding onto the infamous stumbles. Cobb suggests switching the FPR feed to the BOV feed since it gets it's source from the plenum, this should be much better then stock.


Cobb did some awesome data logging a bunch of runs and came up with these graphs for fuel pressures

Stock FPR vacuum feed from the 4th cylinder:



After plumbing into BOV line, as you can see it's a HUGE improvement already.




Now, I do have some concerns against using the BOV feed line, for one it's already being teed into with the infamous barbless blue tee (that you should really replace with the Turkeylord one especially if you do this mod). Second concern is length based, BOV line on it's own is about 19", adding the FPR tee would add another 4-5", giving us a line that's about 24" long. Why is this a concern? Resonance, In the Cobb post they posted a

that talks about the importance of having short FPR vacuum lines and resonance that long lines cause in the vacuum lines. By keeping the lines short the resonance is still there, just above our redline so it doesn't matter, with the line being 24" long that could push the resonance to be at our peak power range, which wouldn't be good.


Unfortunately I can't find a better source, and plastic manifolds don't let me easily tap drill and tap into them. Thus I decided to still give this a go using the BOV line.



BOV on the left and FPR is hidden on the right:



I was hesitant to cut an oem hose, but to keep the lines short did it anyway. Turkeylord's tees to the rescue yet again!



Attached a new vacuum hose (I think it's 5/32") and gave it enough length not to kink, I will probably shorten this line once I switch to different FPR.



For now capped off the feed from cyl 4, I will use this for my boost gauge eventually though. Just a PSA, these caps don't last long at all, they are only good for temp fixes since they crack and leak overtime.



If you do this mod you want to make sure you replace the oem blue barbless tee with a turkeylord one too (or at least put clamps/zipties on it), otherwise if it slips off your FPR will not see positive pressure and send you into lean city. I had my blue tee clamped, but still replaced it.





For now the car seems to stumble slightly less, but it's still not as good as I would like for it to be, this is probably due to the 05 styled fuel pressure regulator that I will be replacing with 04-07 STI in next step.




04-07 STI FPR Retrofit

The stock FPR assembly taps into the feed and return lines. 04-07 STI FPR will fit perfectly into the return line, but that will leave the feed line needing an barb coupler, longer hose, or an inline fuel pressure gauge.


Pulled the fuel pump fuse and started the car. Even though the car would eventually die there was still 20psi in the lines. Keep a few towels handy to collect the fuel spills.



Removed the stock FPR and all of the brackets associated with it



All of the crap on the left replaced with the little thing on the right... This is a prime example of Subaru over-engineering themselves where simplicity is best. Now this is interesting, the U hose goes from the FPR to what looks like a damper. This seems to be done to reduce hampering noise that some people get. If you get this noise after STI FPR install, you might need to add a damper from older WRX's post the FPR.



STI FPR's vacuum nipple faces the strut tower when installed, this would require a long FPR hose (trying to keep it as short as possible for resonance reasons). FPR head is clamped onto the FPR, but wasn't permanently attached, thus I could carefully rotate it with some pliers.



FPR vacuum feed attached to the BOV line, I kept this line as short as possible to reduce resonance.



With the return line now done, you need to now either use a 5/16" (8mm) barb coupler, or you could do an inline fuel pressure regulator. Here is a shot of a friend's car who just did it exactly like that



I personally have my fuel filter in this location since I bypassed the in-tank fuel filter.



This also means I had less room for fuel pressure gauge, thus I put it on the return line. Here is what my finished setup look like:



For comparison sake, here is what my stock setup looked like:



Here is what my friends setup looks like with STI FPR and a fuel pressure gauge. Notice that the feed line is a little short, I think doing a longer fuel line from the tank to the gauge might be a good idea.



Started up the car, disconnected the vacuum to see how much vacuum she was running, and to my surprise it's running 46psi instead of 43. From what I remember stock setup was barely pushing 42psi at idle with FPR disconnected.



FPR Vacuum_001.jpg

FPR Vacuum_002.jpg

FPR Vacuum_003.jpg

FPR Vacuum_004.jpg

















05 LGT 16G 14psi 290whp/30mpg (SOLD)

12 OBP Stock 130whp/27mpg@87 Oct

00 G20t GT28r 10psi 250whp/36mpg

22 Ascent STOCK

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Uploaded SBT's PDF export of the first post here (ran out of file upload amount in first post :lol:)

LegacyGT. com coverrussian's How to Install an STI FPR & Relocate the Vacuum Source on a 4th Gen.pdf

Edited by covertrussian
  • Like 1

05 LGT 16G 14psi 290whp/30mpg (SOLD)

12 OBP Stock 130whp/27mpg@87 Oct

00 G20t GT28r 10psi 250whp/36mpg

22 Ascent STOCK

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Nicely done and thank you for adding to our body of knowledge and how-tos.


Much appreciated and keep up the great input.


FYI: PDF'd your how-to and attached it to your OP. Full author attribution and link-back to this page is included.


Thanks again.



- Pro amore Dei et patriam et populum -
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  • 1 month later...

A local buddy of mine just did this mod on his stock legacy. Adding his pictures to the first post, rewording the writeup to be more clear ditching some redundant pics.


Pics I took of his setup:



05 LGT 16G 14psi 290whp/30mpg (SOLD)

12 OBP Stock 130whp/27mpg@87 Oct

00 G20t GT28r 10psi 250whp/36mpg

22 Ascent STOCK

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  • 3 months later...

I personally didn't, but I have an in engine bay fuel filter and fuel pressure gauge, both work as anti-hampering devices, which could have masked the issues with stock assemblies.


I mainly like it because it reduced complexity of the system and reduced space usage too!

05 LGT 16G 14psi 290whp/30mpg (SOLD)

12 OBP Stock 130whp/27mpg@87 Oct

00 G20t GT28r 10psi 250whp/36mpg

22 Ascent STOCK

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  • 1 year later...
  • 1 year later...

Gauge: Any name brand gauge will work. I don't know that I would buy another summit one though, it turned brown (fuel leaking into it).


Connectors: I didn't mind cutting up up stock fuel lines AND bend the hard line, a lot of people do, so the connectors are need to avoid that.

05 LGT 16G 14psi 290whp/30mpg (SOLD)

12 OBP Stock 130whp/27mpg@87 Oct

00 G20t GT28r 10psi 250whp/36mpg

22 Ascent STOCK

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does the sti fpr bolt to anything or does is it supported by the fuel lines? Doesn't look like it is, if not I will cut off the mounting bracket. And you twisted it about 20 or so degrees shy of 180?
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see my pictures, the spot to the left of the brake reservoir with the other yellow cap is where my clutch reservoir is, for autos its not there and is just a plug. Its between the heater core hoses and the brake reservoir, about the same height give or take several inches. Where my clutch reservoir is should be a plug for autos.
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