Aracheon

It's in the same place on the sedan, in the trunk, driver's side.

Mike, what are you using for fuel pump control?

They don't last forever, evidently. Since it's a cheap and easy replacement, slapping another FPCM in there would be what I'd try next. (Assuming you don't want to plunk down a few Benjamins to replace it with an aftermarket unit.)

The voltage drop across the OEM unit was around 1.5v. It was around 1.0v on the first STi unit I bought. It was 0.1v on the second unit I bought. The second unit completely eliminated my fuel pressure problem, as the pump was finally seeing the current it needed to keep up.

Specifically, the 04 STi FPCM. The internal wiring is nearly 2x larger. I noticed significantly less voltage drop across the STi unit. This is all detailed in earlier posts in the thread, if you care to read it.

Logged in the sense that I watched my fuel pressure gauge rise and then fall as I was doing pulls, yes. I believe I detailed this in an earlier post in this thread. This. +1,000,000,000. eBay is where I bought both of mine... around $40 a pop.

Isn't this issue more prevalent with the 5MT guys? Sent from my SCH-I535 using Tapatalk 2

RTV... is there anything that stuff CAN'T do? I've been meaning to pull the rear door apart and check my wiring as a preventative measure.

Yep! Turned out that the first FPCM I bought was bunk. Bought a second replacement and the problems suddenly went away. Sent from my ADR6400L using Tapatalk

Thank you.

At least it was an easy fix! Sent from my ADR6400L using Tapatalk

What was the outcome?

Where did you get that information from? (I'm not arguing or saying it's wrong, but I don't remember seeing that anywhere in the vacation pics.)

Yeah, I understand the manufacturing and cost logic behind it, but that certainly doesn't make it any less shitty. I used these ones: http://www.connectorsupply.com/catalog/butt-splice-heat-shrink-step-down-16-14-to-12-10-ga-p-480.html I think I paid $53 for the harness locally with tax. So $49 is in the ballpark.

Yeah, I just couldn't deal with it looking like shit. Yep - all 6 wires were spliced into the factory harness. Also yes, that part number is for the complete fuel harness, but it won't work with the Legacy in its entirety. I tried. It doesn't seem to line up. One obvious discrepancy is that the length of wiring from the grommet to the fuel pump plug is WAY too short to work on our car.

I was kicking myself in the ass for messing with the FPCM -> FP wiring in the first place after I started having all the problems, thinking it was something I had screwed up. Even if you look at the Impreza plug wiring, it's still quite a bit smaller than the overkill 10awg wire I'm using. It's probably closer to 14-16. If you can snag that plug out of a junkyard, it would probably be even cheaper than ordering an entire harness from Subaru - but it makes the wiring literally plug and play if you get some step-down butt splices. It's just my luck that I've had this much trouble with the car - it's been a nightmare since day 1. If this had been my first experience with a Subaru, it definitely would have soured me on the brand.

During testing I just shoved the wires into the vacancies left by removing the pins from the plug. My first iteration of soldering the 10awg wire directly to the pin didn't work as well as I'd hoped, as the pin holes barely accommodate the wire, much less the wire + the pin. Plus I wasn't very happy with how flimsy everything felt. The final product is the 08-12 Impreza fuel harness. The FP power wires are much thicker than they are on the '05 LGT harness, so I clipped the Impreza plug off about 6" away, and spliced into the thicker wires.

With the newest module in place and the FPCM drawing power from the factory fuel pump relay, the car starts to lose fuel pressure around 15psi and AFRs begin to rise. Drawing power from the Aeromotive relay, fuel pressure rises and holds all the way to 19psi and the car runs like a champ. So I think we can rest easy in that the first replacement FPCM I ordered was bunk. FWIW: Whilst the harness is completely different and won't fit, the 2008-2012 Imprezas (WRX and STi included) use the same fuel pump basket that the 2005-2009 Legacys do. That being said, the PLUG on the Impreza fuel harness is the same as ours. The reason I bring this up, is because I actually wore out one of the retention pins in my factory fuel pump plug removing and plugging in different pins and test wires. I intended to replace the plug in it's entirety, but wasn't able to find a part number for the Legacy rear fuel harness. But I was able to source the rear fuel cord/harness for a 2012 Impreza for fairly cheap, which not only provided me with a replacement fuel pump plug and a few inches of wiring, but also a plethora of replacement PINS. That harness has a bunch of other plugs on it with pins that are the same size as the ones in the fuel pump plug. I've not been able to find a reliable source for the larger crimp pins that Subaru uses in the various plugs and harnesses around the car (except the ECU pins, which IA Performance sells). Through my exercise of soldering 10awg wire to the FPCM harness, I realized it would be incredibly handy to have a few extra crimps pins on hand as spares. The part number for the Impreza fuel harness is 81803FG010, in case anyone ever finds themselves in the same situation I was in. With the new plug and wiring in place, everything sealed up, the car is running like a champ. I plan to take the car in to a friend's shop next week to hook the FPCM up to a lab scope, as my inner geek doesn't like to just let shit be. I want to have concrete and tangible evidence that it's working like it's supposed to, and not just rely on my butt dyno, as it were. Lesson learned: when acquiring junkyard parts, especially electronics, never assume that they work. Scooby 2.5, Coldsubycz, Integroid, and everyone else who has contributed to this thread, thank you for your help. I'd probably still be banging my head against the wall trying to figure out a fuel pressure problem if it wasn't for your input, time, and research.

That appears to be a nicer unit than the one I'm using. Ah - I misunderstood what you were saying. I'll give that a try tomorrow and report back with the results.

Returning the wiring to stock would be more work than just taking it to a shop and convincing a tech to hook an oscilloscope up to the FPCM to verify the output signal is what it should be. (That being the only thing I haven't been able to accurately test.) Speaking of, Coldsuby, what multimeter did you use for your testing?

I'll grab one tomorrow when I head out to the shop. But in the meantime, this is how the wires are routed: (Pink = power from battery. Yellow = signal from fuel pump relay. Turquoise = power from new relay to FPCM. Orange = new relay ground. Green = FPCM ground.) http://i.imgur.com/NZBrR.jpg

Ignore the messy wiring, haven't had a chance to clean it up or label anything yet. From left to right, Working STi FPCM (2nd replacement), bad STi FPCM, bad stock FPCM: Backside of Aeromotive (Tyco) relay: Top of Aeromotive (Tyco) relay: Backside of FPCM harness:

Surely. I'll snap a couple tomorrow morning.

Update: I ordered another FPCM from an '04 STi which showed up today. Plugged it in, and the voltage behavior I was seeing from before still exists. However - the car was holding fuel pressure at idle without a problem. Took it out for a drive, voltage at the pump didn't change by more than .02v from 33% duty cycle to 100%, but the fuel pressure rose in accordance with boost and the car runs exactly the same as it did when I had the pump wired directly to the Aeromotive relay. I guess I had two bad FPCMs. What I find strange though, is I was seeing the same voltage readings that everyone else was seeing prior to wiring in the Aeromotive relay, and now both of my multimeters (different brands, different ages) are reading the same. Makes me wish lab scopes were cheaper... Because of how much plugging in / unplugging I've done with the fuel pump plug, I also have a replacement rear fuel harness on the way, which I will be rewiring into the car so that the wires and plugs are all clean, fresh, and unscathed.

Exact same issue with the Hella relay, grounding both the FPCM and the relay to the same spot using new wiring. (Tested separately the grounding point at the back of the car, as well as using my 8awg test cable that's hooked directly to the battery.) Results are the same with both modules, except that the voltages on the LGT module are a little lower. Sent from my ADR6400L using Tapatalk