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Old 06-01-2013, 08:58 PM   #1
OC_Nooby
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Default Cylinder 4 running lean due to stock FPR/Bigger Injectors?

So I just wanted to get an idea of what happening with my fuel setup.

I have a 2003 WRX with a 2006 WRX SB and 2004 STI heads. I'm using top feed injector setup on a 2003 WRX fuel rail. The injectors are FIC 1100cc that are fully cleaned out and flowing properly.

Now if I haven't lost you yet here is my problem.

I think its a GR stumble or if anything the bigger injectors are really causing havoc.

If my car is just in neutral in my driveway and I start pressing on the gas lightly I can get the car to stumble at around 2k-2.5k. You can hear it stumble and the motor has the ever so slightly hesitation. Once it passes 2500 rpm it runs like butter. So pre 2k and post 2.5k its great! Air to Fuel is around 14.5, fuel pressure is steady (DEFI gauge)

I've tried everything from adding extra fuel line to tuning out the problem with no luck. Yes I've read and read about this stumble etc. Its more present in the newer Subaru's as they've changed the fuel rail setup if I'm correct and people revert back to the 2004-2007 setup. I think my issue is i've gone with bigger injectors.

I've noticed cylinder 4 is running leaner then the rest (via sparkplug @ idle), is it because the fpr regulator can't supply enough pressure to that injector (last in line)? I ordered up a new aeromotoive fuel pressure regulator and will install it once I get it. I think the stock fpr is not good enough and somehow is being over run?

What do you guys have to say about this, any ideas? Is the stock fuel rail setup that compromised with bigger injectors?

I just wanted to make this thread to see what others have found/done. I'll update my results as soon as I get the regulator.
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Old 06-01-2013, 09:16 PM   #2
ogboot
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Did you reference your injector flowchart and place the injectors appropriately?
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Old 06-01-2013, 10:58 PM   #3
SplatWRX
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Are you running serial or parallel fuel rail setup?
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Old 06-01-2013, 11:49 PM   #4
OC_Nooby
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Stock fuel rails so serial, and yes the biggest injector went in cylinder 4. Even though it was only bigger by 2 cc hahahaha.

I can't convert to parallel as my fuel setup has the fpr right on top of the first injector. Unless I cap it off somehow.
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Old 06-02-2013, 05:25 PM   #5
02nismokiller
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I used this write-up and it's super easy!!! http://www.ludicrous-speed.com/wiki/..._fuel_rail_mod
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Old 06-09-2013, 02:21 AM   #6
OC_Nooby
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Looks like I'll be doing the parallel mod and seeing what happens. Changed the FPR and the fuel filter and I can tell the difference. I still have an ever so slight stumble but it's better. Fuel pressure was set to base at 43.5. I think the fuel filter made the biggest difference to be honest. Anywho I'll do the parallel mod and see what that does for me.

I might also try the volt mod for the fuel pump.
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Old 06-09-2013, 11:37 PM   #7
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Keep us updated on how well the parallel mod helps out. I may be doing this mod to help get even distribution to all cylinders.
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Old 06-12-2013, 01:55 AM   #8
OC_Nooby
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I just finished it tonight. Its not really a 100% parallel setup with equal lines (using stock hardlines, no cutting, really not to bad). Couldn't really test it out as my car is stupid loud and its like 1:30am.

Essentially I put two return lines from just under where the turbo inlet sits. One return is from injector 2 and the other is from injector 3. I used the fuel inlet to feed injector 4 (stock setup) and the return inlet which is now another feed to feed injector 1. Where the stock fpr was I bought an adapter and just made a loop. Pretty much what the fpr did before minus a fpr, just a line.

Fed both returns to my aeromotive fpr and from there returned it back to where it usually went. Ohh also the feeds have a Y fitting to split the fuel. I'll test it out tomorrow. Kind of excited.

Car fired up no problems and I don't know if its random but usually the car fires up at 11 a/f and then slowly climbs up to 14.6 ish. This time it fired up at 13.5 a/f which is nice! I'll report back tomorrow.

Ohh, I also did the fuel pump mod yesterday. It didn't help the situation, that's why I decided to try this semi parallel setup.

BTW, Thanks 02nismokiller
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Old 06-12-2013, 05:54 AM   #9
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Good to hear, sounds like ill be doing this soon, just not looking forward to taking off my inlet to get to it.
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Old 06-12-2013, 08:42 AM   #10
Flat 4 Motorsport
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it has nothing to do with how your rails are currently setup.


Its a tuning issue. you can enrich that specific cylinder during tuning so it see's more fuel.


the FPR thats a different story those GR units IMO sucks balls

check out my GROUP BUY and see what I have to offer, but ultimately its going to come down to getting it tuned out properly.

Get a hold of Junior if he's local to you, of find a competent tuner not some bakyard beefcake thats going to charge you $200 to blow up your car.

-Phil @ F4M
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Old 06-12-2013, 11:36 AM   #11
Z0rr0
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Beefcake!!!

Phil is right. Get a pro tune.

Don't change your rail feed from series... there's nothing wrong with series.
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Old 06-12-2013, 11:39 AM   #12
OC_Nooby
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Quote:
Originally Posted by Flat 4 Motorsport View Post
it has nothing to do with how your rails are currently setup.


Its a tuning issue. you can enrich that specific cylinder during tuning so it see's more fuel.


the FPR thats a different story those GR units IMO sucks balls

check out my GROUP BUY and see what I have to offer, but ultimately its going to come down to getting it tuned out properly.

Get a hold of Junior if he's local to you, of find a competent tuner not some bakyard beefcake thats going to charge you $200 to blow up your car.

-Phil @ F4M
No good tuner is local to me.

The parallel mod didn't do crap. I was always reading around that the issue was mechanical. That's why I tried all those fixes first. I never saw anything in the tune that would suggest only one point of the tune to run worse.
Now that I've tried everything possible when it comes to the damn car I can have a tuner look at it if need be. That way I'm not wasting money on something that say is sensor related or is mechanical.

Things i've done:

Added extra fuel line to the feed of the rail (5 feet)
Added an extra fuel damper to the feed
Fuel Pump Mod (battery voltage/relay)
Parallel Fuel Mod with Aeromotive AFPR

Now i'll data log in my driveway (I can feel the stumble) and see what can be done.

Last edited by OC_Nooby; 06-12-2013 at 11:48 AM.
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Old 06-12-2013, 02:47 PM   #13
Boostedbob
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I did the 3' fuel line mod due to the recommendation of my old tuner as I was getting that nasty stumble and hesitation. He got it to smooth out quite a bit.

My new tuner got the stumble out entirely the car is running smooth as butter now. My case may not be the same as yours but a good tuner will defnitly help a lot.
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Old 06-13-2013, 10:39 PM   #14
OC_Nooby
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One nice thing about the parallel mod was that cylinder 4 is running a bit richer. Before the tip on cylinder 4 sparkplug was white. Whiter then the rest. Now its nice and brown! So its one plus for the parallel mod.

I just want to say that the hesitation is something someone wouldn't notice. Its gotten better driving the car but just free revving it you still notice it being ever so slightly rough.

I'll be fudging with the tune to see what can be done. I'm still not giving up. I feel I'm close lol.
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