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Need advice on diagnosing fuel issues (Misfires, A/F, OL/CL, and fuel pump duty)

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Car Details

06 GT Spec B 5MT with 136k miles. AP tuned by Mike at Innovative for 18 psi, relevant mods are:

  • IHI VF34 Turbo
  • Intake Manifold from 2008 WRX STI
  • Intercooler from 2008 WRX STI
  • Phenolic Spacer for Intake Manifold
  • K&N Typhoon Intake
  • GMS Silicone Turbo Inlet
  • Grimmspeed TGV Deletes
  • Grimmspeed Crosspipe
  • Grimmspeed reworked stock header
  • Xo2 mid/y pipe
  • Nameless Axleback Exhaust
  • Grimmspeed Uppipe
  • Cobb catted Downpipe
  • Grimmspeed Electronic Boost Control Solenoid
  • DeatschWerks 740cc Side Feed Injectors
  • Walbro 255lph Fuel Pump
  • Subaru WRX Single Mass Flywheel


Story So Far

Started getting a misfire in cylinder 2 (P0302) at idle. Doesn't seem to happen all the time, some drives have little to no roughness registered, sometimes I can watched the count rack up at every stop. Engine runs a bit sluggish at low load, hesitating and stumbling, power can feel inconsistent. Mid-high load, and open throttle, the engine performs great, no inconsistency and no misfire (some knowledgeable members already know where to look...)


We tackled all the basic diagnostic steps. New plugs, new coil, cleaned air filter, leakdown test (all good), swapped injectors. No improvement with the maintenance. I had replaced the MAF and front O2 sensor about 4 or 5 months before this started.


Over time, the rest of the cylinders have began registering misfires as well. Cylinder 2 still gets the most, but I now have codes for cylinders 1, 2, and 3.


Since I owned the car, it was always leaning the fuel trim. A/F Learning is often between -7 and -14.7, A/F ratio's looked ok, so we figured it wasn't a problem at the time. MPG has been terrible this entire time, and is getting a bit worse lately, around 15-17 mpg.


I've been just dealing with the current situation for a few months, and I've been trying to put a finger on what is causing the issues. My mechanic was thinking it might be the DW740 injectors, and that replacing or switching back to the OEM injectors would be a good step.


Current Thoughts and Observations

What I've noticed recently is that my power inconsistency issues are happening during the switch from closed loop to open loop fuel control.


Stutter when shifting - goes to open loop while rpms are decenting, will switch to closed when I start applying low throttle engaging clutch (sometimes flips open the closed again). Since my A/F trim is doing so much work, switching between open/closed has a lot of adjustments.


Low-mid load hesitation + inconsistent power - closed loop low throttle, not that bad but still inconsistent (FP duty 33%), then ECU goes to open loop while still at 33% FP duty. Engine runs terribly at this point until I give it enough throttle for the fuel pump duty to go up to 66%, then it runs great!


I realize a lot of this relies on the ECU tuning, but this problem has been getting worse, so I think there's a mechanical issue.


All my sensors seem to be working correctly, AVCS and OCV look fine as well. No one has found a vacuum leak, and we've tried.



All my engine problems are happening while the fuel pump duty is at 33%. Misfires during idle, as well as performance while driving. This is at it's worst when I'm in open loop while FP duty is 33%, but that is usually only for a second or two. Everything seems to run fine when fuel pump duty is 66% or 100%.


What would you test or replace first? Fuel pump, fuel pressure regulator, or injectors?


If there was a leak somewhere, would you be able to see it in the AP's manifold absolute pressure monitor?


I've heard Walbro 255's might have trouble with low voltage, any way to confirm this for my situation?


Should the ECU be going in to open loop when decelerating (no throttle) while in gear? I'm guessing this is something in the tune, but it seems to cause issues when going back on the throttle, as my closed loop adjustments are very different than the open loop trim.



I've been reading a lot here on some of the issues with the Legacy FPCM voltage drop (Thanks Scooby2.5 and others), as well as the details on the open/closed loop fueling system (Thanks NSFW and others). Wouldn't have been able to connect all the dots without the valuable work you guys shared.

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I was doing some testing lasting night, will post the logs later.


I was able to get the ECU into a few more scenarios that might give some insight.


closed loop with 66% fuel pump duty - immediate increase in power and smoothness

open loop with 33% fuel pump duty - bogged down and stumbling

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  • 2 months later...



Will be diving into diagnostics soon.


I ordered the alternator voltage sensor fuse upgrade with the diode installed, to see if boosting system voltage a bit will help, if this happens to be caused by the voltage to the fuel pump...

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  • 9 months later...

For anyone else encountering issues like this, we finally fixed it by replacing the fuel pump.


It had gotten so bad that I was getting lean codes and almost stalling at times when at low RPM, but it would run alright once I was at 66% or 100% fuel pump duty.


Our best guess is the internal wiring of the pump, from the connectors to the motor, were defective.

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