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Old 03-16-2021, 06:22 PM   #27
jimmy12345678
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Member#: 516695
Join Date: Jul 2020
Vehicle:
2008 Impreza 2.5i

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Quote:
Originally Posted by Elbert Bass View Post
So now you have a lean code? I don't see mention of that before - is this something new? So it is possible it is spark knock from a very lean condition. I had mentioned checking fuel trims before. Your lack of response on that led me to believe they were acceptable.
Did you get freeze frame data from when the P0171 set? are both long and short term trims high positive numbers?

Yes, fuel pump is a possibility. You will need to check fuel pressure to verify. It may not be evident at idle and only show low pressure during the rattle under load. In fact, I have seen water in fuel filters cause that. Fuel flows fine under normal load but as soon as you increase demand for fuel flow (accel up hill or heavy load) the water in the filter media will only let's so much fuel through and pressure drops. The amount of water won't be much, could be accumulation of moisture over the years from condensation in the tank. It doesn't take much to soak the filter and block fuel flow.
That is the drawback with a single ended fuel supply and a "lifetime" filter in the the tank on the pump assembly.

The P0171 (which went from "pending" to "current" on my test drive tonight) is a new code for this vehicle. I observed tonight that when the low power/surging conditions happen (along with the rattling) that my fuel trim numbers jump all over the place, from -28 to 28, the car also goes from closed to open loop at times while this occurs, though not always. I didn't have my Autel scan tool at home tonight, only my old Innova 3140 that reads live data, so I'll try and get some data PID pics of when this is occurring. I can also feel a slight miss both at idle and while driving when it surges, it's had pending P0303 codes before, but not enough to set an active code. Have access to an oscilloscope too, so see about getting some secondary ignition waveforms to put up.

I did check fuel pressure with my Actron gauge, teeing into the fuel lines just past the regulator, and I had a steady 50-52 psi running pressure (spec is 49-50, I figure that's within range) even when the low power condition occurs. I also verified fuel volume delivery by jumping the fuel pump relay on the passenger side under the dash and I got roughly 9 quarts in 1 minute, well within the spec, so fuel pressure/delivery isn't my issue.

Thought I was onto something.....
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Last edited by jimmy12345678; 03-16-2021 at 07:21 PM.
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