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This is a discussion on This Normal? within the Tuning: Electronic Engine Management forums, part of the Tech & Modifying & General Repairs category; I think that I'll first try the water/soap version to see if the vacuum sucks it in and makes a ...

  1. #16
    Registered User Obeisance's Avatar
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    I think that I'll first try the water/soap version to see if the vacuum sucks it in and makes a noise.

    Also, as a second check, I drove for another 30-40 mins and there was another small change in the fuel learning value (added to above link) in learning view.

    Before maf cleaning, after a 30 min drive, then another 40 min drive.
    A/F Learning #1:
    0 - <5.60 5.60 - <10.00 10.00 - <50.00 50.00+
    11.67 4.49 3.22 -2.15
    11.47 3.81 2.54 -2.25
    10.89 4.3 1.66 -2.25

    Perhaps it is slowly coming down?

    Edit: later checking shows that there is no slow reduction of the fuel learning
    Last edited by Obeisance; 02-22-2014 at 03:22 PM.
    David - '05 WRX

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  3. #17
    Registered User Obeisance's Avatar
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    Alright, I have now checked for vacuum leaks with soapy water, but saw and heard nothing out of the ordinary. I am viewing this problem as a "perceived problem" since I would not have recognized it without trying to datalog.

    I wonder if fuel composition changes (compared to the injector calibration) could cause my issues. For instance, I've read that winter fuel has added butane, and I'm pretty sure that my 93 octane fuel has 10-15% ethanol. These additives have different densities, and I assume that pressurized fuel is added with a simple pulse width modulation of injectors, so the different density (and possibly compressibility) of the additives should change the actual amount of fuel added at a given PWM (compared to what the injectors were calibrated for). This may lead to a difference between the amount of fuel needed to maintain the 14.7 A/F ratio read by the sensor and the amount of fuel predicted to be needed by the MAF/injector calibrations (and compensated for by the A/F learning). Does what I'm thinking make sense to you all?

    I do not expect that I have an intake leak post turbo, since my car is able to make full boost, and uses 90+% duty cycle on the wastegate (I think if I had a post-turbo leak, that I couldn't build boost easily, or the wastegate would stay closed for more time to help keep the boost up). Maybe there is a tube leaking somewhere, but soapy water didn't reveal anything.

    For now, since I do not have driveability issues, I will continue to monitor the A/F learning periodically without paying someone to diagnose my car.

    My A/F learning is now over 12%
    Last edited by Obeisance; 02-22-2014 at 03:24 PM.
    David - '05 WRX

  4. #18
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    Quote Originally Posted by Obeisance View Post
    Alright, I have now checked for vacuum leaks with soapy water, but saw and heard nothing. I am viewing this problem as a perceived problem since I would not have recognized it without trying to datalog. I wonder if fuel composition changes could cause my issues. For instance, I think that winter fuel has added butane, and I'm pretty sure that my 93 octane fuel has 10-15% ethanol.
    I am fairly certain law requires no more than 7% ethanol or so. Never heard of butane in auto fuel.... I have a good friend that works for one of the largest if not the largest gas and oil companies on the east coast and they don't use butane, although there are certain amounts of diesel and other oil based fuels that are allowed in very very negligible levels.

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  5. #19
    Registered User Obeisance's Avatar
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    Here's what I've found for now:
    1) Michigan fuel commonly includes a fair amount of ethanol (I took this photo at my last fill up)


    2) I found various studies about how the addition of ethanol to fuel influences fuel learning, and liked the plots out of this one:
    http://www.crcao.org/news/Mid%20Leve...G%20050510.pdf

    A short version of my interpretation is that the inclusion of ethanol into fuel does increase the fuel learning. However, if this were a common problem in subaru wrx's, someone else on the forum would have noticed, right? I'm not in a trouble range of the LTFT's yet, but I do worry that my stock car reaches 90+% injector duty cycle on a hard pull. Again, I'll continue to monitor my fuel learning and see if things worsen.
    David - '05 WRX

  6. #20
    Registered User Obeisance's Avatar
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    Well, I've been watching my fuel learning, and it hasn't gotten better as temperatures have increased and as fuel has (I presume) switched away from the winter blend. My fuel economy, however, has returned back to summer-like levels of 25-27 mpg. I do not expect that the moderate fuel learning impacts the fuel economy, though.

    I have noticed another symptom of a possible vacuum leak: when I lift off of the throttle at cruising speed and rest my foot over/on the brake pedal, I can feel a delay in the time from when I let off the gas to when the brake boost begins to build (I can feel the pedal soften and pull away after a delay)- this has two implications: 1) it is potential evidence for a vacuum leak; 2) it is potential evidence for the danger of a vacuum leak beyond engine damage (if I run out of injector overhead at high load). When I have suddenly needed to brake, I feel like my pedal was far too hard and my brake response was too weak for how hard I was pushing.

    Thus, I'm planning on performing a better leak test, as has been suggested to me, next weekend. Here's the plan:
    follow instructions from some other threads to plug off some lines:
    Another DIY Boost Leak Tester - Home Depot and AZ parts - NASIOC
    Just completed the DIY "find your leak" tool. - NASIOC
    Boost leak test instructions

    Their instructions for making the tester mislead me: I tried to purchase 2" PVC components to attach pre-turbo, but these were the wrong size. The inlet after the post-MAF elbow has an outer diameter around 2 7/8" - so I purchased a 3" PVC cap (larger than 3" in both diameters), a 4" to 3" rubber downspout adapter, a couple of worm gear clamps, and a threaded valve stem.



    For the test, I'll use the pressure regulator on an air compressor to pressurize up to about 15 psi and I'll monitor pressure leaking vs time with romraider. I plan to spray my soapy water on parts again, and hope to find a leak.

    My end goal is to repair the leak and observe better brake boost response and reduced fuel learning at low engine load/engine air flow.
    David - '05 WRX

  7. #21
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    Sub'd for later.

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