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P0171 continues intermittently

Discussion in 'Gen 3 Prius Technical Discussion' started by donzoh1, Mar 16, 2023.

  1. donzoh1

    donzoh1 Active Member

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    I cleaned the MAF sensor thinking it might solve the issue but no deal. The LTFT is usually in the high teens but can range from 10% to 30%. MPG is between 40 and 45. (After cleaning the MAF sensor, I didn't disconnect the 12v battery. My scanner says the car has an Evap or other emissions issue but no power train codes, other than the occasional P0171. Is there a way to bench test the MAF sensor? I'm wondering whether the lean fuel and high LTFT are related and if so would that be more likely fuel injectors or fuel pressure.
     
  2. ChapmanF

    ChapmanF Senior Member

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    Yes, the high LTFT means exactly that the ECM is trying to increase the amount of fuel injected to reach the correct mixture, and the P0171 means it is not able to do that enough. So this doesn't have to be a situation where a sensor is lying to you; it can be a situation where the sensors are doing their job and the mixture is really too lean.
     
  3. Tombukt2

    Tombukt2 Senior Member

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    Fuel pump volume ?
     
  4. donzoh1

    donzoh1 Active Member

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    In that case, is fuel pressure (either pump problem or clogging) more likely or dirty injectors not producing good mist? I don't know to diagnose that. Could test fuel pressure I guess. One think I've noticed is LTFT jumps around quite a bit. I've seen as low as 6 and as high as 30. Is that the ECM hunting for a solution or more likely a dying fuel pump?
     
  5. ChapmanF

    ChapmanF Senior Member

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    Maybe @mr_guy_mann will add more. I seem to remember reading that the ECM might store different trims for different operating conditions, which could be part of the story.
     
  6. donzoh1

    donzoh1 Active Member

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    Not sure how to test fuel pump volume either. At this point, even though there is an occasional code for lean fuel mixture, the positive long term fuel trim seems to compensate. But LTFT bounces around quite a bit so maybe the fuel issue is in flux. No misfire codes.
     
  7. CR94

    CR94 Senior Member

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    Are you certain there are no vacuum leaks?
     
  8. mr_guy_mann

    mr_guy_mann Senior Member

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    Ok, so...

    A first step would be to monitor fuel trims while driving the car. Short Term Fuel Trim reacts to the Air Fuel sensor reading. If it reads "lean", STFT makes a (+) correction to add fuel. If "rich", it goes (-) to remove fuel. Fuel trim is normally at 0.

    When ST trim goes too far from 0, Long Term FT steps in. Say STFT reaches +10%. Eventually LTFT will increase (to +10%) and STFT will return towards 0. Add STFT to LTFT to get total FT.

    LTFT values are stored in ECM memory (as long as the battery isn't disconnected). There are different values for different combinations of RPM and load. Think of a chart where the rows are RPM and columns are "load" (MAF). At any point on the chart is a LTFT "cell" or value. Change the RPM or load by a certain amount, and you go to a different cell.

    FT is ideally ("like it left the factory") 0 +/- 5%. 10% is "OK". 15% means something's off but it can compensate. 20+% is where you see codes.

    Monitoring what FT is under different conditions (data logging or "capturing a movie" is really useful for this) can aid in finding a diagnostic direction.

    For example, a vacuum leak can cause high + FT at idle (with low MAF readings), but FT will go toward 0 under moderate accel or cruise. Restricted fuel injectors cause the same FT under most conditions. Most weak fuel pumps get worse under heavy accel (and higher + FT).

    Also see what the A/F and O2 sensors are showing. A/F should stay "around" 3.3V at steady throttle or idle. It goes lower towards 2.0V when "rich", and higher when "lean" - up to 5V during fuel cut.

    The O2 reads near 1V when "rich", and near 0V for "lean". At steady ICE operation on a hot engine with a good cat- the O2 usually reads "about" 0.7V, +/-0.1V

    I don't have "known good" data for a Gen3 MAF sensor. Maybe "someone" can post info from their car (Chapman?) that you can compare to.

    I would connect a fuel pressure gauge and (have someone else) watch it while you drive the car. 98+% of the time, IF pressure stays in spec while the engine is under heavy load, then the pump volume "is good enough".

    Posted via the PriusChat mobile app.
     
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  9. donzoh1

    donzoh1 Active Member

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    No, and I've been too lazy to get out the smoke generator to this point. That's the next thing though. There is a check valve or solenoid? below the air filter housing which I don't think I properly installed when putting the engine in. This attaches to a bolt on the egr pipe to intake manifold but is now pretty much lying on its side. I noticed this when installing an oil catch can (which is capturing excess fluids via the PCV line). So, I need to get that bolted up correctly (vertical). That may or may not fix my problem depending on how that item works.
     
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  10. ChapmanF

    ChapmanF Senior Member

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    Just for identification purposes, that's the purge valve for the evaporative emissions system.
     
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  11. donzoh1

    donzoh1 Active Member

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    I finally got my smoke generator out and injected some smoke just past the MAF sensor (after stuffing a rag into the tailpipe and wherever else I thought it could escape. (I did not prop the throttle plate open.) Soon, I saw smoke escaping from the air inlet clamp area at the throttle body. After a few minutes, there was even more smoke coming from the EGR inlet pipe where it joins the intake manifold. When I removed the pipe, I saw where some shade tree mechanic (it wasn't me, I swear!) had put some RTV because the original rubber gasket had been lost (or stolen). I put in a new gasket and fixed the clamp. Amazing. No more P0171 code, and fuel trims are down from 19% to 3% (at last check). It seemed that the engine computer hunted around a bit before setting the new fuel trim levels but not sure on that. Mileage is between 44 and 48 depending on how I drive. I'm thinking new (or refurbished) injectors might be helpful for additional improvement but happy with how this Gen 3 is doing, especially after all it's been through. Still need to get the HV battery cooling fan cleaned up.
     
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  12. ChapmanF

    ChapmanF Senior Member

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    Chalk up another one for careful checks for vacuum leaks. (y)
     
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