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I have a 2002 Toyota Corolla (1zz-fe engine) and I am getting a P0171 code - lean condition. I am trying to make sense of my fuel trim values and I was hoping someone could help understand these results:

  • I am measuring on Neutral, after the engine warmed up.
  • At idle (about 700 RPM) my LTFT is steady at 31% and my STFT is between 4-6%.
  • At 2,500 RPM, both FTs drop but seem to be doing opposite things: LTFT goes to 22% (remains positive) and STFT drops to -4% - 0% (becomes negative).
  • Going up to 3,200 RPM, the 2 FT are going in opposite directions: LTFT goes back up to 31% and STFT drops to -20%.

I am not understanding why ST and LT seem to be competing (adding / removing fuel), and also why they are going in opposite directions at higher RPM.

If somebody could help, or tell me what to test next it would be really helpful!

Thanks a lotenter image description here

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    Wonderful first question! You include the basic data, what you've tried, and very detailed hard data. You used complete sentences, proper punctuation and formatting. Your questions is perfectly on-topic. You should definitely ask more questions here! – Zach Mierzejewski Nov 20 '16 at 16:52
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    Thanks! I'm sure I'll have more questions coming soon as I just started looking at all that live data! – Florent Nov 20 '16 at 18:24
  • Nice question. Welcome to the site! . +1 – DucatiKiller Nov 20 '16 at 20:26
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Great question, you supplied enough data to give direction and this is a common problem.

This pattern of fuel trim behavior, higher positive at idle than at cruise speeds, is commonly seen when the mass air flow sensor gets dirty. Inspect with a strong light and a minimum of 3x magnification. Look down the small rectangular hole and see if the two small sensors elements are covered with oil and fuzzy looking dirt. If the outside of the MAF sensor appears oily then it should be replaced. This shiny stuff is actually the potting material from inside the sensor leaking out. Carefully cleaning the elements with an purpose made solvent can help but if the potting material is leaking out it will return.

The variation in fuel trim as rpm changes are normal and has no particular significance. The overall trends are what matter. It is best seen by graphing total fuel trim.

The ST being negative at the same as the LT trim being positive just means that this load cell has not adjusted to the new mixture condition. To evaluate the actual trim numbers that the PCM is using add the ST to the LT which results in the total fuel trim.

There are many other possible causes of higher than normal fuel trims. But the MAF would be my first test. If the MAF turns out to be OK then test for intake leaks, low fuel pressure, bad air fuel ratio sensor or restricted fuel injectors.

  • Thanks! I actually cleaned the MAF with a special spray cleaner a week ago, and did not notice an oily appearance. I will check again looking from very close though. I have a graph showing the FTs I measured but I don't think I can attach it to the post... just to confirm, the fact that LT is positive and ST is negative is not concerning to you? Thanks! – Florent Nov 20 '16 at 7:43
  • All tests were done with the gear in neutral, so I'm not sure if sufficient load is present at 2500-3200 rpm to establish light load – Zaid Nov 20 '16 at 10:39
  • @ziad I have done this testing on several hundred of these engines. The fuel trim trend is visible without load on Toyota's of this era. Other engine manufactures write there control software so that it behaves quite differently, Honda, for example, changes fuel trim so slowly that it can be hard to see change trends in real time. – Fred Wilson Nov 20 '16 at 16:15
  • @Florent The total of trim is what I am looking at. In this case the total is +11. The readings in that higher load cell have not yet adjusted to the new lean mixture state. This load cell (high rpm, low load) is not often seen in normal driving. This is the condition that Ziad mentioned in his comment. It would probably adjust itself to around LT +13% and ST -3 if engine were held in that load cell for a few minutes. – Fred Wilson Nov 20 '16 at 22:52

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