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In my Toyota Vitz 2007 (Yaris in some countries), I sometimes get a PO171 (System too lean) trouble code when the vehicle is idling in a traffic.

When my mechanic scanned the vehicle for trouble codes, I observed that the long term fuel trim is about 30% at idle and short term fuel trim is near zero. When the RPM is increased, the fuel trim drops to about 8% to 10%. This is usually an indication of a vacuum leak since the percentage of unmetered air entering due to the vacuum leak at higher load decreases and fuel trim improves.

But, the vehicle does not have a MAF sensor and it has a MAP sensor instead. Since the intake manifold pressure is directly measured to calculate the intake air volume and decide the amount of fuel to inject, a vacuum leak theoretically cannot contribute to a lean condition like this.

Despite this, if I disconnect the PCV hose and create a vacuum leak, I can observe that engine starts to misfire, so a vacuum leak seems to have an effect on the engine.

So, could the reason for the PO171 code be a vacuum leak? What else could be the reason?

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  • Is it possible the MAP is faulty? I'm speaking through my hat, but it seems that if it was faulty, it could result in the calculations being off, with the final product being a too lean / rich mixture. – anonymous2 Apr 27 '18 at 12:01
  • @anonymous2 you very rarely see a faulty map sensor on a toyota. Vacuum leaks and o2 sensors are the most likely causes on non maf cars. – Ben Apr 27 '18 at 12:31
  • @Ben So you mean vacuum leak is a possible cause even when there is no MAF? – Lahiru Chandima Apr 27 '18 at 13:23
  • @LahiruChandima You do realize that a vacuum leak is EXCESSIVE air going in to the engine, right? Where do you think that air is going? And what does EXTRA air do to the calculated air fuel ratio? The engine computer carefully meters fuel from the injectors to the amount of air it expects to see. A vacuum leak throws everything off. – zipzit Apr 27 '18 at 14:05
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    Hint. Vacuum leak air is not being sucked thru air filter. Fuel metering calibration is based on a very specific air flow (filter) restriction observed thru lots of testing. – zipzit Apr 27 '18 at 14:32
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Basically the ECM expects X amount of volume to enter the intake at X RPM. MAF or no MAF. At idle if there's more volume than calibrated for, the AFR/O2 sensor will see a lean condition and the ECM adjusts fuel trim to compensate. Thusly code P0171 sets. It may or may not be the cause of your code, but it can and does happen.

The fact that fuel trim goes into single digits off idle suggests that the fuel delivery system is working as intended.

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If your fuel injection strategy is speed density or (map and no maf) then a vacuum leak should not have any affect to fuel trim. O2 sensors, exhaust leak, baro/map reading, wiring, pcm, fuel quality, fuel pressure, fuel injectors just to name a few of the other possibilities.

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