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?