P0171 continues intermittently

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.

 

Fuel pump volume ?

 

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.

 

Are you certain there are no vacuum leaks?

 

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.

 

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

 

Chalk up another one for careful checks for vacuum leaks.