Fuel trims, MAF and O2 sensors.

Need some advise. Recently my 2006 NHW20R threw a P0172 and engine check condition. With a OBD2 code reader I noticed the long term fuel trim wanting to live at -33.6% which is quite wrong. Anyway, I fixed this as it seemed the MAF (Mass air-flow sensor) was a bit dirty. Had to very carefully clean it twice as the LT-FTRIM crept to -6.3% a while after the first clean. Since the second clean the range seems to be between -0.8% and -2.3%. First question, for a vehicle that has done about 93k miles, is this about right?

At its fault state the engine was not happy. Felt like it was stalling when stopping as opposed to just stopping. (Wierd how the petrol engine is stopped. It's turned at 144 RPM for a second before it's fully stopped.)

When the petrol engine is not turning is a residual 0.07g/s airflow normal or should this be an absolute 0.00g/s with the petrol engine off? I'm assuming 0.07g/s is normal with the engine off because of not-so-cold air rising up the throttle body and surrounding heated components.

Also, in relation with air-flow and the short term fuel trim, what behaviour should I expect from the O2-B1 and O2-B2 sensors compared to other sensors?

My car's fine now. Just seeking advise what to look-out for so to try spot issues well before the engine ECU decides to record pending or actual DTCs.

Thanks in advance for any assistance and advice offered.

 

No damage repair whatsoever. Since the MAF was saturated using CRC Air Sensor Cleaner spray everything has settled to what seems to be healthy figures. Butterfly edge is clean and no corrosion. Maintenance schedule is followed and the next scheduled service will include a spark-plug change. The engine one is Castrol Magnatec 5W-30 A1. Change on the official maintenance schedule.

 

I think people are misunderstanding what I am enquiring about. I know pretty well how the Gen II Prius operates. What I am not sure about is what the typical sensor readings would be. More of how one reading should relate to others.

Working. the O2 sensors seem to be responding correctly compared to Engine RPM, absolute throttle position and MAF readings. I'm pretty sure that any MAF reading will be completely ignored while the Petrol engine is off.

Now, if while the engine is off and the MAF was to rest at something like 0.9g/s or similar then I think I would be ordering a replacement. I understand that the O2 and MAF sensors are critical to the correct operation of the petrol engine.

 

Right.... The 0.9g/s or similar is just conversation to demonstrate I understand. With the EOBD/OBD2 tool's graph showing an 0 RPM on the real-time graph, the MAF is showing 0.07g/s on the same real-time graph. One can plot four streams of data on a Launch Creader 6.

I was wondering if, with the petrol engine off and the car not moving if a MAF of 0.07g/s is normal. I would assume it should be equal or less than 0.01g/s, except that warmed gas/air (heat rises!) will be travelling up the throttle body from the inlet manifold past the MAF sensor and the Prius does not keep the butterfly 100% shut when the engine is off.

With the engine off, obviously, the thermistor next to the MAF sensor registers and increase of temperature because colder air is not being sucked into the engine. The engine is off.

There was something wrong, but a through clean of the MAF sensor's resistors sorted out the problem.

I'm just trying to get to know the figures plotted on the Creader 6 so I can spot misbehaviour by using my brain as opposed to waiting for the ECU to tell me that there is a fault. I suspect the issue with the MAF sensor being dirty was something that was developing over a number of weeks before it got so dirty that a threshold was exceeded. Even then the LT-FTRIM was below -32% for about a week before a pending P0172 was recorded and then another week before the MIL condition was triggered.

After the first clean my brain spotted the LT-FTRIM under -6% so cleaned the MAF's resistors again. The second time my own brain's thresholds were exceeded while my car was happy.

Just wish to learn what to expect from the sensors.

 

Yep, I'm aware. The car is stationary.