I have recently purchased a 2012 Toyota Tundra with the 5.7 L engine with 160,000 miles. At idle (about 700 RPMs), long-term fuel trims are around +15% on both banks and short term fuel trims near zero. It has not yet set a check engine light. Increasing the idle to 2000 RPMs and holding it there does not change them (ruling out a vacuum leak). If it were a defective oxygen sensor or plugged fuel injector, I would not expect such symmetry between banks. What could be causing both banks to run lean like that? Thanks
Thinking about it a bit more, the mass airflow sensor at idle only read about 4.7 g/s. On a 5.7 L engine that’s about 18% off, isn’t it? So, if it thinks it’s only getting 4.7 g of air, when it might be getting 5.7 g (an approximation of the expected amount), would that cause the lean condition?
It's only a rule of thumb, my 3.0 1999 Ford Ranger uses around 3.9 to 4.0 grams of air per second, and it runs almost perfect when using live data.
If it were a defective oxygen sensor or plugged fuel injector, I would not expect such symmetry between banks.
A defective oxygen sensor would trip your check engine light.
All engines get dirty, your MAF sensor could be reading off because of dirt, etc. I had such a problem after cleaning my MAF sensor once, that after I cleaned it, the car wouldn't start. After three tries, the car ran bad for a minute, until the computer realized I cleaned it. Over the next minute or so, it gradually retuned to normal.
Thinking about it a bit more, the mass airflow sensor at idle only read about 4.7 g/s. On a 5.7 L engine that’s about 18% off, isn’t it? So, if it thinks it’s only getting 4.7 g of air, when it might be getting 5.7 g (an approximation of the expected amount), would that cause the lean condition?
I forgot to mention that, prior to the 15% LTFT readings, they were around 7%, so I checked the air filter, cleaned the MAF sensor, changed the oil, and drove it about 70 miles. If anything, the airflow readings went down a little from 5.0 g/s to 4.7 g/s, which would be consistent with higher LTFT readings. The only other thing that might throw off the MAF readings is a carbon filter on the top of the airbox (and seems to be integral to the TRD airbox) that doesn’t look to be serviceable. If that were dirty (and it did not look bad), could it change the air flow patterns enough to make the MAF sensor give an inaccurate approximation of the air going into the throttlebody? How often do MAF sensors loose their accuracy in such a way that cleaning does not get it back? Bad MAF readings could explain how the LTFT of both banks was affected by the same percentage at various RPMs.
measure your fuel pressure. Try some fuel injector cleaner .
