A MAF sensor, as the name suggests, provides the PCM with an air mass reading, not an air volume reading.
As such, it inherently accounts for air density.
You don’t need to know BARO to calculate fuel on a MAF system.
If you go up in altitude, the air is less dense and you will automatically read less air mass from the MAF sensor.
You don’t need to know EGR mass. If you recirculate EGR, you pull less air in from the atmosphere and the MAF sensor automatically reads less.
If your backpressure is lower, the MAF reading automatically compensates, etc. etc.
So why do we want to know BARO?
There are still a few control features that need to know altitude. During crank, the MAF reading is erratic and we calculate fuel based on ECT, rpm and BARO instead of using the MAF reading. For power enrichment, you have to know when you are running out of engine capacity. This is a function of altitude. See the definition of the new LOAD_PCT PID and you will see that it goes to 100% regardless of altitude precisely because there is altitude compensation in it. When you hit 90% or so, you will go into power enrichment. Spark may still be adjusted for altitude because the fuels are generally different at altitude. Any device that uses vacuum needs to be turned off/adjusted at altitude because you run out of vacuum sooner at altitude. Transmission shift schedules may be adjusted for altitude (because of the same logic as power enrichment) – your foot hits the floor sooner at altitude than at sea level. (link)
If scan tool shows seemingly correct values of MAF sensor, but scope doesn’t show any signal coming from the sensor, it means that PCM generates these values based on RPM and Throttle Position Sensor, just to keep the engine running.
Rule of Thumb
1g/ sec/L / 500rpm
A rule of thumb on a MAF’s air flow rate at 500 rpm (don’t use higher than 1000) is 1 gram per second per liter of engine displacement for engines with more than 3 liters.
Watch ECM PID’s
Engine RPM, Throttle Position, Engine Load, STFT, LTFT, All O2 Sensors, Loop Status
When a MAF sensor gets contaminated, it skews the transfer function such that the MAF over-estimates airflow at idle (causes the fuel system to go rich) and under-estimates airflow at high airflows (causes the fuel system to go lean). This means that Long Term Fuel Trim will learn lean (negative) corrections at idle and, learn rich (positive) corrections at higher airflows.
Snap Throttle Test
Calculated Load
Calculated Load PID should reach 100%, otherwise bad MAF.
Maximum MAF Voltage
Perform quick WOT while observing maximum voltage with a multimeter.
Max should reach 4V.
Some Toyotas reach only 3.8V.
O2 readings
At WOT O2 should show RICH condition. If it doesn’t, it might indicate dirty MAF sensor…
