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When it comes to testing MAF sensor on my vehicle, the factory service manual just suggests two PASS/FAIL tests:

  • an out-of-circuit resistance reading at a particular ambient temperature
  • validating that in-circuit voltage varies with unscientific puffs of shop air

Those 2 tests might be good enough to identify total failure, but what about intermediate reduced performance to to degraded internal components (or, as the case may be, a failure to properly clean the sensor elements [glances away in guilty silence])?

Without fancy shop equipment delivering a known quantity of air in a set amount of time, is there some way that a shade tree mechanic can, with some acceptable margin of error, establish that reasonable readings are being obtained from his or her car's MAF sensor?

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Yes. In fact, since you have a MAF sensor, then you likely do already have the expensive equipment required to perform a calibrated test -- it is called an automobile.

The only other tool you will need for this test is an OBDii scan tool that allows you to record multiple PIDs in a somewhat simultaneous way. You will need each of the following PIDs, or their equivalents:

  • Barometric pressure (if your car's ECU doesn't provide this, your smartphone might)
  • RPM (which you will convert to revs/second)
  • MAF reading (converted to atm)
  • Manifold vacuum (converted to negative atm, meaning "the amount by which the manifold is below ambient pressure")

For barometric pressure & the MAF reading, you need to ensure that they are both given in the same unit of measurement. There are several such units: inHg, Torr, Bar, psi, Pascal, and atm. I prefer converting to atm, because its value is easy to understand intuitively: 1.0 atm (standard) is the weight of the atmosphere at sea level when the temperature there is 0 °C.

Finally, you need to know:

  • The displacement of your engine
  • Whether your engine is 2-stroke or 4-stroke
  • The fact that, at 1.0 atm, one liter of air weighs 1.29 grams.

After collecting data from the running engine, there are four basic steps to perform:

  1. Calculate airflow volume per second
  2. Estimate intake pressure
  3. Use (1) & (2) above to calculate an expected MAF reading
  4. Compare (3) with the actual MAF reading, to assess MAF health.

Let's get started with the first of two examples, both done on a 3.4L V6 engine (which like most vehicle applications is a 4-stroke):

EXAMPLE ONE: warm engine, idling at 700rpm

warm engine, idling at 700rpm

  1. Calculate airflow volume per second: the displacement of this engine is 3.4L, but since it is a 4-stroke it only sucks in half that much per rotation. So for this calculation, we pretend it is only 1.7L displacement. 1.7 L/rev * 700 rev/min ÷ 60sec/min = 19.83 L/sec
  2. Estimate intake pressure: there is a reason I call this an estimate, rather than a calculation; I believe that to do this correctly would require an accurate model of your car's specific intake design, a perfectly clean air filter, and a lot of higher mathematics. But our goal here is just to see if we are getting reasonable values from the MAF. So we are going to use a very simplified heuristic here, that I am going to call "estimated intake pressure at the MAF". The idea is that the pressure that the MAF experiences will fall somewhere between barometric pressure (upstream from the air filter) and the pressure downstream in the intake manifold, behind the throttle plate. In my experiments w/ a good vehicle, I find that simply subtracting manifold vacuum from barometric pressure provides a good estimate. (Here I convert 998.3 millibars for barometric to 0.985 atm, and convert 25.1 inHg vacuum to 0.839 atm "below ambient") ESTIMATED INTAKE PRESSURE at MAF: 0.985 - 0.839 = 0.146 atm
  3. Calculate expected MAF reading: Now (1) we know how many liters per second the pistons are collectively gulping, and (2) we can ballpark what pressure those liters are at near the MAF. We also know from Google that a sea-level liter of air would weigh 1.29 grams. So we use all of this info to calculate what we think the MAF should report: 19.83 L/sec * (1.29 g/L * 0.146 atm) = 3.73 g/sec

... and as you can see from the image above, this is very close to the 3.8 g/sec that the MAF actually reported!

Let's do one more example.

EXAMPLE TWO: same engine, moderate acceleration

same engine, moderate acceleration

  • Revs: 1983 rpm
  • Barometer: 977.1 millibars --> 0.964 atm
  • Manifold vacuum: 10.3 inHg --> 0.344 atm ("below ambient")

    1. Calculate airflow volume per second: 1.7 L/rev * 1983 rev/min ÷ 60sec/min = 56.185 L/sec
    2. Estimate intake pressure at MAF: 0.964 - 0.344 = 0.620 atm
    3. Calculate expected MAF reading: 56.185 L/sec * (1.29 g/L * 0.620atm) = 44.94 g/sec

This time, our expected value is about 5% lower than the actual reported value of 47.1 g/sec. But in this case you can also see from the fuel trims (-6.2st+2.3lt) that the ECU thought that the fuel mixture was about 4% too rich, possibly indicating that the MAF's reported value was too high. It's almost like the ECU is agreeing with us!

There are several reasons why this overall technique isn't perfect:

  • The 'estimate intake pressure at MAF' step isn't perfect: This one was mentioned above, but it bears repeating. If your air filter is very dirty, then actual intake pressure at the MAF will be lower than expected. And in any case, our expectation has been generalized, and doesn't take into account the design of any specific intake pathway. It will be more useful when the engine has been held at a steady state when the snapshot is taken, as opposed to immediately after making throttle changes, to give time for the pressures in the intake tubing to reach a homeostasis.
  • These OBDii screen captures are not really instantaneous snapshots of simultaneously collected data measurements: the ECU is continuously reporting a round-robin of PID values, one after another. Example: first it might report RPM, then 13 other don't-care PIDs, and finally the MAF reading. In between RPM and MAF, a significant amount of time has elapsed. So while the last screenshot above implies that the engine was at 1983 RPMs when the MAF read 47.1, in truth the RPMs likely changed in the meanwhile. Again, the advice is to try to hold the engine in a steady state while taking these readings.
  • I assume you have no vacuum leaks

All of the above caveats emphasize that this technique just gets you into the ballpark of answering the question "is my MAF sane?", and needs to be repeated a handful of times before you can treat it as reliable background info on which to base a part-replacement decision.

Finally: be safe, and make good choices. In particular, have a buddy managing the OBDii snapshots while you focus on driving.

  • A less complicated test and rule of thumb is maf g/s at idle is roughly equal to engine displacement. A WOT run in second gear, calc load should reach from 90-100%. Roughly equal to VE. – Ben Dec 28 '18 at 3:55

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