In Front of the Flywheel - August - 2020

MAF Sensor Diagnosis

If you did not know, the engine has to be operating properly before we can diagnose transmission problems. Air flow measurement is a HUGE input on Mass Airflow sensor (MAF) equipped vehicles. It can affect fuel delivery, transmission shifting and more. MAF sensors are one of the most common “Shotgun” parts that technicians swap. During diagnosis mass air flow sensor codes are usually an indication of another underlying problem. In addition, mass air flow sensor issues usually present themselves as a drivability problem with no DTC or MIL illumination. In other cases, a failed MAF sensor will cause lean codes such as P0171 or P0174. Regardless, there are multiple ways to test MAF sensors and I will do my best to go through them all.

Unplug The MAF

I have been in this industry for many years and I can say that unplugging a MAF sensor used to be a good tactic. If we were working on a 1986 Pontiac 6000 that may have been acceptable. Modern vehicles have software strategies that render this technique useless. Some modern vehicles will not even run with the MAF sensor disconnected. I am putting my foot down… this test is basically worthless.

Double The Displacement

The double the displacement rule is a mediocre test. It will alert you to a gross MAF failure but nothing more. The way it works is: at idle the engine should draw approximately two times the engine displacement in grams per second. In the following example (Figure 1) I snapped the throttle a few times on a 2.0 liter naturally aspirated Ford product. When the engine returned to idle the MAF reading is 4.00 grams per second. This is exactly two times the engine displacement. If this test fails there could be an airflow issue but MAF sensors could still pass this test and be faulty.

Snap The Throttle

Another test that can help is a snap throttle test with a scope. This test is only useful on an analog style MAF sensor. What I mean by that is some sensors output a 0 to 5-volt analog signal. Others output a digital square wave signal and the PCM measures the frequency of that signal. The analog style signal is what applies here.

To perform the test, connect a scope to the MAF sensor signal wire. With the vehicle in park, snap the throttle wide open as fast as you can. Observe the voltage level at the first peak (Figure 2.) A general rule of thumb is voltage should exceed 4 volts. On a vehicle with a cabled throttle this test works quite well. However, there is a flaw with this test when used on a vehicle with electronic throttle control. Quite often the PCM will not open the throttle fast enough to achieve the initial gulp of air required for this test’s accuracy. The MAF capture shown is from a vehicle with ETC and does not have a MAF sensor issue. The peak voltage only reaches about 3 volts. This is due to the PCM controlling the throttle movement. As a result, I do not use this test very often.

Forty Times Rule

The forty times rule is relatively simple. Take the vehicle on a wide-open throttle test drive and record the MAF sensor signal in grams per second. The peak MAF reading should exceed forty times the engine’s displacement. In this case, a test drive was recorded (Figure 3) on a 2.0-liter engine. That means that our airflow measurement should exceed 80 grams per second or 2.0 times 40. In the recording we achieved 104 grams per second which is definitely acceptable.

Volumetric Efficiency Measurement

Volumetric efficiency, or VE, is my preferred method for testing Mass Airflow sensors. It is a very dynamic test that is performed by recording scan data during a wide-open throttle test drive. The scan data that you will need to record should include: engine RPM, Mass Airflow (g/sec) and oxygen sensor voltage at a minimum. There may be other data PID’s that you want to observe also. In this case (Figure 4) I have chosen to add the engine load PID as well. Engine load will be addressed next.

After the test drive has been recorded, a cursor is placed near the peak of the RPM trace which I have placed at the top of the image. At this point we will note the engine RPM (6107) and the MAF (104.62 g/sec) readings and apply them to a VE calculation. There are a variety of VE calculators available for PC’s, smart phones and more. They range in price but are rarely expensive. The calculator I use is called DECS and is available from aeswave.com. I use this application because of its versatility and its ability to perform the calculation for forced induction vehicles as well. When we plug our numbers into the calculator (Figure 5) we come up with a VE number of 86%.

On a naturally aspirated engine we would expect to see over 75% VE. On this known good vehicle our VE reading is acceptable. If the VE number were too low we would suspect an air metering or engine breathing issue. We can tell the difference by observing the oxygen sensor reading during our wide-open throttle run. During this section of the drive the oxygen sensor should not flat-line lean.

Basically, if we were to have a low VE and a flat lean oxygen sensor signal we would suspect a MAF sensor or air metering issue. This is because an under reporting MAF sensor resulted in our low VE number and the same inaccurate input to the PCM caused the PCM to inject to little fuel as well. Conversely, if we have a poor VE number and an oxygen sensor that does not report a lean condition then the engine does not have a MAF sensor issue but it does have a breathing issue. Causes of these issues could be contributed to restricted intake or exhaust systems. In these cases, our diagnosis will have to take a separate path.

Engine Load

Engine load, in a few different variations, has been available on vehicles since the 1996 model year. It can be exactly the same as VE, loosely based on VE or calculated other ways depending on manufacturer, model year and more. Many newer vehicles have multiple forms of engine load in the same data list. This data PID can be used in place of an actual VE calculation if you know exactly how the PID behaves on the application that you are diagnosing. On this 2013 Transit Connect (Figure 6) the engine load PID at the same exact point we made our VE calculation is 100%. Compared to our 86% VE calculation it is safe to say that it is not based directly on a VE calculation. Personally, I choose to do a VE calculation every time and record the load PID’s to make some comparisons.

A Couple Parting Thoughts

First, if any of our testing indicates a failing MAF sensor, a thorough inspection of intake tubing should be performed before condemning the sensor. Intake air leaks, especially between the MAF sensor and the throttle body, will cause inaccurate air measurements and can present as a failing MAF sensor. Inspect before replacement.

Second, this entire article pertains to naturally aspirated engine applications. Forced induction applications, turbocharged or supercharged, require some additional steps due to the “extra” air that is being pushed into the engine above and beyond what it could naturally ingest on its own. We will cover forced induction in a future installment of In Front of the Flywheel.