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Mass Air Flow Meter Testing

For the purposes of this article, I will be dealing only with mass air flow (MAF) sensors that use voltage variations as a signal to the PCM and not frequency. The manufacturer's tests for a MAF sensor often deal with checking the minimum and maximum output voltages and a few resistance checks using an ohmmeter. As with any electronic component, an ohm test will verify a part is bad, but it will NOT verify a part is good. In other words, failing an ohm test indicates a defective part, passing an ohm test means absolutely nothing.

Occasionally a MAF will fail completely or be so far out of calibration that it is easy to diagnose, however, that is usually not the case. As MAF sensors age, the hot wire used for sensing becomes dirty or sometimes coated in oil or other residue. This can cause many symptoms including pinging, spark knock, lean codes such as P0171 and P0174, sluggish acceleration, etc.

On Ford vehicles there is a BARO pid available in the scan data that can assist in diagnosing the fault. Since many Fords do not actually contain a BARO sensor, this value is calculated using the MAF sensor at heavy or wide-open throttle conditions. Use a frequency-to-altitude chart to determine if the BARO reading is correct. Most Ford vehicles with a good MAF will have a BARO reading of about 159Hz at sea level. There should be a drop of about 3Hz per 1000 feet of elevation above sea level. For example, a properly reporting MAF sensor will produce around 156Hz in Atlanta, GA and 142.5Hz in Boulder, CO. If a sensor is sluggish or its reading is skewed, it will usually cause a lower than normal frequency giving some of the above lean symptoms. Also note that a dirty MAF sensor on many vehicles can give negative fuel trim values at idle and positive fuel trim values at higher RPMs. This is another indication that the sensor needs to be cleaned or replaced.

The capture below is of a Ford Thunderbird with P0171 and P0174 codes.

The actual BARO reading of 141 Hz is normal if the vehicle were at 6000 feet above sea level. The problem here is that the vehicle is in Atlanta, Georgia at approximately 1000 feet above sea level. The proper reading should be around 156 Hz as seen in the capture below.

The Snap Throttle Test

Now to verify the fault with a scope, use the snap throttle test. This test works on any voltage based MAF sensor, not just Fords. Adjust your scope to .5 volt and 200 ms divisions. This should give you about 2 seconds from one side of the screen to the other. Now connect to the MAF signal wire and monitor the output at idle. Set the trigger to an up slope at approximately .5 volts ABOVE the idle value. If you need to brush up on scope set up, read the Scope Basics article.

Allow the engine to warm up and wait for the idle speed to come down to its lowest point. Turn all loads off (especially the AC compressor). Now, snap the throttle HARD and hold it for a second or so. Make sure you snap the throttle from low idle or the results will be skewed. The engine should reach around but not over 4000 rpm. You should get a pattern similar to the one below from a 1999 Toyota 4-Runner. This customer also had a complaint of a check engine light with P0171 and P0174 codes. He also stated that the vehicle seemed to be down on power.

The initial voltage spike occurs as the throttle is opened and a sudden burst of air rushes in to fill the intake plenum. The airflow then slows as the vacuum in the intake is filled and begins to rise again evenly as the engines RPM increases.

The plenum fill voltage spike is the telltale sign of a sluggish MAF sensor. It should reach around 3.5 volts or more, with most achieving closer to 4 volts or more. Looking at the above capture you can see that the initial plenum fill only reaches ...

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