What is a Mass Air Flow sensor, what does it do and how it does it.
The Air/Fuel Mixture...
The purpose of the fuel injection system is to precisely maintain the proper air/fuel ratio of 14.7 parts air to 1 part of fuel, or 14.7:1. Before the system can deliver the right amount of fuel, it needs to know how much air there is coming into the engine. In the old days when engines used carburetors we could determine the proper fuel mixture with the use of an emissions analyzer. I'd use a four gas, CO (carbon monoxide), HC (hydrocarbons), CO2 (carbon dioxide) and O2 (oxygen). Then I would turn the mixture screw(s) on the carburetor until the mixture was correct. When CO and HC were near 0 and CO2 was as high as possible I knew the A/F ratio was right. Now this was possible because the incoming air was constant. But the problem is the A/F ratio was only correct at idle. When the engine went off idle the A/F mixture changed because the amount of air entering the engine changed. What we needed back then was a mechanic under the hood with an emissions analyzer all the time adjusting the A/F mixture so it was correct all the time.
Air...
Nowadays we have a computer to take the place of the mechanic under the hood (thank goodness, it was getting awful warm under there) to accurately maintain the A/F mixture. The computer gets inputs from various sensors but today we are just going to talk about the Mass Air Flow Sensor (MAF). Now what is 'Air'? Air is a colorless, odorless, tasteless, gaseous mixture, mainly nitrogen (approximately 78 percent) and oxygen (approximately 21 percent) with lesser amounts of argon, carbon dioxide, hydrogen, neon, helium, and other gases. Air has weight, mass and holds water and these are the properties that must be considered when the computer adjusts the A/F mixture.
Enter The Mass Air Flow Sensor...
The early MAF sensor was a vane type sensor. The incoming air pushed a flap that was connected to a potentiometer. The more air coming into the engine, the further the flap was pushed. By reading the potentiometer the computer knew how much air was coming in and how much fuel should be injected. Modern MAF sensors use a "Hotwire" type sensor. A constant voltage is applied to a heated film or heated wire. This film or wire is positioned in the air stream or in an airflow-sampling channel and is heated by the electrical current that the voltage produces. As air flows across it, it cools down. The heated wire or film is a positive temperature coefficient (PTC) resistor. This means that it's resistance drops when it's temperature drops. The drop in resistance allows more current to flow through it in order to maintain the programmed temperature. This current is changed to a frequency or a voltage that is sent to the computer and interpreted as airflow. Adjustments for air temperature and humidity are taken into consideration since they also affect the temperature of the heated wire or film.
Humidity And Temperature...
Humidity makes the incoming air denser so no other compensation for humidity is needed. While air temperature also affects density, cold air is denser than warm air; a similar amount of air can enter the engine at different temperatures. So an Intake Air Temperature (IAT) sensor is incorporated into the MAF to measure the incoming air temperature and that is taken into account by the computer. Some automakers put this sensor in the intake manifold or in the ducts between the MAF and intake manifold.
Operating Parameters And DTCs...
So the MAF sends a changing voltage, or frequency, signal to the computer, which in turn adjusts the A/F mixture. So at idle the signals will be low and at high RPM the signals will be higher. The computer has stored in its memory the proper, approximate, signals for all operating ranges. If any of the signals are incorrect for a specific range a Diagnostic Trouble Code (DTC) is set and the Malfunction Indicator Light (MIL) will be illuminated. If the computer has a fail-safe mode the computer will use these values as a substitute for actual values to enable continued engine operation until the malfunction can be repaired.
Code Procedures...
So when a code is set there is a specific procedure for accessing, determining and understanding the codes. Allow me to outline them here.
1. Using the appropriate method, scan tool or MIL light; determine which code(s) are stored.
2. Using the vehicles service manual look up the code(s) and see what it/they indicates.
3. Read the description of the code very carefully. Many times a code will be misinterpreted and parts end up being replaced needlessly.
4. Many sensors are used to "keep an eye" on other sensors to make sure they are accurate. So, for example, a P0171, P0172, P0174 or P0175 may be due to a vacuum leak, leaking fuel injector(s) or fuel pressure regulator, low fuel pressure or even an engine oil
5. A code indicating a low or high MAF signal can be caused by:
a. A bad MAF
b. A MAF sensor circuit problem such as:
i. Main power feed wire
ii. Ground wire
iii. Output signal wire
iv. MAF or computer connector
v. A bad computer
Some MAF sensors, Nissan for example, use a "burn-off" system. When the engine is turned off a high voltage is sent through the hotwire to burn off any dirt or other contamination that may have accumulated on it. This helps keep the MAF accurate.
