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MAF Quick Tutorial
Mass airflow sensors (MAF), which are used on a variety of multiport fuel injection systems, come in two basic varieties: hot wire and hot film. Though slightly different in design, both types of sensors measure the volume and density of the air entering the engine so the computer can calculate how much fuel is needed to maintain the correct fuel mixture.
Mass airflow sensors have no moving parts. Unlike a vane airflow meter that uses a spring-loaded flap, mass airflow sensors use electrical current to measure airflow. The sensing element, which is either a platinum wire (hot wire) or nickel foil grid (hot film), is heated electrically to keep it a certain number of degrees hotter than the incoming air. In the case of hot film MAFs, the grid is heated to 75 degrees C. above incoming ambient air temperature. With the hot wire sensors, the wire is heated to 100 degrees C. above ambient temperature. As air flows past the sensing element, it cools the element and increases the current needed to keep the element hot. Because the cooling effect varies directly with the temperature, density and humidity of the incoming air, the amount of current needed to keep the element hot is directly proportional to the air "mass" entering the engine.
MAF sensor output to the computer depends on the type of sensor used. The hot wire version, which Bosch introduced back in '79 on its LH-Jetronic fuel injection systems and is used on a number of multiport systems including GM's 5.0L and 5.7L Tuned Port Injection (TPI) engines, generates an analog voltage signal that varies from 0 to 5 volts. Output at idle is usually 0.4 to 0.8 volts increasing up to 4.5 to 5.0 volts at wide open throttle.
The hot film MAFs, which AC Delco introduced in '84 on the Buick turbo V6 and have since used on the 2.8, 3.0 and 3.8L V6 engines, produce a square wave variable frequency output. The frequency range varies from 30 to 150 Hz, with 30 Hz being average for idle and 150 Hz for wide open throttle.
Another difference between the hot wire and hot film sensors is that the Bosch hot wire units have a self-cleaning cycle where the platinum wire is heated to 1000 degrees C. for one second after the engine is shut down. The momentary surge in current is controlled by the onboard computer through a relay to burn off contaminants that might otherwise foul the wire and interfere with the sensor's ability to read incoming air mass accurately.
An engine with a bad MAF sensor may start and stall or be hard to start, it may hesitate under load, idle rough or run excessively rich or lean. The engine may also hiccup when the throttle suddenly changes position.
Often, a dirty or faulty MAF sensor will cause the engine to set a LEAN code and turn on the Check Engine Light. If the MAF sensor wire becomes dirty or is contaminated with oil (from an aftermarket reusable air filter), it will be slow to react to changes in airflow. This may cause the MAF sensor to under-report airflow, causing the engine to run lean.
On OBD II vehicles, the input from the MAF sensor is combined with those form the throttle position sensor, MAP sensor and engine speed to calculate engine load. If your scan tool can display calculated engine load, look at the value to see if the load is low at idle, and higher when the engine is running under load. No change in the reading or a reading that makes no sense could indicate a problem with any of these sensors.
If you suspect a MAF sensor problem, scan for any fault codes. Trouble codes that may indicate a problem with the mass airflow sensor include:
- P0100....Mass or Volume Air Flow Circuit
- P0101....Mass or Volume Air Flow Circuit Range/Performance Problem
- P0102....Mass or Volume Air Flow Circuit Low Input
- P0103....Mass or Volume Air Flow Circuit High Input
- P0104....Mass or Volume Air Flow Circuit Intermittent
- P0171....System too Lean (Bank 1)
- P0172....System too Rich (Bank 1)
- P0173....Fuel Trim Malfunction (Bank 2)
- P0174....System too Lean (Bank 2)
- P0175....System too Rich (Bank 2)
On older Pre-OBD II vehicles, you can use a scan tool or manual flash code procedure to read the codes:
GM Pre-OBD II: Code 33 (too high frequency) and Code 34 (too low frequency) on engines with multiport fuel injection only, and Code 36 on 5.0L and 5.7L engines that use the Bosch hot wire MAF if the burn-off cycle after shut-down fails to occur.
Ford Pre-OBD II: Code 26 (MAF out of range), Code 56 (MAF output too high), Code 66 (MAF output too low), and Code 76 (no MAF change during "goose" test).
Of course, don't overlook the basics, too such as engine compression, vacuum, fuel pressure, ignition, etc., since problems in any of these areas can produce similar driveability symptoms.
Unlike vane airflow meters with their movable flaps, MAFs have no moving parts so the only way to know if the unit is functioning properly is to look at the sensor's output, or its effect on injector timing.
With the Bosch hot wire sensors, sensor voltage output can be read directly with a digital voltmeter by probing the appropriate terminals. If the voltage readings are out of range, or if the sensor's voltage output fails to increase when the throttle is opened with the engine running, the sensor is defective and needs to be replaced. A dirty wire (which may be the result of a defective self-cleaning circuit or external contamination of the wire) can make the sensor slow to respond to changes in airflow. A broken or burned out wire would obviously prevent the sensor from working at all. Power to the MAF sensor is provided through a pair of relays (one for power, one for the burn-off cleaning cycle), so check the relays first if the MAF sensor appears to be dead or sluggish.
On GM MAF sensors, there are a couple of quick checks you can do for vibration-related sensor problems. Attach an analog voltmeter to the appropriate MAF sensor output terminal. With the engine idling, the sensor should be putting out a steady 2.5 volts. Tap lightly on the sensor and note the meter reading. A good sensor should show no change. If the analog needle jumps and/or the engine momentarily misfires, the sensor is bad and needs to be replaced. You can also check for heat-related problems by heating the sensor with a hair dryer and repeating the test.
This same test can also be done using a meter that reads frequency. The older AC Delco MAF sensors (like a 2.8L V6) should show a steady reading of 30 to 50 Hz at idle and 70 to 75 Hz at 3,500 rpm. The later model units (like those on a 3800 V6) should read about 2.9 kHz at idle and 5.0 kHz at 3,500 rpm. If tapping on the MAF sensor produces a sudden change in the frequency signal, it's time for a new sensor.
On the GM hot film MAFs, you can also tap into the ALDL data stream with a scan tool and read the sensor's output in "grams per second" (gps) which corresponds to frequency. The reading should go from 4 to 8 gps at idle up to 100 to 240 gps at wide open throttle.
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