EML and MAF-/MAP- Sensors
All EFI ( Electronic Fuel Injection ) equipped cars need to monitor the air going into the engine in order to get the air/fuel ratio correct. This monitoring can be done by using a MAF- ( Mass Air Flow ), a MAP- ( Mass Air Pressure ) sensor or both. The sensors supplies information about the air entering the system to the ECU ( Engine Control Unit ), so it can calculate the amount of fuel the injectors should deliver for optimal Air/Fuel-Ratio. A malfunctioning sensor can create all kinds of problems. Sometimes the problems are obvious, but sometimes hidden and except for a slight loss of performance the fault can be difficult to notice.
Depending on type of fault in the Air-Sensor and the ECU software, the result varies from indication on the dashboard by a lit EML ( Engine Management Light ) or Check Engine light to a total stall of the engine.
A very common type of MAF-sensor used in cars is the Hot Wire, which often also come with an internal AIT-Sensor ( Air Intake Temperature ), which is supplies the ECU with information about the air temperature.The HotWire MAF-sensor has a wire suspended in the engine's air stream, to which a constant voltage is supplied. The current flowing through the wire heats it and increases the wire's resistance, which according to Ohm's law ( I = U / R ) varies the current flow through the circuit.
Since the Voltage ( U ) is constant, the current flow ( I ) will stabilize as the wire reaches a certain resistance ( R ).
Air flowing past the wire cools it and decreases its resistance, which allows more current to flow through the circuit. The current increase or decrease is proportional to the mass of air flowing past the wire, and the integrated electronic circuit converts the measurement into a calibrated signal that is sent to the ECU.
If air density increases due to pressure increase or temperature drop, but the air volume remains constant, the denser air will remove more heat from the wire, indicating a higher mass airflow.
This means the HotWire sensor also responds to air density and air temperature changes.
Some sensors also have a HotWire Burn-Off cleaning circuit, wherein a relay applies a very high current through the wire for roughly a second, when the engine is turned off. This current burns or vaporizes any contaminants stuck to the platinum hot wire element.
In most cases the EML light illuminates when the MAF-sensor is faulty, but this is not always the case. As long as the fault in the sensor is within the ECU's tolerance limits the ECU will not indicate a failure.For example:
The MAF-sensors measured values are within the tolerance limits, but the sensor reacts slow due to fault or contamination. This will not be a reason for the ECU to illuminate the warning light until any other sensor reacts to wrong air/fuel ratio.
If the management light comes on during load, acceleration or high speed it is usually the oxygen sensor ( Lambda ) that informs the ECU that the fuel mixture is lean or rich.
In these cases the ECU can sometimes mistake the oxygen sensor to be the cause and blame it in the DTC-code.
Insensitivity can also cause faults that never illuminates the EML, but still severely affects the performance. This is especially noticeable in turbocharged vehicles, where the MAF-sensor information never reaches the point where the ECU decides to let the turbocharger kick in.
- Engine turns over but won't start up. Excessive black smoke from exhaust while engine is turning over.
- Engine starts but stalls soon after. Can be kept running a few seconds by moving throttle.
- Engine drags or hesitates while idling or under load.
- Engine hesitates and jerks during acceleration.
- Engine hiccups.
- Engine displays unusually lean or rich idling. Excessive black smoke while idling ( rich ).
- Turbo-charger fails to kick in.
Repairing the MAF-sensor, in the sense changing parts within it - no, not to my knowledge.Cleaning the sensor on the other hand is possible, and in many cases it is the only "repair" needed. The first thing to try when you suspect a faulty MAF-sensor, is to clean it.
! ! CAUTION ! !
NEVER install the MAF before it has dried 100%.
Any remaining chemicals or moisture can lead to damage when power is supplied to the sensor.
Q: My EML illuminated whilst driving. Can i continue or should i stop?
A: If the engine performance isn't changed so that it might create a hazardous situation, You can drive the car home or to a garage.
The light is an indication that the engine is in "Limp Home" mode, meaning that the car uses factory presets to function - not the information from the sensors.
Leaving the fault unattended will lead to more problems, so check up the engine as soon as possible.
On vehicles equipped with a MAF-sensor that is mounted on the hose between the air cleaner box and the throttle body, the MAF-sensor can be disconnected and the engine forced into Limp-Home mode.
If the engine starts but stalls immediately or as soon as throttle is given, the MAF-sensor might be giving the ECU wrong info regarding Air Flow.
Forcing Limp- or Safe- Mode by disconnecting the MAF-Sensor might make the vehicle perform good enough to be driven home or to a garage. In this mode the ECU is in "Open Loop" and uses factory presets ignoring information from sensors like the MAF-, MAP- and oxygen- sensors.
A light on the dashboard will indicate that the vehicle is in "Limp" mode, and the reason should be attended to as soon as possible to avoid further damage.
DISCLAIMER
If you are inexperienced working on projects like those mentioned on this blog, especially but not limited to
electrical or mechanical projects,please remember to read all safety precautions and use safety equipment.
Also remember that during the work, it is very possible to cause more harm to the vehicle than it initially had.
You might also void your vehicle insurance, create a hazardous condition, harm or even kill yourself or others.
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