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Omega Common Issues and FAQ / Mass Air Flow Sensor
« on: 09 August 2006, 13:22:52 »
The Mass Air Flow Sensor (MAF sensor) is key to the operation and mixture control of the engine system.
Because the combustion process is simply speaking a chemical reaction which is triggered by heat (in the form a of a spark) or compression (on a diesel) its essential to know how much air the engine is using in order to inject the correct amount of fuel and ensure that complete combustion occurs.
Air mass is measured in preference to air flow because the density of air (and hence the oxygen content) varies with humidity, altitude and temperature. Its the oxygen that we are using in the combustion process.
The MAF has evolved considerably over the years and started life as a moving vane/flap type arrangement (as used on older fuel injection systems i.e. the 24V straight six Carlton/Senator engine) which could only measure air flow to the hot film method which is used across all the Omegas engines (with the exception of the very early 2.5 diesel power plants). This gives the advantage of no moving parts, greater reliability and more accurate air measurement.
Note: add phott here.
The principle of operation is simple, a small hybrid circuit is placed into the the air flow. The membrane has a thin film temperature sensor printed on the upstream side, and one on the downstream side. A heater is integrated in the center of the membrane which maintains a constant temperature. the current through this varies as the film is heated and cooled by the air flow (the resistance of the film varies with temperature). Without any airflow, the temperature profile across the membrane is uniform i.e. both temp sensors see the same temperature. When air flows across the membrane, the upstream temp sensor cools differently than the downstream side (the upstream one will see the heat from the heater). The difference between the upstream and downstream temperature indicates the mass airflow.
Points to note:
1) It is not unusual for people to get MAF fault codes raised and there is not a fault with the sensor, the ECU will raise an error if the readings from the MAF are lower than what it is expects given the operating conditions of the engine.
i.e. the MAF might be reporting a typical idle air flow reading when the engine is doing 3000rpm, this may well be an air leak and un-metered air is entering the system.
or
At idle (not so relevant on Omega engines but, true of the new breed of power plants), the engine might be idling badly and a MAF code could be raised, reality is it could be that the EGR valve is stuck open.
Reality is with MAF faults that further live values should be viewed as part of the overall diagnostics.
2) The MAF sensor elements are very sensitive, you don't want any dirt getting into them. This can be helped by ensuring the air filter is always fitted and is changed at the correct service intervals. Also, cone filters tend to be of the clean and re-oil variety, this is not good as the oil can get sucked through and deposited on the MAF sensor.
3) The MAF sensor is easy to find, it is normaly located in between the airfilter housing and the throttle. In teh case of the V6 it is just below the first 90 deg bend as the pipe exits the air filter. The devcie it self is a tube of betwen 50 and 150mm in length (later ones are smaller i.e. on the 2.2)
Because the combustion process is simply speaking a chemical reaction which is triggered by heat (in the form a of a spark) or compression (on a diesel) its essential to know how much air the engine is using in order to inject the correct amount of fuel and ensure that complete combustion occurs.
Air mass is measured in preference to air flow because the density of air (and hence the oxygen content) varies with humidity, altitude and temperature. Its the oxygen that we are using in the combustion process.
The MAF has evolved considerably over the years and started life as a moving vane/flap type arrangement (as used on older fuel injection systems i.e. the 24V straight six Carlton/Senator engine) which could only measure air flow to the hot film method which is used across all the Omegas engines (with the exception of the very early 2.5 diesel power plants). This gives the advantage of no moving parts, greater reliability and more accurate air measurement.
Note: add phott here.
The principle of operation is simple, a small hybrid circuit is placed into the the air flow. The membrane has a thin film temperature sensor printed on the upstream side, and one on the downstream side. A heater is integrated in the center of the membrane which maintains a constant temperature. the current through this varies as the film is heated and cooled by the air flow (the resistance of the film varies with temperature). Without any airflow, the temperature profile across the membrane is uniform i.e. both temp sensors see the same temperature. When air flows across the membrane, the upstream temp sensor cools differently than the downstream side (the upstream one will see the heat from the heater). The difference between the upstream and downstream temperature indicates the mass airflow.
Points to note:
1) It is not unusual for people to get MAF fault codes raised and there is not a fault with the sensor, the ECU will raise an error if the readings from the MAF are lower than what it is expects given the operating conditions of the engine.
i.e. the MAF might be reporting a typical idle air flow reading when the engine is doing 3000rpm, this may well be an air leak and un-metered air is entering the system.
or
At idle (not so relevant on Omega engines but, true of the new breed of power plants), the engine might be idling badly and a MAF code could be raised, reality is it could be that the EGR valve is stuck open.
Reality is with MAF faults that further live values should be viewed as part of the overall diagnostics.
2) The MAF sensor elements are very sensitive, you don't want any dirt getting into them. This can be helped by ensuring the air filter is always fitted and is changed at the correct service intervals. Also, cone filters tend to be of the clean and re-oil variety, this is not good as the oil can get sucked through and deposited on the MAF sensor.
3) The MAF sensor is easy to find, it is normaly located in between the airfilter housing and the throttle. In teh case of the V6 it is just below the first 90 deg bend as the pipe exits the air filter. The devcie it self is a tube of betwen 50 and 150mm in length (later ones are smaller i.e. on the 2.2)
