Posted on 12 May 2012

If you are experiencing problems with your BMW, such as poor running, loss of power or intermittent issues such as Turbo Lag or poor mid range acceleration on BMW Diesel models, then the chances are, you have a failed or worn MAF (Mass Air Flow Sensor).

The BMW MAF, is a highly sensitive electronic device which is located in the Air Intake system, just upstream of the Air Filter Housing, the job of the Mass Air Flow Sensor, is to constantly monitor the  flow rate of air into the engine, in order for the electronic engine management system to adjust parameters such as fuel, timing and stroke (and on Diesel models – Turbo VNT Adjustment) in order to maintain optimum combustion, exhaust emissions, power and acceleration.

On some BMW’s the Mass Air Flow Meter serves a dual purpose as it also has a built in temperature monitor, which also feeds back information as to the temperature of the outside air, again this information can be used by the Engine Management system in order to adjust fuel levels whilst starting on cold mornings, and general winter running.

Of course, failure of the MAF temperature sensor, can lead to over fueling,  (which caused symptoms similar to an older vehicle running with its choke fully out) if the sensor goes faulty and constantly feeds back an incorrect temperature reading, and the engine is fooled into thinking its 35f when the air temperature is actually 80f, hence the fueling can be adversely affected.

In short, the MAF sensor, forms an important part of your vehicles’ Electronic engine management system, and loss of calibration caused by age and dirt or a complete failure can cause a very noticeable loss of power, and the feeling that the vehicle simply isn’t running properly or as good as it used to. These symptoms are even more noticeable in Diesel BMW’s, as the Mass Air Flow sensor also forms part of the operation of the Turbo. A Failed MAF Sensor on a BMW 320D or BMW 330D can cause the Turbo to spool up at a lot later RPM than normal, coupled with general loss of power and Torque and this is generally more noticeable when setting off and moving through the lower gears, and during Mid Range acceleration.

As a general rule, a MAF sensor will generally begin to deteriorate after around 60,000 miles of service, although they have been reported to fail as early as around 40,000 miles. So if your BMW has traveled above these mileage milestones, and has never had its MAF sensor replaced, then it is likely to be the cause of  running problems, poor idle or increased MPG.

Cleaning the MAF is rarely practical or effective and generally any improvements are only a short term solution, and often will not improve the situation at all, especially if the failure is electrical, and the problem is likely to return. Although considered relatively expensive, the Mass Air Flow sensor is often considered a consumable, especially on diesel variants, where failures are the more common. Generally when a MAF begins to fail, and a performance loss or general sluggishness is noted, then it is time to replace it.

The MAF sensor, is considered a critical component as it provides real time running parameters back to the engine management system, therefore buying cheap ‘white box’ MAF sensors, (often imported from China) from Motor Factors and Car parts suppliers should be avoided, purely because you could end up with a “new” sensor which makes your car run far worse than the failed original!. However you don’t need to resort to buying cheap Chinese unbranded parts, in order to save money, there are other alternatives that you can use which are equally as good as the original Bosch, and also manufactured in Germany.

If you don’t want to pay well into three figures for a Bosch MAF then fortunately, there are quality well made alternatives, which cost less than original Bosch MAF sensors from BMW. Manufactured in Germany by Pierburg, these low cost, but high quality MAF sensors will give a much better performance than the original Bosch OEM Part, and save you money at the same time. (Pierburg is a well respected manufacturer who supply components to Mercedes). The Pierburg sensor on the link above is suitable for 2.0 litre and 3.0 Litre Diesel engines, including 325D and 525D models upto 2007, and also 116i petrols.

However, if you wish to use the original Bosch Replacement MAF, then it can be bought much cheaper than from a Dealer – HERE

Replacing the Mass Air Flow sensor on a BMW is relatively straight forward whether you use the Pierburg or Bosch version, as it is easy to get to, being mounted at the top or side of the engine, on the Air Intake System. All that is required is a TORX hole style bit set, and around 20 minutes of time. The exact location of the MAF sensor can be found in the BMW TIS parts manual, which is for sale as part of our BMW DIS Diagnostic Package.

If you are replacing the Mass Air Flow Meter on any Diesel 2.0D or 3.0D Variant, such as the 320D, 325D or 330D, then you will also need to clear the adaption settings of the old MAF Sensor, from out of the Engine Management System. This is extremely important, as the slow deterioration of the old sensor, will have caused the engine management to make various adjustments or fall back to a preset set of running parameters in an attempt to correct the errors and compensate for the loss of accuracy / MAF Sensor failure, and so these will need clearing and resetting.

You can access and clear MAF Sensor adaption settings, by purchasing our BMW DIS Package

Some screenshots of the MAF Adaption Clear Process from within the Servicing section of the BMW DIS diagnostic Program are shown below

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