Sunday, 8 November 2009


Fault Diagnosis:

This article relates to the Volkswagen Golf mk4 1.8T VW MAF fault diagnosis but the general information is also likely to be useful for many VW / Audi group vehicles.

Noticeable variations in fuel consumption and the occasional subtle or not so subtle hesitations when accelerating could well point to a faulty mass air flow sensor or VW MAF sensor. This is a relatively common issue with the VW Golf 1.8T engine. More serious issues which can be caused by a faulty VW MAF sensor include really poor running, not revving through the whole rev range and stalling / cutting out.

The purpose of the VW MAF sensor is to measure the amount of air which is being drawn into the engine. The measured air flow value is used by the engine management system (ECU) to adjust the air / fuel mixture for optimum performance, ensuring the engine does not run too lean (surplus air) or too rich (surplus fuel). On the mk4 VW Golf 1.8T engine the VW MAF sensor is located between the air filter and turbo intake pipe here:

A faulty VW MAF sensor will cause incorrect information to be supplied to the ECU causing the engine to run incorrectly. Although poor fuel consumption and engine hesitation are classic symptoms of a faulty VW MAF sensor, these symptoms could very well be caused by a variety of other reasons.

The first step to establish whether a vehicles poor running performance is related to the VW MAF sensor is to electrically disconnect it. This is easily done by operating the tab on the electrical connector and pulling away from the VW MAF sensor, as shown:

Electrical disconnection of the VW MAF will cause a warning lamp to be permanently illuminated on the dashboard (traction control / ESP) and the ECU will now use default estimated air flow values associated with other engine running characteristics instead of actual measured air flow values from the VW MAF sensor. If disconnection of the sensor causes the engine to run better, it’s an obvious signal that the sensor could be the cause of the problem.

Whilst this could be a step in the right direction in diagnosing the fault, it does not conclusively prove the VW MAF sensor to be faulty. If the engine runs better with the MAF sensor disconnected, what has actually been proven, is that with the MAF sensor connected, the ECU was being provided with inaccurate readings causing the engine to run poorly – this is not necessarily the fault of the VW MAF sensor.

Whilst it would be reasonable to assume the VW MAF sensor faulty and install a replacement in an effort to resolves the issue, the sensor could actually be functioning perfectly well and the issue lie with a leak in the pipe between the sensor and the turbo inlet or a leak in the pipe between the turbo outlet, intercooler and engine manifold. Any of these leaks would make the VW MAF sensor appear to be faulty.

A leak between the VW MAF sensor and turbo inlet will pull additional air in through the leak (and into the engine) that has not been measured by the sensor. The ECU will not be aware of this extra air and will, in turn, calculate the wrong air / fuel mixture, causing poor running issues.

If there is a leak on the other side of the turbo ie. from the turbo outlet , through the intercooler and into the engine manifold, then some of the air measure by the VW MAF sensor will be lost to atmosphere through this leak and there will be less air going into the engine than communicated by the VW MAF sensor to the ECU which will cause the wrong air / fuel ratio to be calculated and poor engine performance.

Other useful tools to diagnose a VW MAF fault on the Volkswagen Golf 1.8T include the VAG-COM software tool which connects to the vehicles OBD-II port via a laptop and special connection cable. This software allows the actual value of the VW MAF sensor to be monitored and any deviations from the normal range of values assessed.

Replacement of the VW MAF sensor is pretty straightforward, disconnect the electrical connection and the clips onto the airbox and turbo inlet pipe pull the VW MAF sensor out and fit the new one.


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