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Hello Rob,

My '02 Jetta TDI surges when driving. I thought it was my automatic transmission but I had it checked at a shop and it was found to be operating properly. The technician said it must be something else so I changed fuel filter, the air filter, changed out vacuum lines, cleaned my EGR valve, and cleaned my air sensor chip.

I'm looking for advice as it still surges. Can you shed light on this? I like the car, it has ample power and runs well except for the surging.

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Thank you very much, Herman

Herman, it's too bad you didn't have a chance to read my May 31, 2010 column on using the "shotgun approach" to car repairs. This approach is costly, time consuming and frustrating. Instead, consider the symptom from a systems point of view – that is, the diagnostic routine should be focused by possible historic and frequently common failures.

Look for technical information that points to common driveability failures with diesel Jettas. I found a Service Bulletin from Volkswagen listed at Alldata that points to a faulty MAF sensor (Mass air-flow).

I am impressed that you thought to clean the "sensor chip," assuming that this is the piece located inside the MAF sensor – so good call!

For the uninitiated, an MAF sensor is a tube placed in the air intake system between the air filter and the intake manifold. Some early fuel injection designs used this component to measure the amount of air being taken in by the engine. Inside the tube is a tiny sensor and a control module that sends air-flow signals to the Engine Control Module (ECM). The ECM uses this info to calculate fuel, ignition and emission control actuation.

Herman, it appears that your diesel suffers from a malfunctioning sensor, but let's understand that this sensor is now nine years old and has put up with a large amount of vibration over the years. This is what typically destroys these devices. However, (here is where cause and effect gets interesting), because the MAF measures air getting into the engine, it can be affected by circumstances that seem unconnected.

Cases in point:

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• Leaking EGR valve

• Leaking crankcase breather

• Leaking vacuum hoses (some of which you have addressed Herman)

• Oil dip stick not fully seated

• Intake system leaks

Each one of these problems will affect the amount of air entering the engine and anytime air enters the engine – at any point – that is not measured by the MAF, driveability issues will pop up.

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MAFs are not used often anymore is that there are too many sources of possible problems.

With focus on the MAF, it should also be noted that the service bulletin goes on to illustrate a full inspection of the mechanicals of the engine as well as the electronic control systems of the diesel. Mechanical integrity of an engine cannot be over-stated. Without it, there is no way to expect proper operation of the engine or the support mechanisms attached to it.

So Herman, take your Jetta to a competent shop – one that has the electronic diagnostic equipment to properly test and validate the MAF sensor and the electronic support systems attached.

Send your auto maintenance and repair questions to globedrive@globeandmail.com

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