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Matt, Engineer
Category: VW
Satisfied Customers: 21995
Experience:  Mechanical Engineer with 20 years experience in the auto industry, 8 yrs in formula 1 engine testing
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VW Jetta: jerking..accelerate..the spark plugs are all worn

Customer Question

Good day,

I own a VW Jetta 5, 2.0 FSI (2006 Model), recently it has started jerking as you accelerate. I thought that perhaps it could be becausae the spark plugs are all worn out and had them all replaced with new ones bought directly from VW itself. This discomfort is still continuing and i'm now not sure what else to check in order to determine the root cuse of this. please help
Submitted: 6 years ago.
Category: VW
Expert:  Matt replied 6 years ago.

This could be an airleak after the airflow meter, any air dragged in here isn't 'seen' by the ECU and so not compensated for and can lean the engine out and can also allow the engine to rev up when not desired causing rough running.
As its a mechanical fault it tends not to turn on the fault light and you can sometime hear a 'hissing' noise with the engine running.

Check the hose clips for tightness and inspect the trunking for any cracks or splits and also all the vacuum system, the small bore pipes and fittings for cracks and missing parts.

The best way to locate a leak is to have the engine running and warm and then spray lighter gas /propane around each joint in turn. If the engine rev's up you've found your leak.
Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.

It’s also worth getting the fuel pressure checked as if this is low due to a blocked filter or faulty regulator or even a poorly pump will all result in insufficient fuel being delivered to the engine

Might also be worth checking the wiring and connector to the airflow meter for any signs of corrosion or damage. you can do a quick fault find if you unplug the meter and run the engine without it.
if the engine condition is the same then chances are the meter or the connection to it is faulty

Air leaks are very temperature dependent as gaps can open or close up as things expand with heat, so they can be effected by the weather and engine temperature.

This leads them to be quite intermittent in the case of mild leaks

its also worth cleaning the airflow meter by removing it from the car and spraying the exposed sensor wires inside the tube with a brake or switch cleaner - ensure that the cleaner is one of the old fashioned, non Eco type that does not leave a residue

On no account touch the sensor wires with anything physical as they are extremely fragile
Customer: replied 6 years ago.

Will it help if I replace my airflow meter as it was damaged during a car accident - with a cone filter instead... will this have any ramificatios to the car's perfomance?

Expert:  Matt replied 6 years ago.

If the airflow meter has been damaged then it should certainly be replaced as this sensor is the primary source for all the fuel calculations by the ECU.

While you can can do away with it and just have a filter plonked on the end of the intake the ECU will be relying on other back-up sensors to get the car to run and performance , fuel economy etc will all be sub-optimal