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Matt
Matt, Mechanical Engineer
Category: Audi
Satisfied Customers: 20575
Experience:  BEng hons Mech engineering, in auto industry 22 years
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I have a 2008 A6 Avant which starts and drives perfectly

Customer Question

Hi, I have a 2008 A6 Avant which starts and drives perfectly smooth with no power loss or excess smoke. On the face of it, it seems to be in a very good condition despite the 210k miles. It was serviced (full) 4 months ago but even prior to that, I was experiencing the fault I'm about to explain. When the car has been running for a long period (especially through town) and it comes to a stop (just idling), it causes a severe shuddering/vibration. When the vehicle sets off again you wouldn't know there was an issue. When in neutral and it's shuddering/vibrating, a tiny pinch of throttle smooths it out but as soon as you remove the throttle, it returns to shuddering/vibrating. Any thoughts?
Submitted: 5 months ago.
Category: Audi
Expert:  Matt replied 5 months ago.

Hello

I'd agree that checking the engine & gearbox mounts is a good idea as they have an 'active' mount on these which can leak its oil out over time - if you inspect the mount and check for signs of leaking black thick oil from around and behind the mount

Also check the tightness of the engine and gearbox mounts and their condition - this is best done by using a jack to take the weight of the engine off the mounts and seeing if any cracks or splits open up in the rubber.Also check the tightness of the mounts bolts .

and if this is all OK then I'd say its worth checking for an airleak after the airflow meter, any air dragged in here isn't 'seen' by the ECU and so not compensated for causing rough running.

As its a mechanical fault it tends not to turn on the fault light and you can sometimes 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.

Now you might think that spraying lighter gas around a hot engine isn’t wise, however the flash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.

Work your way through each possible joint one at a time and you should find it. I use a slightly flattenedpiece 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.

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 the weather and engine temperature can effect them.

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

ncident.

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.

Expert:  Matt replied 4 months ago.

Hi

do you still need help?

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