I have just bought a friend's 1999 model A4 B5 1.9 Tdi Avant (auto) for a smallish sum. It's very high mileage (175,000 ), but everything works, there's no rattles, the bodywork is in good condition, and it drives well. It was maintained up to 120,000 by the local Audi dealer, and then by a big fleet-maintenance operation, so has been looked after. Has only been used for motorway journeys.
The only thing I could find wrong when I test-drove it, was a vibration on the over-run, most pronounced at around 2200rpm, most noticeably when cruising at 60-70mph. Same vibration is also evident when revving the engine whilst stationary. So, it seems there is some coupling of the engine to the bodywork in some way, particularly when the drive-train is driving the engine. I reckoned it could be engine mounts, exhaust mounts or routing, or similar.
I took it along to my local independent Audi specialist for them to diagnose the problem, but, having spent a half-day looking at the car, they have informed me that they are not sure what the cause is. It's not the exhaust, or any pipe-work, They feel it may well be the engine mounts, but rather than waste my money on trial and error fixes (they didn't charge me anything, which was nice), they suggested I try to find someone with some other ideas who may be able to pinpoint the exact cause, eg maybe Audi dealer with access to a database of problems and symptoms.
So, whilst applauding the honesty of the independent Audi dealer (and also the fact that he didn't charge me anything), I am back to square one.
Anyone have any ideas? What's the cost of a speculative engine-mount change?
Paul
|
One thing that costs very little to try is to make sure that the engine mounts aren't pre-stressed against each other. To eliminate this as a cause, you slacken the bolts on all the engine mounts, backing off the nuts just enough to allow the mount to move. Then, go for a brief drive round the block, making sure that you use full drive and over-run. When you get back, tighten them all up.
This is something that I've done many times with various cars, and jiggling the mounts around into their relaxed positions can sometimes make a difference.
Number_Cruncher
|
One thing that costs very little to try is to make sure that the engine mounts aren't pre-stressed against each other.
Thanks, good suggestion and easy to try.
|
|
|
|
Vibrations on the overrun make me wonder about things like flywheels...
|
On this power unit installation there is a "snubber" mount at the front - bottom centre. It disengages when you pull the lock carrier forward to the service position to change the cambelt.
Check this first - the rubber is prone to fret because it's not a bonded mounting (well done, VAG) and is cheap to fix.
If you feel a cambelt change is coming on, that would be an idal time to take a look.
659.
|
|
|
Vibrations on the overrun make me wonder about things like flywheels...
Not sure I understand the logic behind your observation. Can you explain further?
I should also add that the vibration (really just a thrumming noise) is definitely related to whether the engine is driving the car or vice-versa. Driving along the flat, the thrumming comes and goes as the road undulates, even while the accelerator pedal is kept steady. Revs are hardly changing at all when doing this, pointing to torque reaction in the drive-train as the cause.
|
Pauly
There is a clear reference to this being a vibration "on the over-run" in your first post.
This engine will likely have a dual-mass flywheel.
They eventually start to come apart and vibrate....
...replacement is phenomenally expensive....
...the usual advice is to "Get rid - quick...."
...you've just bought this car......
See where I'm going?
|
Don't panic at this stage. The symptoms are consistent with a snubber which has fretted to death on one side. Due to my friend Mr. Newton's third law, the power unit winds itself nose up on drive and nose down on overrun. Have a look here first.
659.
|
Ok, thanks, will take a look.
|
|
|
|
This engine will likely have a dual-mass flywheel. ....
Ok, I understand what you're saying having had a quick "Google". Would a DMF fault give the same symptoms when stationary? Ie is the vibration linked to engine revs or under/over-run, or both?
|
Pauly
They can do many things; not all predictable or expected...
Stick with the easy things first, as 659 suggests.
|
Stick with the easy things first as 659 suggests.
Snipquote!
Ok, thanks. Will post back later with the verdict.
|
|
|
|
|
The O.P. says it is an auto, so surely no DMF but another possible "box of tricks"
|
Frogeye
Well spotted. So that's eliminated one possibility.
[Glasses - must get new glasses...]
|
|
|
|
|
|
|
