I have an XJ6 2.7 litre diesel - year 2008. There is a really bad vibration which I have been unable to identify. The characteristics of it are strange - see below :-
1 There is no problem at slow speeds e.g. 30mph - never. Because most of my driving is in built up areas, it could therefore very well be that the problem had been there for some time - just unnoticed.
2 There is no problem when the engine is cold - or at least for the first few miles of a journey - irrespective of speed.
3 However, if the car is driven at (say) 50 mph or above, after (say) the first 6 to 10 miles of any journey, there is a really bad vibration, felt mainly via the steering wheel, which rapidly turns a fraction of an inch to the right and then to the left. It would not be exaggerating to describe the experience as "scary". It is not pleasant to hold the steering wheel, because of the vibration (but of course you have to).
4 If you select 'neutral' with the transmission lever, whilst this is happening, (hence disconnecting the engine from the drive train) it does not make any difference to the vibration. This surely suggests that it is NOT the engine that is running unevenly - because the engine has now gone down to idling speed.
5 However, If you then slow down to (say) 25mph-30mph, the vibration disappears - or at least reduces to a really miniscule movement of the steering wheel, which perhaps you would not really be worried about.
Any ideas? Any help would be greatly appreciated. The obvious - i.e. balance the wheels - has already been tried. It made no difference whatsoever.