www.jec.org.uk

So, when I say “beats”, I mean the effect when you have two similar but slightly mismatched frequencies that interfere to give a new vibration at the interference frequency.

My ‘98 XK8 recently had a serpentine belt and tensioner change. Was silky smooth before - changed belt due to age. But now, when the car is in gear, but held stationary on the footbrake, there is a palpable beat felt in the whole car (seat, pedals, steering wheel).

Any ideas what might be causing this gents?

Assuming the job has been done properly, it's unlikely that just changing the belt and tensioner for the correct quality components would add a 'beat'. Hence, it would be worth checking visually that nothing's been accidentally displaced around the front of the engine. If that's the case, is it possible the 'beat' is actually an engine misfire? Are any dashboard lamps illuminated? Mike.

They were OEM components, though I can't speak to whether the job was done properly other than to say I hope so since that's what I paid for!

Nothing looks obviously displaced, no warning lights, and the car runs fine when moving. I can't tell if the vibration is still there under motion due to the added road vibrations, but I can't feel anything over and above the road vibrations.

Without hearing/feeling the 'beating', it's difficult to make any other suggestion than to return the car to the garage that changed the belt and ask for a diagnosis. A Jaguar V8 which isn't silky smooth isn't normal behaviour, especially if there are no warning lights displayed. Mike.