Hey guys, after hours of research I am still looking for answers. My 2010 S5 dips in and out of purring like velvet or tapping away, which I believe is down to the RON 95-99 petrol I am using. My local Sainsbury's only delivers 97 RON. This hasn't helped at all but after discovering Tesco 99 RON the engine has improved however it's still unpredictable and will more often than not knock rather than purr. Can anyone share their experiences to get me back to purring. 

Hello Craig,

I could well be wrong, but I would have thought that your knock, no-knock symptoms are not purely down to where you get your petrol. My suspicion is there is something unreliably temperamental going on here - dodgy camshaft sensor or the like perhaps. Worth getting this plugged in when you have the symptoms. 

Having said all of that, I've given up using supermarket fuel in our cars (as of this year) and all in the mind it might well be, but I would certainly say that the cars are performing better on the supposedly like for like grades of fuel. A pal has an aging diesel and was plagued by the EML light frequently coming on. He changed to non supermarket fuel and after a short spell, that car has decided to give up on illuminating the light and gives a lower fuel consumption. OK, I appreciate there will be many with alternative views, but for me it's now non supermarket fuel at a neighbouring station at around the same price.

Kind regards,

Gareth.

Hi Gareth,

Thanks for your feedback, I will get it plugged in to check so That I can rule out anything sinister. I don’t have many other options for fuel unfortunately but will probably stear clear of supermarkets. 

Thanks