Hi all I just picked myself up a cracking 97 S8 I love the car but she dose have a few issues and I am just wondering if anyone else has been in the same boat and can shed any light as to what it might be.

So from cold she fires up perfect and runs perfect until I shut the car off then when I start the car back up when its hot the CAT light comes on and has a missfire and will not go off again.

Thoughts are as follows...

OBD scan - Got a cheap scanner and came up with missfire cylinder 2 which led me to think coils - So I swapped the 1st and 2nd coils over but the missfire didn't jump (Still might be coils though and I am looking into buying new ones)

Ignition amplifier unit - Read online they can go bad randomly I bought a new one as it was cheap enough and already fitted but no differance.

Spark plugs - Took them out car has a full set of iridium plugs fitted but they all seem pretty fresh and good (might get another set also to rule this out)

My main thought is this when I took the engine covers off to check the coil's etc I found these two plugs on the top on the engine for the knock sensors they are totally borked and fell to bits in my hand Ive ordered two new knock sensors but I also don't know if that would be cause the missfire but they are getting replaced reguardless.

Weird thing is I can leave leave the car to cool off or overnight and she will fire right up again no problems plus the car will run perfect so long as I dont shut it off after a run while its hot. Its really got me scratching my head???

But thank you for any input it would be greatly appreciated

Cheers Johnny.

Looks a lovely car. The joys of an old Audi is that they end up with electrical gremlins mainly. 

On 5/28/2023 at 1:31 PM, Steve Q said:

Looks a lovely car. The joys of an old Audi is that they end up with electrical gremlins mainly. 

to true that is

I am currently experiencing similar problems, did you ever find out the cause of the misfiring?

Roni.

Johnny has not visited the forum for over a year so it is unlikely he will respond.

You can send a Personal Message the original poster Roni. Hopefully he will reply, and you can then let us know.

Kind regards,

Gareth. 

new plugs i put in and was sweet as nut eversince

Have had similar problems with my 1997 S8.  Fine when cold, but misfiring when hot and cat light on  

Managed to solve it my changing the misfiring injectors, but also rebuilding the alternator, which seems to have improved the current to the injectors and is making them more reliable.