just got a 1998 a4, nice and clean but runs rough. so any help would be greatly appreciated. Let me say on my way home the cats went, they were shooting out hot embers, took a peek under and they were glowing red. went home, cut them off. I was hoping that's why it was running so rough but sadly it was not. So then I looked into the throttle body, I tried a few of the alignment/reset procedures to no avail. went though and replaced all the vac lines(they were falling apart), found the boot that the tb slides into to connect to the intake tube was half off. cleaned the tb gunk off and redid all the lines up nice. still the same symptoms, which are -very low idle, will die if I do not tap the gas. if I pump the pedal I can get it to rev out. every once in a while I get a little backfire pop from the intake. every so often when I let off the throttle it will run sweet and idle perfect for a second, then it dies or goes back to running rough. it has the cable and elec throttle. since it runs perfect every so often I would think it has to be something electronic. any ideas guy?

 

Get ahold of a scan tool and scan it to see if any codes come up - that's your first step. Running that poorly I would think there should be something in there to at least partially indicate what is causing it to run so badly.