on my 98 s4 i have had problems with the headlights not working so i went on a forum and was shown how to fix it by taking off the steering wheel and taking out the switch for the headlight and cleaning it. Before i did that i had to unhook the connections for the battery so the airbag wouldn't blow up in my face and when i fixed the switch and turned the car back on it idles funny. when it starts up it will jump from about 500 RPM to 1000 RPM and when i come to a stop it will almost die, sometimes it even does. was there something in the steering colum or switch that i messed up or was it when i hooked the battery back up and it screwed somthing up in the electrical.

 

Hmmm I don't think you could've messed anything up when you were poking around in the steering wheel. If you unhooked the battery you would'nt be able to short anything out either. Sounds to me like a mechanical issue but I dont know the older S4s to well. It could have something to do with your timing belt or maybe an injector or 2?

 

I don't know anything about your car but I do know there are some vehicles out there that have a special idle relearn procedure if the computer lost its memory of it, which could have happened from having battery unhooked long enough. You could call a dealer and ask them. It may eventually come out of it. This is just speculation though so good luck!

 

Could also check for vacuum leaks. I've seen many A4 1.8 and 2.8 behave like this with vacuum leaks.

 

You might need to resync the TB. Turn the on, but don't start it, leave it alone (don't touch ANYTHING) for 5min. Turn off, then start.

 

if all else fails get it checked for fault codes, even though the CEL isn't on it will store soft codes