Probably have to remove the tailgate trim & give the lock actuator & wiring a good spray with WD40 as it's a water repellent as well as a...
Water around the tailgate lock wiring would be my guess.
These guys are highly regarded in the VAG world. http://www.r-techpower.co.uk/
I always liked the Revo stage 1 myself. http://www.revotechnik.com/
At 100k it's barely run in. I've remapped several 100k+ VAG 1.8t's without out issue. You don't need to upgrade any parts to go Stage1.
Coolant temp sensor & MAF would be my first suspects, both of which can be faulty without triggering the EML. Only way to really tell is to get...
Had a similar problem with a Bora which was caused by the Tandem pump leaking.
Niether an exhaust or an oil leak would show up on diagnostics....
Hopefully an S4 owner will be along to enlighten you better than me, but IIRC rattling timing chain on start up & shut down is the giveaway sign...
Looks like a nice one, but you have to be absolutely certain the engine,& gearbox are good. Deep pockets needed regardless as running costs aren't...
Timing chain/tensioner issues are common on S4's, & seriously expensive to fix so beware.
Link doesn't work.
You really need to get a diagnostic done to be sure of what's failing.
In the mean time.
Are your brake lights working? As the Traction Control &...
I got a used Denso AC Compressor on eBay for £50 about 3 years ago, & it's still going strong.
This may help you. http://www.audizine.com/forum/showthread.php/327299-DIY-B6-A4-Window-Regulator-amp-Motor-Replacement
Injector harness is a common failure, & is often evident at around 1800 rpm.
Does it stop juddering when you press the clutch?
Try AVF as it's the same rocker cover as the AWX.
TPS don't always serve the public, but if you say you're a taxi driver or an independent mechanic they usually do.