hi everyone! i'm new here so please be gentle & patient. here's the long story:
i have a 99 a4 avant quattro 2.8 manual trans w/ about 92k miles. i've had it for about a year now & although it's got some minor quirks its been almost trouble free until now. last week , after parking it in the garage I noticed an intermittent humming noise coming from the front. it thought it was the aux fan so I left it alone. I come back after a couple hours & I could still hear it so i popped the hood. both fans aren't spinning but the noise is still there. it was coming from under the airbox & would go on for about a minute then stop only to start again after a couple minutes. I started the engine & everything seemed ok so i left it alone. the next morning the battery's dead. I went online to get some answers & found some here. it seems that the audi has a secondary air injection pump & based on my car's symptoms & what i've read here so far i might have a faulty relay. next thing i did was remove the battery so i can find out if i can still recharge it or replace it. it was still taking a charge & keeping it. i start the engine to check everything & it seems to be ok - it was idling fast, as usual, for the first 30-40 sec with the air pump going but then it (pump) turns off & the idle becomes rough & loping around 600-800 rpm. i turn off the engine & remove the relay (the gray one marked 373 from the ecm box below the driver side wiper) so i can get a new one. the next morning i started the engine just to see any change without the relay. same thing - idle is fast w/ the pump on then going low & loping after it turns off. i get a new relay & install it & same thing. at this point i have no idea if either of my relays are faulty or could it be a different problem altogether? did i cause more problems by the battery running down & disconnecting it? i read somewhere that disconnecting the battery "erases" the ecm's memory & all info will have to be "re-flashed" by the dealer. could it be an ignition or vacuum problem that's causing the loping idle? all the time i was starting the engine the CEL never went on (after first turning on & off at initial startup).
again, thanks for your patience & reading my problem. i try to provide all the info i can to make the troubleshooting easier. thanks for your replies.

 

If the pump is going on with the relay removed you probably removed the wrong one.
As` far as the idle , just drive the car around until the ecm relearns. Should be fine unless there is another problem lurking.

 

When you pulled the battery, over the course of the next period of time the caps in the ECU discharged and put the ECU in relearn mode. The loping idle is caused by an unsynchronized throttle position sensor. On the drive by wire ATQ engine the throttle body would need an adaptation. On your AHA cable-driven car it's simply the throttle position sensor. You can force-adapt it with vagcom or passively reset it by hooking up the battery again after a period of a few hours. Turn the key on but don't start the car. Let it sit a couple minutes. Turn it off for 5 minutes or so and do it again. Do it a third time and you'll find the car is either recovered or will be after a short stint of driving.

 

thanks for the help, guys. i did what you adviced & there's improvement. the idle is a bit smoother & the loping is less but still not like how it was before. i'm gonna keep driving it to see if it gets better.