Coming off the highway today & stopping at the light at the end of the ramp, my 154k mi 2001 S8 T6 shuddered, feeling like it was going to stall, then started sounding like a diesel. There is a noticeable sluggishness and the sound gets slightly louder under load.

When I take my foot off the gas while driving, the sound is mostly gone, but it is definitely there at idle (normal, 750 rpm, not surging) and definitely there under load.

Getting home, letting it idle and lifting the hood, it sounds like the normal valve train, only amplified about 4x in volume.

Any ideas that I want to hear (as opposed to the really expensive ideas I'd rather not hear)?

 

Usually a burnt valve. Get codes read if you have a CEL.

 

I managed to put a reader on it tonight & there are three codes:

P1171 Fuel Adaption, Bank 1
P0154 02 Circuit No Activity Detected (Bank 2, Sensor 1)
P0446 Evaporative Emission System Vent Control Circuit

 

I finally had the opportunity to get the car to my mechanic. He listened to it and had a look; the timing belt is loose (likely a bad tensioner or idler) and he seems to think that it slipped causing the timing to either advance (or retard) causing the diesel sound.

He won't have an opportunity to get into it until Monday, but if that's the case, it seems I may have dodged a huge bullet (provided the valve train checks out once the timing belt is replaced).

 

Ultimately, the tensioner completely failed causing the belt to skip a couple of teeth disrupting the timing but not enough for valves and pistons to have a meeting of the minds.

We replaced the belt, both pulleys, the water pump and put on a new cover and all is right with the world.

I'm still working on removing the horseshoe from my @ss.

 

You won't really know that for another 10 to 15,000 miles.