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About 4 months ago, I bought a used Audi A5 3.0 TDI Quattro (247hp) having 75k miles at the time. A few weeks ago I was driving home (oil was at temperature), and wanted to downshift from 6th to 4th at speeds around 45-55mph. I missed and went into 2nd. (Just thinking about it still hurts me…I love the car and would hate to damage it by doing something stupid)
I’m not sure about these things (everything went very quickly) but I believe the revs went up to 5.500 rpm (redline starts at 4.500 and counter stops at 7.000). I think I could hear the limiter as well. Before knowing what was going on, I got power back, accelerated the car and shifted back up to 6th gear. The ride home went without any noticeable problems.
The day after I noticed a sound I had never noticed before (wether paranoia was making me notice every little sound, or it was actually new… I’ll never know). When accelerating from 45mph to 55mph in 6th gear, I could hear a low rumbling sound from the engine. (In any other car, this sound would be a pleasant deep sound from a good engine… but the engine bay in an Audi A5 is so well insulated you normally don’t hear a thing when driving)
Since that day, I’ve driven more than 620miles. The sound is still there, and now comes on at lower speeds, stays to higher speeds and is noticeable in gears 4 through 6 (it changes pitch when going faster). Maybe you can hear it in 3rd as well, but I never drive in 3rd gear at the speed the noise start becoming noticeable. I’m also starting to feel a slight vibration in the throttle.
Two additional symptoms:
– When braking at high speed (75mph) the steering wheel vibrates hard. I believe this was already there before my mishap, but not so hard.
– I was revving the engine in neutral to hear if the noise would appear standing still. I did not, but when I keep the revs up (2.000rpm) for a few seconds, and then release the throttle, the car shakes at the moment the revs drop below 1.000 rpm (engine idles at 600 rpm when warm)What I’ve tried to diagnose the problem:
– Looked at fuel consumption: no change.
– Looked at oil consumption: no change, but I have to rely on the computer since there is no dipstick in the car.
– Kept the car at a fixed speeds when I could hear the noise for a few seconds, and then release the clutch. The noise is still there (but becomes less noticeable for some reason).I first thought I ruined the engine and created loss of compression in one or a few cylinders (V6 engine). However, ECU does not detect any misfires (or equivalent term for diesels), fuel and oil consumption don’t change… I’m hoping I can eliminate this possibility.
I’m now thinking it’s a damaged wheel bearing or wheel noise (the tires don’t have to much thread left on them). I initially heard the sound at speeds around 44mph. Running on 19″ wheels, this makes the wheels turn at speeds around 25 rotations per second. A damaged bearing would thus create a noise equal to (or multiple of) 25Hz. This is approximately the low rumble frequency I hear. But then, what causes the vibration at idle? Just some ECU logic injecting a little fuel to keep the car from stalling when the revs fall down quickly? (I never did this before so I don’t know if it’s always been there, or new) Or do the symptoms then point to things like the engine mount?
I’m not a mechanic and won’t try to repair any damage myself. I will take it into the dealer. With this story to tell however, I fear they’ll just tell me to replace all kinds of expensive things without really knowing if it will solve the issue (engine replacement, clutch, gearbox, differential are all extremely expensive replacements for this car and brand). I would like some expert opinions what things could cause the symptoms, and which things I can eliminate (either directly or by doing some simple test). I will then take it to the dealer and hear what their mechanic has to say about it.
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