Document A4 8D, Rumble/Drone Noise (Solution)

Loud rumbling/throbbing noises, even causing vibrations in the seat, when accelerating in the upper gears.
The engine speed is between 1400 and 2000 RPM. The engine is running unevenly, and the automatic transmission is in emergency mode.

Up to chassis number: Fg.-No. 8D XA 261 212.

Cause:
1. Tensioner roller for the timing belt on the camshaft drive is difficult to turn, seized, or the timing belt tensioning element is defective. This can cause deviations in the valve timing. In extreme cases, this can result in error code 00550, "Injection start control - deviation in engine control unit." In vehicles with automatic transmissions, this can trigger the transmission to enter limp-home mode.

Performance variations between individual cylinders lead to discrepancies in fuel injection amounts during idle and overrun conditions, which in turn cause undesirable resonance vibrations.


Solution:
1. Remove the timing belt tensioner roller and check its smooth operation on the mounting bolt; replace if necessary. Check the timing belt tensioner mechanism to see if it can be compressed. The timing belt tensioner is oil-damped and should only compress slowly with even force. The timing belt tensioner mechanism must automatically extend; replace if necessary.

Check measurement block 13+14.

If the tolerance range is outside of the specified limits and the compression pressure is within acceptable parameters, then the fuel injectors must be replaced. Afterwards, re-check the measurement data block.