2003 Passat shakes when idling after timing belt replaced

We recently had the timing belt replaced in our 2003 Volkswagen Passat (1.8T). Now it shakes and vibrates badly when we put it in drive and idle. It ran smoothly before we had the belt replaced. The mechanic hasn’t found a solution yet. Can you help explain this?

The most likely explanation is that he installed the timing belt one or two notches “off”.
If that is what happened, it would affect the valve timing and lead to uneven idle. Longer term, it can lead to engine damage.

Another possibility is that he failed to reconnect a vacuum line or two when he put everything back together. Still another possibility is–if it was necessary to loosen motor mounts (and I am not familiar enough with VWs to know if this is necessary) when installing the timing belt–it is possible that the mounts are not secure at this point.

I strongly suggest that you not drive the car.
Leave it with the mechanic until he figures it out.

I second VDCdriver and the mechanic should connect a vacuum gauge to the intake manifold.
It’s quick and easy to do and if the belt is off a tooth or two the gauge should reflect that.

The bad, and sad, part is that many mechanics do not use a vacuum gauge.