Engine Mount Failure

This situation may pertain to any vehicle after 1992 which use the fluid type motor mounts. Do you ever hear a rumbling noise when your BMW is idling with the A/C on? The early BMW's all have rubber engine mounts. Previously, a failed mount was evident since this could be seen as a mount broke from its base plate or compressed to a point where the sides would bulge or show signs of cracking. In the case of the new fluid style mounts that are encased in a metal housing there is little visible evidence of failure. The mount actually looks the same as a new one, only it would be dirty and slightly compressed. Not having a new engine mount in your possession for comparison would make a compressed mount difficult to detect. What occurs is the mount will compress internally to a point where the internal metal components begin to make contact during acceleration or when the A/C is engaged at idle.

Above is a photo of one of the mounts from our project car after 100,000 miles. The taller mount on the left is a new replacement unit. As the mount compresses, the engine oil pan may also make slight contact with the chassis sub frame causing additional noises. In many cases as the engine drops, A/C compressors will contact sway bars.

Since the engine is at a 10-degree angle towards the passenger side, the mount on the passenger side will typically be more severely compressed than the driver's side. You may also want to take a quick look at your transmission mounts. These mounts are the traditional rubber type, which will break down due to oil saturation that may occur from a slight oil leak at the rear transmission seal.