The E46 rear drive-line and suspension are very similar to that of the E36 chassis. An unfortunate side effect of this is that the E46 also shares some of the same problems as the E36. One of these problems is the rear mounting points for the sub-frame, which can rip out from their spots in the sheet metal. This happens because the rear differential transmits the torsional load from the engine through the sub-frame, then into the chassis. This constant loading and unloading weakens the sheet metal and causes it to fatigue and separate from the chassis. BMW tried to fix this problem in the E46 by placing a cross member on the front two mounting points of the sub-frame. This cross member keeps the front mounting points from having problems by placing the bushing and the stud in double-sheer, thus reducing the twisting action on the sheet metal. The unfortunate side effect of this is that the load from the front of the sub-frame now gets transmitted to the left rear of the sub-frame. BMW has also tried to prevent the issue by injecting foam into the open space between the underside of the chassis and the top of the trunk. This seems to work but that does not address the issues of the spot welds failing on the sheet-metal. It can also make any additional work there harder and almost impossible to weld against.
Doing it alone in the parking lot, was a hell of a job, but not impossible.
The job was done under a slim budget, with improvised tools since I am not a shop owner. I didn't wanted to buy any expensive tools for a one time job.
- remove the brake caliper and hang it somewhere ´´safe´´. In order not to loose the braking fluid and then to remove the air from the lines, I preferred to leave it on the chassis.
- next remove the brake disk so you can loose the handbrake cable.
- sometimes you cant remove the handbake cables from the subframe, because they are stuck. If this is the case, you can loose them also from the hand-brake lever. You´ll figure it out how.
- loose the lower control arm from the chassis.
- loose the prop-shaft 6 screws from the joint with the diff.
ATTENTION! This ones are VERY tight so you I recommend a pneumatic tool to loose them, or if your biceps is strong enough, you could unloose them with some leverage. Just make sure the torx wrench is well sitting on them and not at an angle. If you loose the wrench from the screw because it was angled, you make the nut round and your chances to remove the nut close to 0.
- with the car still standing on the ground you loose the end of the shock absorbers from the trunk.
- remove the sub-frame 4 screws and the reinforcing aluminum cross-bar.
- you can now lift the car up on jack-stands.
- DO NOT GO UNDER THE CAR IF IT IS NOT ON JACK STANDS!!! Wood supports or regular jacks can fail and you could have 1.5 tons of steel on top of you. Never go under a car or put your legs under a car that is not properly sitting on jack stands.
- after the car is up the suspension should come off along with the entire assembly like in the pic above.
Now you can do your welding at the chassis and change the bushings.
Although BMW are very good quality cars, way better than most of the cars that are on the market today, it can happen that things like this get omitted, or not considered when they test them. What surprises me is that how come the guy who designed the subframe didn't saw this coming. It´s so logic. The diff stay in the subframe, the subframe stays in the chassis. Where do the forces from the diff go? In the subframe. And then`? In the chassis. Can the chassis take it? OF COURSE NOT! It´s just a piece a thin metal sheet with NO re-enforced areas at all. How can it?