A note to all E46 owners.
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The subframe failure is the result of bad subframe bushings. The rear subframe is secured to the floor pan with 4 studs which are screwed in. What happens is the 4 subframe bushings go bad (typically the fronts) and then allows the rear subframe to twist everytime you accelerate and decelerate. This twisting loads and unloads the force of the subframe onto those studs which in turns leads to the metal fatiguing and cracking. Then continued use will result in your rear floor basically tearing itself out of the car.
First, address your driveline bushings. Typically motor and diff mounts fail quickly and are the first to go. Keeping the engine, tranny, and diff mounts, guibo, and center bearing in good shape will reduce the stress on your rear subframe bushings.
2004 BMW started using structural foam in the rear floor which helped.
The problem is there's no way to really see if the rear subframe bushings (that I know of) have failed. Most won't do preventive replacement due to the cost.
IMO, the fix is preventively reinforcing the 6 mounting points and installing solid rear subframe bushings. The solids won't add much NHV and will last a lot longer.
That's not true, my subframe bushings were replaced at 65,000 as preventative maintenance and again now. They never went bad nor did they have enough time to do so. Also BMW never added the structural foam from the factory. The structural foam was only used as a fix for cars with 2cm or smaller cracks, the rest had the entire rear floor pan removed. There are only theories on what causes these floor pans to crack, no true facts on what it is.
Silver Grey E46 M3
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That's not true, my subframe bushings were replaced at 65,000 as preventative maintenance and again now. They never went bad nor did they have enough time to do so. Also BMW never added the structural foam from the factory. The structural foam was only used as a fix for cars with 2cm or smaller cracks, the rest had the entire rear floor pan removed. There are only theories on what causes these floor pans to crack, no true facts on what it is.
So wait...you already had a crack then replaced the bushings and then didn't fix the crack at that time? Then you let the crack go for another 20k miles until this happened?
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No racks at 65,000 miles, bushings replaced anyway. Subframe inspected again 40,000 miles later, crack city, bushings replaced again, subframe fixed and reinforced.
Silver Grey E46 M3
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Yes, bushings can fail in 40k miles. Not unheard of. Especially if the diff mounts were on their way out. Usually if your subframe bushings are bad then your driveline mounts will be suspect.
I've went through motor mounts in my E46 330 (s/c'd) in a year. I had the powerflex rear subframe bushings which seemed to be fine after about 35k miles and 40 hours of track time.
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