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.

AvusHatesCones said:

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?

bigjae1976 said:

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.

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.

It is definitely a concern for all e46s but truth be told, not all have been affected.  I have yet to see one facelifted E46 affected by the subframe cracking issue the pre-facelifted E46s have.

Edit:  If you want more details on this, pop over to forum.e46fanatics.com

michael1972 said:

What year is your M? It's was my understanding that by the 2003, BMW had strengthed the subframes because of all the problems with the earlier years.

Unfortunately, that's not the case at all. And mine is an 04. There are plenty of people with 06's that have the same issues.

REKIII said:

Remember, the three seris is the most basic BMW.  It's designed to be a 318 all the way up to an M3.  The M3 USUALLY gets different subframes, but the attachment points are the same.  The additional forces that the M's put on themselves is much more than a 318.  These extra forces work on paper, but not after lots and lots of cycles.

The guys running HPF kits and whatnot much be tearing their cars apart.

It's my understanding that this is just as common on 323/325/330's as well. It just snow balls once you get a little extra movement

The guys running HPF kits and whatnot much be tearing their cars apart.

AvusHatesCones said:

Yep, Jake has become quite the fabricator. There are a few theories on what causes these cracks, I'm assuming its simply bad/too small of load points for the subframe mounts. This is know to happen on E36's also, just not to the same extent.

He started out working for my Dad when he was 19. I used to use jake ALL THE TIME. But now I do mostly everything on my own. Not the transmission though. F that.

Jake is actually really good at what he does guys. And his prices are very fair. Especially for those jobs that you look at and say...I can do that...but i don't want to.

Do you think that happened as a result of BMW trying to keep the weight down or something?

jockhater2 said:

Really? Jake does this work? That's pretty cool! Hopefully he gets to my transmission soon.

I'm sorry to hear about this. I had no idea the E46 body can crack. How is that possible?

Yep, Jake has become quite the fabricator. There are a few theories on what causes these cracks, I'm assuming its simply bad/too small of load points for the subframe mounts. This is know to happen on E36's also, just not to the same extent.

StewCrew said:

Wow, that got real bad real quick! Out of curiosity, how much did it cost to get it welded? That's quite a crack.

Typically its right around 1k to have it all done. That includes tearing apart the whole rear end of the car, chasing cracks, welding, and installing the reinforcement kit. Which in all honesty, isn't all that bad considering most shops charge near that just to swap out a clutch.

I'm sorry to hear about this. I had no idea the E46 body can crack. How is that possible?