Subframe / trunk floor / differential mount failure
This thread concerns an issue with the Z3 known as "subframe failure." Subframe failure is a known problem and all Z3 owners and potential owners should be aware of it, be able to diagnose it, and be familiar with the solutions.
Please don't post any replies to this thread, but instead post replies here.
The Z3 chassis was originally designed for the 1.9 M44 four cylinder engine developing 138 horsepower. From 1997 to 2002 BMW introduced progressively larger displacement/horsepower engines, from the 2.5 M52 six developing 170 horsepower through the final 3.2 S54 ///M version in 01' and 02' developing 315 horsepower. However, BMW did not re-engineer the chassis to handle the additional horsepower. There was also a problem of undetermined origin at the Spartanburg plant that effected the spot welding of the trunk floor and frame crossmember. These two issues combined have led to many failed trunk floors, differential mounts, and the frame cross member that joins them to the trunk floor. This issue has led to a catastrophic failure of the system on several Z3's, and can potentially destroy your car, leading to cost prohibitive repairs into the many thousands of dollars.
These engineering defects and production flaws combined are referred to here as the "subframe issue," although they in fact do not involve the subframe directly. The good news is that there are complete solutions. The bad news is that they can be very expensive to have done (many hundreds to thousands). There are also lesser solutions that cost less.
BMW North America has systematically refused to recognize this issue publicly, although privately they addressed several instances by replacing the floor, cross member and mount with OEM replacement parts. The OEM replacements also fail. BMW Europe has released a reinforcement, but it is not substantial and doesn't address the whole problem.
By universal recognition in this and other forums, the definitive and best solution is called the Randy Forbes Subframe Kit (RF kit), and was developed and refined by fellow member Randy Forbes, who remains active--both on the forum and in his shop in Florida. He sells his kits and installs them as well. The kit is a steel weld in reinforcement of the cross member and floor, and redesign of the differential mounting system. There are several "certified" RF kit installers throughout the country and across the world--RF can direct you to the nearest one to you. There are other weld-in solutions and kits, and links to these are provided below, but they are universally recognized as being less effective than the RF kit.
It is generally agreed that a part of the overall problem (and this does involve the subframe) is that the subframe bushings mounting the subframe to the body are too soft, allowing too much play, and placing an undue stress on the differential mount. There are polyurethane subframe bushings available in the aftermarket to address this aspect of the problem, with the Ireland Engineering (IE) bushings being generally recognized as the best option. Many believe, and there is anecdotal evidence to back this up, that going to the IE poly subframe bushings can prevent the problem from developing in the first place.
The problem appears in one of two (or a combination of both) forms: spot welds in the trunk "popping," and stress cracks in the differential mount. Below are pictures of the differential mount, and what to look for there, and also are pictures of spot welds popping, and what to look for in the trunk. You can also see pictures of what it looks like when the system fails.
There is a school of thought that advocates reinforcing the first popped spot weld or two in an effort to stave off the problem. There are links to this discussion included below as well.
There is a long standing decade old and many many page thread called the "Definitive Subframe Strategy Thread" that documents the growing awareness of the problem, BMW's shirking of it, and the development process that led to today's solutions. Feel free to post in that thread, it is a living evolving thread and an active discussion--this post was developed in that thread and is intended to put you on notice of the problem, how to diagnose it, fix it, and possibly avoid it.
Below are links to threads addressing various points, from inspection, to the RF kit, to all the other options and opinions. Also following are various pictures you need to reference to thoroughly understand and diagnose the issue (or lack of it).
One final note: the problem appears more often in the higher horsepower Z3's (like the ///M's). If you own a 1.9 automatic, it is unlikely that you need to be concerned with this issue. In fact, pretty much any 1.9 or automatic is likely not an issue. Some small number of 2.5's (2.3's) have been reported as having the issue, it is more common, but still rare in 2.8's, increasingly common in 3.0's, and quite common in the 3.2 ///M (both S52 and S54 engines). If you plan on supercharging or turbocharging your Z3 (forced induction, or FI), you need to address this issue first.
PICTURES (courtesy of Randy Forbes)
This a typical view of the trunk floor exhibiting spot weld issues:
Here is a close up of some bad welds, so you know what to look for:
Here are some stress cracks in a differential mount:
And here is a total system failure--note that the cross member has completely seperated from the trunk floor and exhibits its own stress cracks--you can see where the spot welds have ripped out of the trunk floor:
On this example, the differential came down and dragged on the ground, you can see how it looks compared to a good cover:
There is seam sealer in the trunk, and some models have sound deadening, which can make it difficult to diagnose the problem:
LINKS TO OTHER THREADS (courtesy of Reddy Kilowatt):
Steps to access trunk floor
Pictures of asphalt mat removal and re-install
Repair photos & bushing recommendation
Link w/ pictures of cars post-repair
More repair photos
Reinforce the popped weld