We have classified the 2 complaints from 1998 BMW Z3 about SUSPENSION into the following categories.
I have a 1998 z3 m roadster and the trunk welds holding the subframe is failing. this is a commonly documented issues. bmw dealers acknowledge the issue but bmw north america will not fix it. this is a welding/manufacturing defect that can lead to the differential and suspension components in the rear to completely separate.
I purchased a used 1998 bmw z3 with the 1.9 liter engine on 10/14/2010 with 90564 miles. in mid november i took the car to a mechanic for a check engine light which was serviced. the mechanic informs me there is a problem with z3's along with other model bmw's with the way the rear differential is attached. the problem seems to be flexing in the sheet metal where the differential is attached causing failure of the spot welds holding the whole assembly together. he showed me in my trunk where two of the six spot welds had broken. i have researched this on the different bmw forums and bmw is claiming this failure is due to performance modifications or cars that have been taken to a track or autocross. i do not know the previous history of my car but it appeared to have been taken care of. on 6/30/2011 i had the car in the shop again for service work and my mechanic found two more broken welds and some separation in the rear chassis. this car is pretty much un-drivable now due to this condition. i understand there is a couple of companies that make a repair kit for this that will strengthen the trunk area and prevent this from happening again.
Rear stabilizer bar repaired several times with three replacements. mjs