This is a common problem with the V8 engines in the 540/740s. It is either the radiator neck or expansion tank which WILL fail in the range of 38 to 80k miles (sorry). The prices for DIY have come down quite a bit from before. There will eventually be a aluminum radiator but it is still being developed. At least it was covered.
9-04 Mt. Hamilton
2000 740i Impala brown