The constant pressure valve (CPV) on the S54 engine comes from the factory with an o-ring made of a rubber not suitable for the high temperatures this engine regularly sees. This o-ring will harden and flatten, allowing oil to seep past it and down the engine block.
This leak is often misdiagnosed as a leak rear-main seal, as the oil accumulates at the bottom of the engine at the meeting point of the engine and transmission. Fortunately this is an easy fix.
We offer a replacement o-ring made of Viton fluoroelastomer with a temperature range exceeding 400F.
Simply remove your CPV (located on the right side of the engine, at the back of the block, near the transmission), carefully cut the original o-ring off, and replace with this o-ring, with a small amount of oil added to lubricate it for install.
The original o-ring will likely be very brittle and simply crack off using a pick or razor blade.
The CPV can be removed using a large flat head screwdriver and an adjustable wrench for leverage or preferably a flat bit or drag link socket. It can be replaced with everything still installed on the car, though it is nestled behind the rear header.
All E46 M3, Z3M, and Z4M.