Complex sentence: Valve stem seals often harden and start to pass excessive oil, causing a smoky exhaust on cold starting and after the engine has been allowed to over-run-this will worsen over time as the seals deteriorate further.
Split into simpler sentences: Valve stem seals often harden and start to pass excessive oil, causing a smoky exhaust on cold starting and after the engine has been allowed to over-run. This will worsen over time as the seals deteriorate further.