A more modern everyday engine
Hydraulic tappets, Motronic management and a belt-driven camshaft make the M40 feel like a step into the late 1980s. It is not a performance icon, but in a light 316i or 318i it offers useful economy, straightforward parts availability and enough pace for relaxed road use.
Camshaft wear
A pronounced top-end clatter, reluctance to rev and visible scoring under the oil cap can point to camshaft and follower wear. Oil quality and neglected spray-bar lubrication are relevant. Diagnose before buying parts: noisy injectors, a loose exhaust shield or tired accessories can imitate internal noise.
Belt history is non-negotiable
As with the M20, documentary proof matters. Replace an unknown belt and tensioner immediately and inspect oil seals that could contaminate the new belt. A clean cooling system and correct thermostat help the small four-cylinder work at its intended temperature.