BMW E46 / N46 – in cold conditions the engine cranked as if there was no compression

The car started and ran normally in warm weather or after spending a few hours in a warm garage. After being left outside in winter conditions, however, the engine cranked unusually freely — almost as if compression was missing completely.

• Initial suspicion: The symptom suggested a serious mechanical problem such as valve timing issues or actual compression loss.
• What made it unusual: The fault depended strongly on ambient temperature, which pointed away from a straightforward permanent engine failure.
• Finding: A piece of plastic from a broken timing chain guide had become lodged in the oil pump pressure relief valve.
• Effect: The debris disturbed pressure regulation and caused excessive oil pressure. When cold, thicker oil made the effect worse, overfilling the hydraulic lifters and preventing the valves from fully closing.
• Resulting symptom: Effective compression was lost during cold cranking even though the engine was not mechanically “destroyed” in the obvious way the symptom first suggested.
• Fix: The obstruction was removed and the oil pressure system was checked.

This is the kind of case where the first impression can look very close to major engine damage, even though the real cause turns out to be oil pressure control and a small plastic fragment in exactly the wrong place.