This video documents the diagnostic and teardown process of a BMW E91 M57 engine that was brought in for a transmission upgrade. Mechanics discovered the engine was extremely difficult to turn over by hand, despite having been driven into the shop with no apparent running issues other than a hard-starting complaint. After systematically eliminating external components and the valvetrain as the cause, a full teardown was performed. The investigation revealed a significantly bent crankshaft with a measured runout of 0.05mm. This imbalance led to consequential damage, causing the main bearing caps to fret or "wander," resulting in aluminum transfer from the engine block to the steel caps. The root cause is suspected to be a previous hydrolock event from a malfunctioning water-methanol injection system.
Diagnosing and Uncovering Hidden Damage in a High-Torque M57 Engine
0:00 - The Initial Problem: While attempting to perform a transmission upgrade on a BMW E91 335d, the mechanics discover that the engine is seized and cannot be turned over by hand to access the torque converter bolts.
1:10 - Deceptive Symptoms: The car had been driven into the workshop without issue and had completed a 2,000-3,000 km trip recently. The only symptom was a slow, difficult start, which the owner had mistaken for a bad battery. The engine ran smoothly with no abnormal noises or loss of power.
2:34 - Isolating the Fault: To diagnose the source of the seizure, all auxiliary components, glow plugs, and injectors were removed. Since the camshafts turned freely, the problem was isolated to the engine's lower rotating assembly (crankshaft and pistons).
3:01 - Suspected Cause: The team hypothesizes that the damage originated from a past hydrolock incident. A faulty check valve in the water-methanol injection (WMI) system had previously allowed the engine to fill with fluid, which likely bent the crankshaft or a connecting rod upon starting.
4:57 - Engine Internals and Oil Quality: During the teardown, the mechanic notes the exceptional cleanliness of the engine's internals after 20,000-30,000 km. He attributes this to the high-quality Motorex oil used, which effectively suspends soot and prevents sludge buildup.
5:48 - The Culprit Found: After removing the pistons and still facing resistance, the crankshaft was removed and measured on a lathe. It was found to be significantly bent, with a runout (wobble) of 0.05mm.
6:55 - Evidence of "Wandering" Bearings: The primary damage from the bent crankshaft was found on the main bearing caps. The imbalance caused the caps to micro-move, or "wander," leading to fretting and visible transfer of aluminum material from the engine block onto the steel caps.
7:55 - Next Steps: The bent crankshaft will be sent to a machine shop for professional analysis to confirm the extent of the damage and to be professionally balanced or replaced before the engine is rebuilt.