A wheel loader was brought in with a terrifying symptom. Under heavy load-like pushing into a gravel bank-the engine emitted a deep, heavy metallic knock that sounded exactly like a spun rod bearing. The shop immediately shut it down and scheduled an engine out-frame overhaul, fearing the worst.
Before pulling the engine, the foreman decided to do one final diagnostic test. He put a stethoscope on the torque converter bellhousing instead of the engine oil pan. The knock was distinctly louder at the bellhousing. He pulled the engine inspection cover and locked out the engine from starting, then had a mechanic bar the engine flywheel over by hand.
There was a slight "clunk" felt in the rotation. Using a flashlight, he found the culprit: a cracked flexplate. The torque converter bolts to a thin, flexible steel plate (the flexplate), which is bolted to the engine crankshaft. Over thousands of hours of harsh directional changes, the flexplate had cracked around the crankshaft mounting bolts. At idle or under light load, the two halves of the flexplate stayed aligned. But under the massive torque of pushing a gravel bank, the cracked halves sheared slightly against each other, causing the heavy torque converter to lag behind the crankshaft for a fraction of a second and slam into the bolts, creating the deep, metallic rod-knock sound. A $100 flexplate replacement saved a$15,000 engine overhaul. If an engine knocks only under severe load, always verify the flexplate integrity before condemning the bottom end.
