A friend of mine took his Chevy Impala into a local GM dealer for service, claiming it had a vibration in the engine at certain speeds. The dealer techs confirmed the vibration and also noticed that the oil pressure light intermittently flickered on and off, mostly during idling. This seemed to go away as the engine RPM increased.
The service crew installed a new oil pressure sender, but that didnt solve the problem. They checked the oil pressure, and although it was very low at idling, it came up enough at higher RPMs to be adequate. The engine ran fine, so they decided to concentrate on the vibration, which only occurred at certain RPMs and didn't change much whether it was being driven or free revved in the shop.
After a week of fruitless attempts to get rid of it by changing the damper, the alternator, the water pump, the power steering pump, and even the flywheel and torque converter, they were no closer to figuring out the problem. They returned the car, with a promise to look at it again once they heard back from GM. A week later, the engine made a horrible racket and died abruptly in traffic. The owner had the car towed back to the dealer, where the service crew tore it down and discovered the reason for the oil light, the vibration, and the sudden demise of the motor.
The crankshaft was broken in half. During initial assembly, someone had forgotten to install the center lower main bearing shell. The cast crankshaft had been flexing up and down at that point until it finally failed.
This entry was submitted by Dave James and edited by Rob Spiegel.
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