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 didn’t 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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The clue was lower than normal oil pressure in addition to vibration; though one might suspect an oil pump or oil pressure relief valve issue before an internal leak (e.g. a missing crank bearing shell, block casting pinhole/crack into an oil gally) in the oil distrubution system, and might reasonably suspect two unrelated problems, one in the oil system another in rotating part balance. A missing bearing shell is a pretty unlikely cause, though *perhaps* could have been caught if the pan and perhaps valve covers too were removed to investigate the oil pump and distribution system).
Clearly though once broken and the missing bearing shell was found, assuming prior engine overhauls this is clearly a manufacturing defect, even if beyond the usual drivetrain warrenty when the problem (engine drive train vibration at sertain RPM's) was first logged.
Mydesign; First, I doubt it was the service person that forgot to install the bearing support. The engine was probably built at the Engine Plant, and then shipped to the Assembly Plant. At the Engine Plant this engine may have been partially assembled and then shift change happened, and the worker that continued to assemble the engine didn't notice the missing support and continued to the next step of the assembly. On the assembly lines that I have seen, one missing or extra part would not be noticed, especially not right away.
Glenn, how can they miss a part while assembling, that too crankshaft bearing. If it's a mistake from the service person, then obliviously they have to bear the expenses for repair and replacing the damage parts. Am not clear about when this story had happened; now a day's most of the service stations are automated and they used to maintain a check list for the spare parts.
Fortunately, it was still under warranty. This was quite a while ago - I think the engine was a 350. 350's were (and are) pretty tough motors. You really have to beat on one to hurt it. I've watched people try to blow them up (no oil, no water, full throttle - for an hour). It still ran. Smoked a bit though...
Pretty tough to diagnose this sort of thing. One always assumes that things are built correctly in the first place, and something has gone wrong.
Where should one start when diagnosing a vibration like that? When do you give up and pull the motor? I wouldn't want to be the one to make that call.
I once changed a water pump on the 350 in my Suburban. The motor shook like crazy with the new pump. How much rotating mass could there possibly be in a water pump? Evidently enough to shake the motor. A real GM pump solved the problem.
Many years ago (too many to admit) I worked for a local Heathkit Electronic Center on the "Jiffy Bench". I would take a look at kits that customers couldn't get to work, and try to fix them 'in a jiffy'. Usually it was just crappy soldering.
Sometimes, it was a bit more involved, and they had to sit in the Service Dept. for a few weeks before we could look at them.
One thing about fixing a kit - you must use a totally different approach to problem solving, as you could never assume that it was built right in the first place.
Some true horror stories (and some very funny ones) were generated in that place...
One kit I fixed had horrible soldering, but had worked for 25 years (poorly). I re-soldered everything, and turned it on. Bad idea. Some of the wiring mistakes weren't actually there due to bad joints. It blew up when I turned it on. Oops.
Good observation, Chuck. Quite amazing. I wonder if the car company would take responsibility for this even outside of a warranty. Seems this type of error would extend responsibility to the car maker even outside the standard warranty.
Beth, good observation. I have had lots of excperiences like this, although I was doing the wprk myself. After replacing a number of parts it tuens out that a major assembly has a problem that one did not expect. Once it was a cracked head. The push rods were bending. It turns out tha the head had been milled to raise the compression. This was a valid performance enhancement on another type of engine, but not on the one I had (an MG B). So, similar to the leaving out of a bearing, my friend had done something that was not valid. I treated the symptoms, not the disease. This is all too easy with an internal combustion engine.
I would hardly call a missing crankshaft bearing support 'obvious'. If the technicians had suspected the engine was assembled improperly or incompletely, my guess is that they would have replaced the engine and sent it to the engine manufacturer for analysis and tear-down. Even if they had a suspicion of a crankshaft support problem, I doubt they could have found it by dropping the oil pan for a visual inspection.
When your lab is only one scope, one meter, and one homebrew function generator and power supply, the scope is stage center. But this one wasn't working right.
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