either when the timing belt broke some teeth off the valves hit the pistons and its running rough.....(worst case scenario)
or when whoever replaced the timing belt and balance shaft belt, they didnt time one of the balance shaft belts correctly.
believe it or not there are 2 balance shaft belts, one to the left of the crank(the one the balance shaft belt goes to) and one behind the oil pump. the one behind the oil pump MUST be timed correctly when putting the timing belt on, even though the marks may be lined up the balance shaft will not be lined up. the only way to make sure it is in time without removing the oil pump is to pop a bolt out of the side of the block and insert a screw driver into the hole far enough to hit the balance shaft and then some.
99% of the time when i was working at a Kia dealership, when i got an optima in with an engine vibration, they usually just had a timing belt done at an independent shop and they obviously didnt know about this little trick.
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