could someone please help me with this problem? I have a 1994 acura integra LS. The crank pulley came off while it was being driven. i would like to know what could have caused this problem so that i can start the process of getting it fixed.  

First question; Did you get a new timing belt installed recently, say, within a month? That is the only thing that would come to my mind as a reason for the pulley coming off. The service tech that performed the job may not have torqued the pulley bolt to spec. Or he may have neglected to double check the bolt while doing other things in the general area. It happens some times to the best techs, so please don't be too harsh on the tech if this is the reason. He should have double checked his work and there is no excuse, but again, it does happen to the best techs as well.
Second, if you did not have a timing belt installed, what other work may have been performed in the general area that involved the crank pulley being removed.
Again, there is no other reason why it would come off unless there was work done involving it to be removed. The other thing that comes to mind is, and may be more impossible but not completely... was it impacted by some thing like wood in the road that you may have gone over or some other foreign object? The way it sits in the engine bay makes it a little tough for any thing like that happening. You may need to see if the remaining portion of the bolt is still on the crank, if that is what happened. If so it is completely repairable, assuming that nothing is wrong with the crank and surrounding components. Let me know what you find.  

I forgot one other possibility... some automotive companies use dampening material, like rubber, sandwiched between two pieces of metal. The rubber portion of the crank over time breaks down due to dirt, greasy oil deposits that fall on it, temperature extremes from engine heat to weather, etc. causing it to over time fall apart and giving you this issue. I'm not too familiar with what was used as a pulley on your particular model, but it's worth looking into. If this is the case it is no ones fault but time. Removing what's left of the pulley on the crank and replacing it with a new pulley is all that's required, again, assuming nothing esle was damaged when the pulley fell apart. Hope this helps you!  

thanks for the reply.i had a timing belt done about three years ago and a water pump about two years ago.i was told that the inside of the pulley was worn down or that the part where the bolt goes in may be worn. i have both the bolt and the pulley.do you think i might need a new crank.and also at times my car would rev unexpectedly on acceleration.i thought that it was my transmission slipping.do you think this had something to do with the crank?  

With out looking at your car I can't positively give you a real answer. Generally, though, with what you're telling me it is a possibility that the reving is due to transmission issues. I'm going to assume that you drive an automatic. If it's a manual the problem is in the clutch and requires replacing. Similar symptom, different repair. Your pulley and bolt are in hand? wow! I don't know what to say to that. I wouldn't drive any where!! AT ALL! There is a small piece of metal that is on the crank, it's called a keyway shim. If that is still in there, please, please, please don't turn it on to go any where. That helps keep the timing pulley, found under the plastic cover behind the crank pulley, from spinning out of timing!!! Assuming it hasn't fallen out, you may still have a real chance of a repair! Again, assuming nothing is damaged at the crank. The crank it self will need to be visually inspected for damage. Again, I can't be sure 'cause of me not being there. If you're not too mechanically inclined you will need to gwet your car to a shop any way. With both the pulley and the bolt in hand it is not safe for the engine to drive. It's time for you to get your car towed! Who knows, If there is nothing damaged, the repair may only be to replace the crank pulley with a new bolt.  

I forgot to mention... knowing how long it's been since you replace the timing belt helps in not blaming the tech who did the job for you. It's been quite some time that it's been done. If it was his fault the failure would have happened a lot sooner than now. A LOT SOONER! Another possibility is that you may have some one that hates you and purposely loosened the crank bolt or maybe some one is trying to get your attention... romantically. Things to consider, seeing the issue at hand 'cause it's hard to beleive that it would come loose like that all by it self after all the time it's been there from your last engine repair. I'm curious... did you feel a vibrationcoming from the engine at idle? Because if it was loose for a while before it fell off that would've been an early symptom giving you, or a tech, a false diagnosis. Things like that rarely, if ever, happen so diagnosis for a loose pulley is never really looked at. I hope this all helps you. Get back at me.  

Just browsing through, saw the post. Is the motor still intact? *** comes unbalanced and other fun stuff when that happens  

That was one of the reasons I told LilladyLS to get it towed instead of driving it. I'm not sure if those engines are non-interference engines but if it is, all it would need would be to realign the timing marks and set it. Assuming no other damage was done and the key way shim wasn't missing!