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This shop is taking you for a ride. The timing belt won't cause this problem. If you have them replace it you'll still have a problem. It's a maintenance item. You need to get the van scanned so I can see what code it has stored inside the computer. If you have an Autozone or Advanced auto parts store near you they will scan the vehicle for free and give you the code. If you get that code I can get you an answer and try to fix it.
It sounds to me like you have overfueling problem OR a vacuum leak causing this problem. I would check fuel pressure on bank 1 (rear bank) and check for vacuum leaks to start.