hesitates at acceleration

I had my mechanic replace the knock sensor on my mercury villager 96 at a cost of $500 labor and part. After repair I noticed that at acceleration the car hesitates and then goes. My mechanic is still working on the problem. It looks like when he was putting everything back together after replacing the sensor, he made a mistake. Any idea what could be wrong?

First you should have not replaced the knock sensor. It rarely ever fails. The only reason its there is to warn you of other problems. Look at some other threads. Post some previous info and I can help you.