We have a bathtub in our upstairs bathroom. The faucet has always leaked a little so we tried to fix it this week. We bought new stems and put them on. (I think that's what they're called... there's one for hot and one for cold, they go straight into the pipes and then the handles go onto them). While replacing the knobs, we turned off the water supply to the house. Upon replacing the stems and turning the water supply back on, the bath faucet runs immediately like someone is taking a bath. Even when the hot and cold knobs are in the completely OFF position. I think the problem is with the cold knob as that was the leaky one before and the water coming out of the faucet is cold.
For now, we put a plug in place of the cold stem so that we can have the water on for the rest of this house. But we really really really need to get this problem fixed immediately. Any ideas?
Thank you in advance!

There are only two knobs and no shower head. Just a plain old bathtub.

I think you may have bought the wrong stems. Check to see they are 100% exact replacements and that the seats and washers are in place. Did you do anything with the valve seat or just left it in place?

I spoke with the plumber about this problem yesterday and he suggested that we replace the T-looking pipe in the wall. The stems were exact replacements as the hot side worked great. After posting my first message, I felt around inside the stem hole and it felt like sharded metal in there. I was told that the brass seats inside were probably bad and I needed to replace the wall pipe. It comes in tomorrow, wish me luck!

Did you know that you can repair the seats for less money and less time than replacing the faucets? You can buy a repair reseating tool kit for about $10 and it comes with a variety of washers. That T looking pipe in the wall probably has nothing to do with your problem so I am not certain your plumber understood the issue. That was not a good suggestion for him to make.

I just had the same thing today with a set of Crane stems.
I had them reversed (hot and cold) I took the stems and switched them around and it worked perfectly.
Not sure if you have the same problem but its worth a shot.