How To Fix a Leaky Toilet
What to do When Your Toilet Flushes or Runs Continuously
If you hear your toilet "flushing" or refilling by itself, it probably has a leak. You may even hear water constantly trickling out of the tank. Water is leaking from the tank and when the water level gets low enough, the fill valve automatically starts refilling the tank, just like after a flush.
The leak typically occurs in one of three places. If you have water on the floor, start by inspecting the tank bolts, which fasten the tank to the bowl. Next, inspect the water valve where the water supply line connects to the tank. If water only leaks when you flush, then the leak is at the gasket between the tank and bowl. Note: there are other leaks that can result in water on the floor, but they are not accompanied by the toilet refilling periodically.
If there is no water leaking onto the floor, then the leak is located at the drain flapper. One way to test this is to put some food coloring in the tank and then check the bowl after 20 to 60 minutes. If you see food coloring in the bowl, the flapper is leaking.
The two next possibilities involve a problem with the fill valve. If the fill valve and float are not adjusted correctly, the tank may fill continuously. Once the tank is completely full any excess water flows out the overflow tube. Adjust the float so that the tank stops filling before the water level reaches the overflow tube.
Finally, the fill valve may be malfunctioning. This is not as common of a problem, but when a fill valve fails, it may periodically or continuously let water flow into the tank. just as in the case above, when the tank gets full, the excess water runs out the overflow tube and flows into the bowl. Replacing the valve will resolve this problem.