If your toilet fill valve won't shut off, you've got a running toilet, and that has more than one possible cause. In fact, the culprit might not be the fill valve at all, but the flush valve, because, if it's leaking, the fill valve has to keep running to maintain the water level in the tank. Before exploring that possibility, it's best to dispense with possible malfunctions of the fill valve itself.
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Try Adjusting the Float
A Fluidmaster fill valve, which sits on the left side of the toilet tank, is made entirely of plastic, and the float rides up and down on the valve body instead of hanging out on the end of an armature like a traditional ball-cock-style fill valve. The float is connected to the valve by a plastic rod, and by adjusting the length of this rod, you can adjust the water level in the tank.
If the fill valve won't shut off, and water is spilling into the overflow tube in the center of the tank, lengthening the rod to make the valve shut off sooner may be all you have to do. You'll see a nut on top of the rod that you can turn either by hand or with the help of a flat-head screwdriver. Turn it counterclockwise, then flush and wait for the tank to refill. If the valve stops before the water reaches the top of the overflow tube, your job is done.
Flush Out the Fill Valve
If you can't make the fill valve stop by adjusting the float, it's possible that debris has lodged inside the valve that prevents it from closing. You may be able to clear this by flushing the valve.
- Turn off the water.
- Grasp the valve cap with one hand and the valve tube with the other and turn the cap in the counterclockwise direction. It should pop off in your hand.
- Turn the cap over and hold it over the valve, then turn on the water for a few seconds and let the spray hit the cap and fall into the tank.
- Replace the cap, turn on the water and check the valve operation. Repeat until you're sure this strategy isn't going to work.
Service the Flush Valve
If you can't fix the fill valve, you can always replace it. but before you do that, make sure the toilet flapper isn't leaking. There are several ways to test it, but it has to be leaking badly to keep the fill valve running all the time, so the easiest test is to just fill the tank, turn off the water and note whether the water level drops. If it does, lengthen the flapper chain by two or three links to make the flapper seat properly. Or you can replace the flapper. If that doesn't improve the situation, you may have to replace the entire flush valve.
Some toilets, particularly those from Kohler, have canisters instead of flappers, and when the flush valve leaks, you simply have to replace the seal on the bottom of the canister. You can remove the canister by turning it counterclockwise. If you pull off the old, worn seal and install a new seal, the leak should stop.