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Sometimes my toilet starts running for a few seconds at a time, like it's just finishing filling up the tank.
Usually this happens when the bathtub is being drained, or when I turn on the facets in the sink, but I think it also happens sometimes when no water is being used anywhere else.
As far as I can tell the flapper in the tank is working fine, and I don't see any leaks anywhere.
The typical toilet tank will have a flapper which seals the water in the tank. When you flush the toilet, the flapper is lifted and water is allowed to move from the tank into the bowl. Once a set amount of water has exited the tank, the flapper falls back down and re-seals the tank.
The most common issue with toilets randomly flushing is that the flapper has become brittle or sediment has formed on the flapper/tank which prevents the flapper making a complete seal. This will allow water to slowly drip from the tank into the bowl. If enough water is permitted to leak out of the tank, the filling mechanism will be triggered and the tank will refill. If enough water enters the toilet bowl, the self-siphonage will be triggered and the bowl will drain into the sewage system.
Here's a picture of a typical assembly:
My problem was that the chain was too tight and was exerting constant upward pressure on the flapper mechanism. By adding a link to the chain, the problem was resolved.
I had this problem before. It was due to the big plastic nut that hold the flapper unit to the tank was not tight enough, thus water was slowly seeping out. This may be your problem if the other suggestions are not it.
I think pdd's answer is the right idea. Almost certainly your flapper no longer seals properly. The fix is to replace the flapper. It's a $5 part and requires no tools to install. It's a 5 minute fix. I recommend replacing with a flat rubber flapper since a rounded/shaped flapper may not fit your drain hole well. They also make flat hard plastic flappers with large rubber washers attached, but I've found these do not seal as well as flat rubber.
If you want to confirm that your flapper is leaking, pour some food color into the tank and check the bowl a few hours later. If the water in the bowl is discolored, the flapper is not sealing as it should. If you have no food color, you can get blue plumbing dye from any home supply retailer or hardware store.
Put a lead fishing sinker inside the hollow flapper to better seal when the flap is down. Be sure the lead weight is big enough not to fall out through the hole in the flapper.
Always make sure the refill tube going into the overflow tube does not extend down below the fill line. If it does, the refill tube may slowly siphon water from the tank into the bowl causing water loss and intermittent running of your toilet to refill the tank. This mimics the effect of a leaky flapper. Some toilet repair kits come with an extremely long refill tube and many novice or DIY repairmen just shove the refill tube into the overflow tube extending below the fill line by several inches. The refill tube should always be cut and clipped to the top of the overflow tube.
It can happen for one of the following reasons
Generally the thing stops automatically unless there is an internal fault. Try draining all water and refilling the flush to see if there is any leak or anything.
I had the same problem with an upstairs toilet. A plumber came over and fixed a Pressure Reducing Valve (PRV) valve in the basement that adjusted the water pressure into the house and havent heard a thing since.
I had a long chain that was stuck between the stopper and was causing it not to seal. I'd recommend checking around the stopper.
Another thing not mentioned here, but along the lines of the PRV...when you turn on a water faucet then turn it off abruptly, there is a slight, momentary increase in pressure, which may cause the fill valve seal to lift. This motion will also cause the float ball to 'bounce', causing a run-stop-run-stop situation at the valve. Usually only lasting a few seconds. I hope I explained this satisfactorily.
I have seen this happen when there are no vent pipes. The water flowing down the pipe can create a vacuum and draw water out of your p-traps and toilets. If that happens you will usually know because you will get odours coming from your sinks and bath tub drains. If you have no way to vent them there are vents they make that you can install under your sinks. They can draw are in but will not flow back so water won't back flow through them and sewer gas cannot come up through them.
