The toilet is only 2 months' old. The water keeps

The problem may be the flapper that seals between the tank and bowl is leaking or the overflow pipe may have a crack in it. I wouled replace the flapper first and check the overflow pipe for cracks.
Hope this helps.
Bob

You have a partial clog, as if someone tried to flush paper towels or something that shouldn't have gone in the drain.

Try plunging, first, then a toilet snake, or you may have to get a pro to pull the toilet off the floor and run a larger snake if it's further down...

just replace the flapper with a universal flapper

i had the same issue few months ago take the lid off the back of it and check the floater if it is not where you set it will move to the overflow and run into the toilet. if that is not the issue then it is the plug flap either the toilet is not sealed properly or is deteriorating. the plug is easy to replace ($15 or less at walmart).

An installation guide for your toilet can be found here:

http://admin.totousa.com/Product%20Downloads/0GU013Z,%20ONE%20PIECE%20TOILETS,%20V.08.pdf

See the top of page 5 for guidance on setting the water level. Make sure you are turning the screw the correct direction (clock-wise) to reduce the fill level.

Is the refill tube properly connected? Should be connected to fill valve nipple and the other end clipped to direct flow to the inside of the overflow pipe.

Is water flow only through the overflow tube or is there leakage from the flush valve? Being only 3-months old you should not be having problems due to breakdown of rubber seals; however, check to make sure the toilet flapper is properly sealing against the flush valve.

If none of this helps with your problem you made need to replace the fill valve. On page 10 of your guide is a parts list; you can identify your part numbers and shop on-line for an idea of replacement costs:
https://www.totostore.us.com/e2wShoppingCatalog.aspx?parentId=3100066448&parentLink=2100000107:3100066448

How old is the toilet? Might have lime buildup in the housing that's keeping the float from raising completely. The whole assembly is relatively inexpensive at Wal-Mart, and very easy to change. It would probably be less hassle than trying to fix the old one. I found that out after wasting a lot of time trying to mix my old one. :-)

There are cheep fill valves and ther better built fill valves, so make sure you buy aa quality fill valve. Also buy a pressuer gauge at ahard ware store and check water pressure level at a faucet outlet. Screw gauge onto water fauct leave a little room for air to escape when water is drippng out tighten the hose bib on the gage turn on the faucet and read the pressure. If it's more than 50 PSI you need to lower the water pressuer. I need to know if your on a, on site underground well,or are on city water to tell you what you need to do to to fix this problem. If there is one. thanks for using Fix-ya if you want more info post here.
Avid101

Kohler is correct. The most likely cause of this problem is an obstruction in the common vent stack that both toilets share. When you flush the old toilet it need more air to flush and will get it from the easiest source (your new water saver toilet). First adjust the float level in the old toilet so it won't use as much water or just stick a brick in the tank to take up space.Second, run a snake down the vent stack from the roof to check for obstructions. Birds will sometimes nest in there.