I have been working on a surge problem in my upstairs toilet. It started about a month ago and I finally go around to working on it. I replaced the flapper valve, checked the seat which is clean and smooth. I replaced the float valve and float, adjusted the level to both extremes and many levels in between. The fill tube is installed correctly (I think) it is clipped to the top of the tube and can't siphon back.

I drained the whole system plumbing system and let it refill to purge any air, I've checked the water for excess air and it seems OK.

Still the problem persists, I get a surge of fill water every 30 sec. or so...

The water company installed back flow valves in past few months (not sure exactly when), but deny any problems.

I'm out of ideas except to unsolder the stand pipe associated with the toilet and see if it's clogged in some way.

Help!!

Just an update. I tried the food dye and the new flapper valve was leaking. I reinstalled it and it may have corrected the surge. I'll update if it reoccurs. Thanks to who ever suggested the diagnostic.

I am sure that flapper valve was the problem it is good that you replaced.

Also Check the "critical level" Mark at should be at least an inch over the rim of the white overflow tube.

Check the tank water level, it should be at the mark in the tank, 1" below the rim of the over flow tube when the tank is filled and the ballcock shut down, also check that the refill tube is connected into the over flow tube, see the images below for info,Thanks.

John