Why won't my toilet flush all the way?

These two problems feel similar but call for different fixes, so sort them first.

A clogged toilet stops the water from going down. The bowl fills high, drains slowly or not at all, and may threaten to overflow. The obstruction sits in the trap (the curved channel built into the toilet) or in the drain line just past it. A plunger or a toilet auger is the right tool.

A weak flush is different. The water does go down, but the swirl is lazy and waste or paper is left sitting in the bowl. The bowl is not on the edge of overflowing. This points to a delivery problem inside the tank or at the bowl's water inlets, not a blockage in the drain.

A quick test settles it. Pour about a gallon of water from a bucket straight into the bowl, fast. If that bucket of water flushes the bowl clean, your drain is clear and the problem is on the tank or rim-jet side. If the bucket water backs up or drains slowly, you have a clog. This separates a delivery problem from an obstruction in under a minute.

This is the Phoenix problem. Under the rim of the bowl are small angled holes called rim jets, and at the bottom front is a larger opening called the siphon jet (or flush jet). Together they aim the tank water to create the swirling siphon that pulls the bowl empty. Hard water leaves calcium and lime scale that slowly narrows or plugs these holes, and the flush gets weaker month by month until it cannot finish.

You can spot it. Hold a small mirror under the rim and look at the jets. Healthy jets are open holes; scaled jets show white crust and may be fully blocked. A flush that pours straight down on one side instead of swirling all the way around is a strong sign the rim jets are clogged.

To clear them:

• Ream each hole. Turn off the water and flush to lower the level. Work a short piece of stiff wire, an unbent paperclip, or an Allen wrench into each rim jet and the siphon jet to break up the crust.
• Dissolve the scale. Heat white vinegar to roughly 120 degrees (not boiling, which can crack the bowl), pour it into the overflow tube in the tank so it runs through the rim channel, and let it sit. Many people leave it overnight. The acid dissolves the mineral.
• Flush and recheck. Run several flushes to clear loosened debris, then inspect the jets again with the mirror.

In very hard Phoenix water this buildup comes back, so plan to repeat it once or twice a year.

If the jets are clear, the next suspect is how much water the tank delivers and how long it holds the valve open. Fluidmaster, the largest maker of toilet repair parts, lists these among the leading causes on its weak-flush guidance, noting that for a strong flush the tank simply has to release enough water at once.

Low water level in the tank. A flush is only as strong as the water behind it. Lift the tank lid and find the overflow tube, the open vertical pipe in the middle. The water should sit near the fill line marked on the tank or about one inch below the top of the overflow tube. The rule of thumb: if the water sits more than one inch below the overflow tube, it is too low and your flush is starved. Raise it by adjusting the fill valve. On a modern Fluidmaster-style valve you turn the adjustment screw or twist the cap clockwise to raise the shutoff level; on an older float-ball valve, gently bend the float arm up. Raise the level until it sits just below the top of the overflow tube, never above it.

Flapper closing too soon. The flapper is the rubber seal at the bottom of the tank that lifts when you flush. If it falls shut before the tank has finished emptying, only part of your water reaches the bowl and the flush dies early. The usual cause is too much slack in the lift chain or a flapper waterlogged with age. Adjust the chain so it has about one-half inch of slack when the flapper is closed, enough to seal but no more. Too much slack and the flapper drops early; too little and it cannot seal. If the flapper is warped, stiff, or coated in scale, replace it. Hard water and in-tank bleach tablets both shorten flapper life, so Phoenix toilets tend to need this part more often.

While the lid is off, glance at the fill tube, the small hose clipped to the overflow pipe. If it has popped loose, the bowl is not getting its refill and the flush will feel weak even when everything else is right.

If the jets are open, the tank is full, and the flapper is healthy but the flush still struggles, look past the toilet itself.

A partial clog in the trap or drain. Waste, paper, or a foreign object can lodge in the toilet's built-in trap or the branch drain and leave just enough room for water to seep through slowly. The bucket test above flags this: if poured water drains sluggishly, work the spot with a toilet auger (closet auger), which is built to reach through the trap without scratching the porcelain. A flange plunger can clear softer blockages. Skip chemical drain openers in a toilet; they rarely work on a trap clog and can damage the bowl and harm the people who later service the line.

A blocked or restricted vent. Every drain ties into a vent stack that runs up through the roof. The vent lets air in so water and waste flow freely, and the International Plumbing Code requires venting to hold the pressure swing at a trap to within one inch of water column. When a vent is blocked by a nest, leaves, or ice, the draining flush pulls a vacuum instead, which slows the flow and can make the bowl gurgle or bubble. A telltale sign is cross-fixture behavior: the toilet gurgles when a nearby tub or sink drains, or other fixtures react when you flush. A vent that runs to the roof is not a safe do-it-yourself fix for most homeowners and is a good point to call a plumber.

Multiple slow or weak fixtures at once, or a lower fixture backing up when you flush, point toward a developing main-line blockage rather than a single toilet problem.

Work these in order, from easiest to hardest:

1. 1Run the bucket test. A gallon poured in fast tells you whether the drain is clear. Clear drain means the problem is in the tank or jets.
2. 2Check the tank level. Raise the fill valve so the water sits just below the overflow tube if it is sitting low.
3. 3Check the flapper and chain. Set about one-half inch of chain slack and replace a warped or scaled flapper.
4. 4Clean the rim jets and siphon jet. Ream and soak with warm vinegar. This is the highest-value step for most Phoenix homes.
5. 5Auger a suspected clog. If the bucket test was slow, use a closet auger.

Knowing the scale of the waste helps you judge urgency. The EPA's WaterSense program reports that toilets account for about 30 percent of an average home's indoor water use, and states that "household leaks can waste nearly 1 trillion gallons of water annually nationwide." A flush that needs two or three tries quietly drives your water bill up, so a weak flush is worth fixing even when it is only an annoyance.

Call a licensed plumber when the bucket test shows a slow drain you cannot clear, when you suspect a vent problem on the roof, when more than one fixture is weak or gurgling, or when the toilet is old and the jets stay clogged no matter how often you clean them. A pro can scope the line, clear a main blockage, or recommend replacing a worn fixture.

For related tank issues, see our guides on how to fix a running toilet and why is my toilet gurgling.