Why does my kitchen sink gurgle when the dishwasher drains?

The plumbing code spells out the connection. "The waste connection of a residential dishwasher shall connect directly to a wye branch fitting on the tailpiece of the kitchen sink, directly to the dishwasher connection of a food waste disposer, or through an air break to a standpipe." It adds that "The waste line of a residential dishwasher shall rise and be securely fastened to the underside of the sink rim or countertop."

That rise is the high loop. Whirlpool explains why it's there: "It prevents siphoning, which occurs when water is drawn back into the dishwasher due to pressure changes." A high loop or an air gap keeps sink water from running back into the machine.

So each drain cycle sends the dishwasher pump's output into the sink's tailpiece or the disposal, then down through the sink trap and into the branch drain. If anything along that path is partly blocked, or the vent behind it isn't giving the water enough air, the discharge pushes air back up through whichever trap is closest. In a double sink it can be the other bowl. The gurgle is that air breaking through the water. Oatey, a plumbing manufacturer, describes the general mechanism: "when a blockage closes off the pipe completely, the air will escape via the drain instead. And that's what's causing the gurgling."

Start at the kitchen and work outward.

1. The garbage disposal. If you have a disposal, the dishwasher hose usually connects to it. GE's troubleshooting advice is direct: "If a disposer is installed, run the disposer to clear the drain system." GE also warns you not to put your hands into the disposal to check it, and to run it before starting the dishwasher. If the disposal is slow or humming, see garbage disposal draining slow.

2. The knockout plug, after a disposal install. A new disposal has a plug in the dishwasher inlet that must be removed. InSinkErator says "The knockout plug is a small piece inside of the garbage disposal at the dishwasher inlet. It needs to be removed in order to properly hook up the dishwasher drain hose." KitchenAid puts the consequence plainly: "A retained plug blocks drainage completely." If the gurgling began after a disposal was installed or replaced, this is the first thing to check.

3. The sink trap and tailpiece. Grease, coffee grounds and food scraps build up under the kitchen sink. KitchenAid recommends you "examine the sink tailpiece, trap, and nearby plumbing for buildup that can slow or stop flow from the dishwasher." A slowly draining kitchen sink is the usual companion symptom. Our guide to unclogging a kitchen sink covers the trap and how to clear it.

4. The high loop or the air gap. If your dishwasher drains to an air gap, the cylinder on the counter next to the faucet, remove the cap and clean it out. GE says "Clearing the air gap will allow water to flow better as the dishwasher drains." If there's no air gap, the hose needs its loop. Whirlpool's guide says the loop should ideally be at least 20 inches from the floor, and that a loop lower than 20 inches calls for an air gap. Sagging hose or a low loop after under-sink work is a common find. KitchenAid recommends you verify "the drain hose maintains a high loop after any under-sink work or disposal replacement." For the parts and how they work, see what a dishwasher air gap is.

5. Kinks in the hose. GE says to "check the drain hose where it connects below the sink for a blockage or a kink in the hose." A kinked hose can show up as a dishwasher that leaves standing water, which we cover in dishwasher not draining.

If the disposal, trap, air gap and hose are clean and the sink still gurgles when the dishwasher drains, the restriction is further along. Here's how to sort it.

• Only the kitchen sink gurgles, and it drains slowly on its own. A partial clog in the kitchen branch, usually grease, is the likely cause.
• Only the kitchen sink gurgles, and it drains fine. That leans toward a vent that isn't supplying enough air to the kitchen line. What a plumbing vent does explains the parts.
• Other fixtures gurgle too. If a toilet or bathroom sink also bubbles, the problem is likely the main line. Check why a toilet gurgles and main line versus branch clog.

The other appliance-and-fixture gurgle patterns follow the same logic. For a sink that gurgles when the toilet flushes, see why a sink gurgles when you flush the toilet, and for one that gurgles when the washer drains, see why a sink gurgles when the washing machine drains.

Don't keep running cycles hoping it clears, and don't reach for chemicals.

Don't run the dishwasher repeatedly to see if it clears. If the line is blocked, each cycle pushes more wastewater toward a sink that can't drain it, and it can overflow into the cabinet. KitchenAid recommends turning off the dishwasher's power at the breaker before you disconnect anything. Skip drain-opening chemicals inside the dishwasher too. KitchenAid warns: "Do not use drain-opening chemicals inside the dishwasher; they can damage components and create safety hazards."

Once it's fixed, a few habits cut the odds of a repeat. KitchenAid advises: "Run hot water at the sink before starting a cycle to help dissolve grease and improve wash and drain performance." It also says to clean the dishwasher filter regularly, since a routinely cleaned filter prevents a clog from developing. If you have an air gap, take the cap off now and then and clear out debris. After any work under the sink, look at the hose and confirm the high loop is still fastened up under the counter. None of this takes more than a few minutes, and it's far cheaper than a clog that has time to harden.

For the kitchen line itself, what not to put down the drain is the way to keep grease and scraps from coming back.

Call a plumber if the sink keeps gurgling after you've cleared the disposal, trap and air gap, if water backs up in the sink during the dishwasher's drain cycle, if other fixtures gurgle too, or if you smell sewer gas. Those point toward a branch clog, a vent, or the main line.

Here at HQ Plumbing & Air we're licensed and insured, our estimates are free, and you get the price up front before we begin. A kitchen drain backing up is an emergency, and we take those calls 24/7.

When you call, tell the plumber whether the sink drains fine on its own, whether an air gap or disposal is installed, and whether the gurgle began after any recent work under the sink. Those three facts narrow the search fast.

The dishwasher is just the messenger. The sink drain is telling you it's out of room.

For other causes of gurgling drains, see why is my drain gurgling.