Why does my sewage ejector pump run but not pump?

Liberty Pumps' troubleshooting chart for its LE-series sewage pumps has a row for "Pump runs or hums, but does not pump." It lists these causes:

• Air lock. The chart says: "Pump is airlocked." The fix is "Turn pump off and let set for several minutes, then restart." It adds "Add baffle to reduce trapped air bubbles."
• A stuck or backward check valve. "Check valve is stuck closed or installed backward." The fix is "Remove check valve(s) and examine for freedom of operation and proper installation."
• A closed valve. "Gate or ball valve is closed." The fix is "Open gate or ball valve."
• A blocked discharge line. "Discharge line blocked or restricted."
• A clogged impeller. "Obstructed impeller or volute." The fix is "Remove obstruction."
• A pump too small for the lift. "Total head is beyond pump's capability." Head is the height and friction the pump has to push against. The fix is to "Route piping to a lower level. If not possible, a larger pump may be required."

Zoeller's chart for its submersible pumps has a similar row, "Pump operates but delivers little or no water," and it adds electrical causes to the list: "Low or incorrect voltage. Incorrect motor rotation. Capacitor defective." A pump that hums but does not turn often points to the motor or capacitor rather than the piping.

Air trapped in the pump case is the classic cause, and the pump is built to move liquid, not air. Once the case is full of air instead of sewage, the impeller spins without moving anything.

Zoeller explains the fix built into the plumbing. Its manual says that if a check valve is part of the installation, a vent hole of about 3/16 of an inch "must be drilled in the discharge pipe below the check valve and pit cover to purge the unit of trapped air." Zoeller adds: "Water stream will be visible from this hole during pump run periods." It also says the hole has to stay open: "This vent hole should be checked periodically for clogging and cleaned as necessary." And it names the cause: "Trapped air is caused by agitation and/or a dry basin."

Liberty describes the same idea inside the pump: "Some LE and LEH-Series pumps come equipped with an air bleed hole to help prevent airlock. A small spray of water from this hole is normal while pump is running." After any clog is cleared, "the anti-airlock hole should be cleaned."

If your pump has been air-locking, the vent hole or air bleed hole is the first thing a plumber checks. Liberty's fix for an air lock is the one already quoted above: turn the pump off for several minutes, then restart it. If it happens again, the hole may be clogged or missing, and that needs a plumber.

Check three things, all from outside the basin, with clean hands and dry feet. First, look at the basin level through the lid or alarm if you have one. Second, find the valve on the discharge pipe above the basin. The plumbing code requires one there: the 2024 International Plumbing Code says "A check valve and a full-open valve located on the discharge side of the check valve shall be installed in the pump or ejector discharge piping between the pump or ejector and the gravity drainage system." If the handle on that valve, the full-open one, sits across the pipe instead of in line with it, someone may have closed it. Third, listen for a hum versus a spin.

Then stop. Note what you saw and heard: whether the motor hums or spins, whether the basin level is rising, dropping or holding, and whether a small stream shows at the vent hole, if your system has one, while it runs. Those three observations tell a plumber which of the causes above to test first.

Anything beyond that means opening a system full of sewage. Taking the check valve apart, pulling the pump and cleaning the impeller all mean opening a system full of sewage. Liberty warns "Sewage and effluent systems produce and may contain flammable and explosive gases" and "Do not enter a pump basin after it has been used. Sewage and effluent can emit several gases that are poisonous." Zoeller tells owners of its submersible pumps: "Do not attempt to service or otherwise disassemble pump."

Turn the power off at the breaker before anything else, as covered on our page about a sewage ejector pump tripping the breaker. Liberty's manual says "Always disconnect pump(s) from power source(s) before handling or making any adjustments." And don't keep flushing. If the basin is full and the pump isn't moving it, every flush adds to it.

No. A pump that runs and never shuts off is a different row in Liberty's chart: "Pump runs, but does not turn off." It lists air lock, a control panel switch left in Hand, a float that cannot move because it is jammed against the basin, and a defective float switch. Our page on replacing a sewage ejector pump float switch covers that case.

The difference matters. If the basin level drops, the pump is working and the float is the suspect. If the level stays put or rises while the motor runs, the pump is not moving sewage, and the causes above apply.

Call as soon as the level is not dropping, and sooner if a toilet or drain has started to gurgle or back up. See sewage backing up into a tub or shower for what that looks like. A plumber will check the vent hole, test and clean the check valve, clear the impeller, confirm the pump can handle the lift, and look at wiring or voltage, which may need an electrician.

Prevention is mostly routine checks. Liberty's quarterly list includes "Check for free and unobstructed float switch operation and float switch condition" and "Inspect for proper check valve operation." Two quick looks a few times a year catch a sticky float or a tired check valve before they leave you with a full basin.

HQ Plumbing & Air is licensed and insured, and our residential plumbing service covers the plumbing side. We give a free estimate, and you get the price up front before work starts. We take emergency calls 24/7. For how these pumps differ from a sump pump, see sump pump vs ejector pump vs lift station, or why a sump pump is not turning on.