Why won't my portable water pump prime?

Start with the water in the pump

A portable engine-driven pump that will not prime is usually short of water in its chamber, drawing air through the suction side, or trying to lift water higher than it can. Most fixes take a few minutes and no tools beyond a screwdriver or hose-clamp driver.

The first check is the simplest. Before starting the engine, remove the filler cap from the pump chamber. Fill the pump chamber to the undersurface of the discharge port with water.^[1] Put the cap back on and tighten it. The water in the chamber is what lets the impeller start pulling air out of the suction hose, so a pump started dry has nothing to work with.

If you have already run it dry for a while, stop. Operating the pump dry will destroy the pump seal.^[1] If the pump has been operated dry, stop the engine immediately, and allow the pump to cool before priming.^[1]

Hunt for air leaks

Air getting into the suction line is the most common reason a full pump still will not draw water. The pump cannot create enough vacuum if air is coming in somewhere along the way.

Look at each joint on the suction side. Use a hose clamp to securely fasten the hose connector to the suction hose in order to prevent air leakage and loss of suction.^[1] Verify that the hose connector sealing washer is in good condition.^[1] A flattened, cracked or missing washer can let air in.

Then look at the hose itself. The suction hose must be reinforced with a noncollapsible wall or braided wire construction and cannot contain any perforations.^[1] A garden hose or a thin-walled discharge hose used on the suction side tends to flatten as soon as the pump pulls on it. Do not use a hose smaller than the pump's suction port size.^[1]

Check the strainer and the end of the hose

The strainer on the end of the suction hose keeps leaves, mud and gravel out of the pump. Never operate the pump without the strainer installed.^[1] A strainer packed with debris, or sitting in mud on the bottom of a pond or ditch, starves the pump just as well as a leak does. Lift the hose end so the strainer sits in clear water, and make sure the whole strainer is under the surface so it does not pull air.

Reduce the lift

Pumps can only pull water so far. Minimizing suction head (placing the pump near the water level) is also very important for reducing self-priming time.^[1] Honda's manual notes that the suction head can never exceed 25 feet (7.5 meters),^[1] and in practice a long run of hose and a high lift make priming slow or impossible.

Move the pump down to the water, set it on firm, level ground and shorten the suction hose. Keep the hose rising steadily toward the pump so air is not trapped in a high spot.

Use full throttle and give it time

Once the chamber is full and the suction side is tight, start the engine, then move the throttle lever to the FAST position for self-priming.^[1] Priming is not instant, and your pump's manual says what to expect. If the pump takes far longer than that with water in the chamber, stop and look again at the clamps, the washer and the hose rather than letting it run dry.

Safety while you work

Run the pump outdoors. Do not start or operate the engine in any closed or partially enclosed area, such as a garage.^[1] Carbon monoxide is an odorless, colorless and toxic gas,^[2] and the EPA lists generators and other gasoline powered equipment among the sources.^[2] That includes a pump pulling water out of a flooded basement: keep the engine outside and run only the hoses indoors.

Pump only the liquids the manual allows. Pumping flammable liquids, such as gasoline or fuel oils, can result in a fire or explosion, causing serious injury.^[1] And if the discharge hose crosses a driveway, remember that driving over a discharge hose while the pump is running, or even possibly when the pump is stopped, may cause pump case failure.^[1]

Stop and call a pro if

• The pump ran dry and now leaks from behind the pump body. The pump seal is the part the manual says dry running destroys,^[1] and replacing it means taking the pump apart. A small engine shop can do that.
• The engine runs but the pump never draws water even though the hoses and strainer are sound. The cause may be inside the pump, such as a worn impeller or a damaged casing, which is shop work.
• The engine will not start or will not hold speed. That is an engine fault, not a priming fault. See why a chainsaw won't start for the fuel and spark checks that apply to most small engines.
• The pump is cracked or has been driven over. A cracked case will not hold prime and should not be run.
• You smell gas or see fuel leaking. Stop, move away from ignition sources and have the pump serviced.

Getting help in Virginia

If the pump is working but weak, why a log splitter is slow or weak and how to fix a pressure washer that won't build pressure cover similar pump and engine checks. Browse more small engine repair answers or look at small engine repair pros near you.

When you are ready, describe your water pump and local small engine shops can quote the repair.