Priming is the process of filling the pump and suction hose with water so that the impeller can create the vacuum that lifts more water in. A pump that runs at full speed without picking up is either not sealed on the suction side, not filled, or being asked to lift from too far down. The checks below go from the most common cause to the least, and most pumps are back in action by step three.
Quick answer
Fill the pump casing completely with water through the priming port, make sure every suction-side connection is airtight with its gasket in place, confirm the suction hose is rigid and not collapsed or kinked, keep the suction lift under about 20 to 25 feet, and clear the strainer. Give the pump a few minutes at full throttle to prime. If it still fails, suspect the mechanical seal, the impeller clearance or a stuck check valve.
In this guide
Step 1: Is the casing full?
Self-priming does not mean the pump can start dry. It means that, once the casing is full, the pump can evacuate the air from the suction hose by itself. Remove the priming plug on top of the housing and fill it until water overflows. On a long suction hose, fill the hose too if you can. Running a pump dry for more than a minute or two also damages the mechanical seal.
Step 2: Find the air leak
Even a tiny air leak on the suction side will defeat priming, because the pump pulls air in through it instead of lifting water. Air leaks are the single most common cause.
- Check that the gasket or O-ring is present in every suction coupling. A missing gasket at the strainer coupling is a classic.
- Tighten every hose clamp on the suction hose, and check the hose for cracks, pinholes and worn ends. Flex it and listen.
- Tighten the priming plug and drain plug on the casing.
- Check camlock arms are fully closed and their seals are intact.
- Make sure the strainer and hose end are fully submerged, with enough depth that a whirlpool does not pull air down.
Step 3: Suction hose and strainer
- The suction hose must be rigid or wire-reinforced. Lay-flat discharge hose collapses under vacuum and the pump will never prime.
- Look for kinks and sharp bends, and avoid high points in the hose where air can collect.
- Use a hose the same size as the inlet, and keep it as short as possible.
- Clean the strainer and keep it off the bottom so it does not clog with silt or weeds.
Step 4: Suction lift
Atmospheric pressure can only push water up about 25 feet in practice, and most engine-driven pumps are rated for 20 to 26 feet of lift. The higher the lift, the longer priming takes and the less flow you get. If the water surface is more than about 20 feet below the pump, move the pump closer to the water or lower it on a platform. Altitude reduces the limit further. Our guide to pump head and flow explains the physics.
Step 5: Give it time and speed
A long suction hose at high lift can take several minutes to prime. Run the engine at full throttle; a pump at idle may never build enough vacuum. If the pump primes and then loses prime repeatedly, you have an air leak that opens as the pump vibrates, or the water level has dropped below the strainer.
Step 6: Check the discharge side
A pump trying to push against a closed valve, a kinked discharge hose or an extremely high discharge head may struggle to expel the air from the casing. Open the discharge fully during priming, and if the pump has a discharge valve, crack it so air can escape.
Step 7: Internal wear and faults
If everything outside the pump is right and it still will not prime, look inside.
- Mechanical seal. Water dripping from the weep hole between the engine and the pump means the seal is worn, and a worn seal lets air in. Pumping sandy water or running dry kills seals. Replacement is a routine repair.
- Impeller and volute wear. Grit widens the clearance between the impeller and the housing until the pump can no longer create vacuum. Some pumps have an adjustable wear plate; otherwise the impeller or volute needs replacement.
- Check valve or flapper. Many pumps have a flap valve at the inlet that holds water in the casing. Debris stuck under it, or a torn flap, lets the water drain back and air in.
- Cracked casing or priming plug threads. Look for seepage around the housing when it is full.
Priming checklist
- Pump level, close to the water, suction lift under 20 feet if possible.
- Rigid suction hose, same size as the inlet, strainer fitted and submerged.
- All gaskets in place; clamps and couplings tight.
- Casing filled to overflowing; plug refitted tightly.
- Discharge open and hose laid without kinks.
- Engine at full throttle; allow several minutes.
For choosing the right pump in the first place, see our water pump buying guide.
Frequently asked questions
How long should a pump take to prime?
Usually under a minute with a short suction hose and a modest lift. With 20 feet of hose and a high lift it can take several minutes. If nothing has happened after five minutes at full throttle, stop and look for an air leak.
Can a pump lose prime while running?
Yes. The water level dropping below the strainer, a loosening clamp, a cracked hose, or a worn seal all let air in. Air entering with the water also shows up as spluttering, surging discharge.
Why does my pump prime, pump for a while, then stop?
Either the strainer has clogged or lifted out of the water, the suction hose has collapsed as it warmed up, or a vortex at the surface is pulling air into the hose. Lower the strainer, use a rigid hose and keep it well submerged.
