What Causes a Pump to Lose Prime (and How to Fix It)

If your jet or transfer pump keeps losing its prime, the problem is almost always on the suction side. A pump loses prime when air gets into the suction line or the pump can no longer hold water, usually from a leak, a bad foot valve, a low water level, or worn seals. Find the air, and you find the fix.

Below are the common causes, how to re-prime, and when a self-priming pump is the better long-term answer.

What Losing Prime Means

Suction pumps need to be full of water, or primed, to move water. They pull water up by creating low pressure, and that only works if the pump and suction line are free of air. When air sneaks in, the pump spins but moves little or no water. That is losing prime.

Common Causes of Lost Prime

• Air leaks on the suction line, from loose fittings, cracked pipe, or bad thread sealant
• A failing foot valve or check valve that lets water drain back down between runs
• Low water level, so the intake pulls air instead of water
• Worn seals or a worn impeller that can no longer hold suction
• A clogged intake screen or foot valve that starves the pump

How to Re-Prime a Jet Pump

With the power off, remove the priming plug on top of the pump and fill the pump body and, ideally, the suction line completely with water. Replace the plug, then start the pump. It should catch prime within a minute or two. If it will not hold prime, you have an air leak or a bad foot valve to track down.

When a Self-Priming Pump Is the Answer

If you fight prime constantly, a self-priming pump is designed to re-prime itself by clearing air from the suction line automatically. They are ideal for irrigation, water transfer, and any setup where the pump sits above the water source. It will not fix a serious air leak, but it removes the daily headache and protects the motor from running dry.

Preventing Lost Prime

Seal every suction fitting with the correct thread tape or compound, keep the foot valve and intake screen clear, and make sure the intake stays below the water level. On shallow wells, confirm the water table has not dropped below the pump. A little maintenance keeps prime holding season after season.

Final Thoughts

Lost prime is a suction-side problem. Check for air leaks and a failing foot valve first, re-prime correctly, and if the problem keeps coming back, a self-priming pump ends it. Not sure which pump suits your setup? Browse shallow well pumps and deep well jet pumps or ask our team.

Frequently Asked Questions

What does it mean when a pump loses prime?

It means air has gotten into the pump or suction line, so the pump spins but cannot move water. Suction pumps must stay full of water to work.

Why does my jet pump keep losing prime?

The usual causes are an air leak on the suction line, a failing foot or check valve draining water back, a low water level, or worn seals. Check the suction side first.

How do I prime a jet pump?

With the power off, remove the priming plug, fill the pump body and suction line with water, replace the plug, and start the pump. It should catch prime in a minute or two.

Will a self-priming pump fix the problem?

A self-priming pump re-primes itself automatically and removes the daily hassle, which is ideal if you fight prime often. It will not cover for a major air leak, so still seal the suction side.