Why a Jet Pump Loses Prime and How to Troubleshoot

When a well jet pump—whether a shallow well jet pump or convertible jet pump—loses prime, it's easy to focus on restoring water flow and getting the system back online.  

However, simply re-priming the jet pump and failing to inspect its components and the water system can leave the true underlying cause unaddressed. Because jet pumps rely on a sealed suction system and a continuous water column, a variety of pump and system-related conditions can lead to recurring prime-loss issues. 

The most effective approach is to start by re-priming the pump to verify operation, then systematically inspect the components that commonly contribute to lost prime. By identifying the root cause early, contractors can improve system reliability, reduce repeat service calls, and help prevent unnecessary pump replacements.

How to Prime a Jet Pump After It Loses Prime 

If a well jet pump loses its prime, restoring the prime to verify pump operation should be the first step in the troubleshooting process. 

A loss of prime can be caused by something as simple as a temporary system condition rather than a pump failure. For example, an extended power outage can allow water to drain back toward the well, introducing air into the system and causing the pump to lose prime. Once the pump is re-primed, normal operation may be restored.

1. Turn Off Power to the Pump

Before servicing or troubleshooting the pump, disconnect power at the circuit breaker and verify that power is off. This critical safety step helps prevent accidental pump startup while the system is being inspected or primed. Because well pumps typically operate on 230-volt power, contact with energized components can result in serious injury and damage to the pump. 

Confirm that power has been disconnected by checking that the pressure switch does not activate when manually engaged or by using an appropriate voltage tester at the pump. Never begin work on the system until power has been verified off. 

3. Fill the Pump with Water

Using clean water, slowly fill the pump through the priming port with a funnel, hose, bucket or hand primer pump. Continue adding water until the pump casing and suction line are completely full and the water level remains at the top of the opening. Depending on the pump size and the length of the suction piping, this may require one to two gallons of water or more. 

Fill the system gradually to allow trapped air to escape. Adding water too quickly can create air pockets that make it more difficult to establish and maintain prime.

4. Replace the Priming Plug 

Reinstall the priming plug and tighten it securely to create a watertight seal, taking care not to overtighten and damage the threads. If recommended by the manufacturer, apply thread sealant or PTFE (Teflon) tape before reinstalling the plug. 

A properly sealed priming port helps prevent air from entering the system, which can make it difficult to establish prime or cause the pump to lose prime over time. 

5. Turn the Pump Back On

Restore power to the pump at the circuit breaker, then open a nearby faucet to allow trapped air to escape from the system. As the pump begins operating, you may notice sputtering or bursts of air and water at the faucet. This is normal during the priming process. 

If the pump successfully establishes prime, it should begin moving water, build system pressure and shut off automatically once the pressure switch reaches its cut-off setting. A steady increase in pressure and consistent water flow are good indicators that the pump has been properly primed and returned to normal operation. 

If Re-Priming the Jet Pump Does Not Work 

If the pump does not begin delivering water within approximately 30 seconds, disconnect power and stop the pump. Running a jet pump without water can damage internal components and should be avoided. 

Refill the pump through the priming port and allow the water to sit in the casing for several minutes before restarting. This gives water additional time to fully fill the impeller, volute and suction passage. If the pump still fails to establish prime after several attempts, further troubleshooting is needed. 

If the pump has been properly primed but the system still will not establish or maintain prime, the root cause is likely elsewhere in the water system. 

We still recommend inspecting other parts of the system even if re-priming the pump returns it to operational status, as there could still be an issue within the system that will cause the pump to lose prime again once you leave the jobsite.

Troubleshooting Lost Prime in Jet Pumps 

If a well jet pump does not re-prime or struggles to maintain prime, there may be an issue with the pump’s components or within the water system. 

Air Leaks in Suction Lines 

Air leaks anywhere along the suction side of a jet pump system are a common cause of lost prime. Unlike pressure-side leaks, which often show visible signs of water loss, suction-side leaks can be difficult to identify because they may allow air into the system without producing any obvious symptoms. Even a small opening in the suction line can draw in enough air to disrupt the continuous water column the pump relies on to establish and maintain prime.

Common sources of air intrusion include loose fittings, worn pump seals, deteriorated valves, damaged threaded connections and improperly sealed priming plugs. Over time, these components can wear or loosen, creating pathways for air to enter the system. Symptoms may include increased operating noise, longer pressure build times, intermittent priming issues, reduced performance or complete loss of prime. In some cases, subtle hissing sounds may be present. 

Because suction-side leaks are often difficult to locate, the entire suction path should be carefully inspected. Identifying and correcting even minor air leaks can help restore reliable operation and prevent recurring priming problems.

Clogged Intakes or Screens and Wear on Hydraulic Components 

Some jet pumps incorporate serviceable hydraulic components, including nozzles, venturis, injector screens and diffusers, that are essential to proper water delivery and pressure generation. 

Cracked or Aging Pipes or Pump Casing 

Cracked, deteriorated or aging pipes can cause a jet pump to lose prime by allowing air to enter the system—even a small crack or damaged connection can break the water column and prevent the pump from establishing or maintaining prime. As piping ages, it should be inspected for signs of wear, corrosion or damage that could introduce air into the system. 

The pump casing should also be inspected. When water trapped inside the pump freezes, it can expand and crack the housing, creating pathways for both air intrusion and water loss. Even hairline cracks can lead to recurring priming issues and should be addressed before returning the system to service. Properties in USDA Plant Hardiness Zones 5 and colder face elevated seasonal risk of pump housing freeze damage when systems are not properly winterized. 

Waterlogged Pressure Tanks, Incorrect Pre-Charge Levels and Leaking Air Valves 

Pressure tank issues can also contribute to recurring priming problems and inconsistent system performance. A waterlogged pressure tank, often caused by a failed bladder or diaphragm, reduces the tank's ability to store water under pressure and can lead to frequent pump cycling. Over time, this excessive cycling places additional stress on the system and may contribute to conditions that make it more difficult for the pump to maintain prime. 

Other common pressure tank-related issues include an incorrect air pre-charge and leaking air valves. If the tank's air pressure is set too high or too low, the system may not operate as designed, resulting in poor pressure performance and increased pump cycling. Likewise, a leaking air valve can gradually release compressed air from the tank, eventually causing a properly functioning tank to become waterlogged. 

Changes in Water Level and Pump Performance 

Changes in well conditions can also contribute to lost prime issues. Seasonal drought, periods of high water demand or declining water tables can lower the water level enough for the suction line or foot valve to draw air. When this occurs, the pump may lose its ability to maintain a continuous water column, resulting in symptoms that resemble a priming problem. 

Additionally, as pumps age, they begin losing efficiency. Pumps that have operated reliably for years may begin experiencing prime-loss issues even though the pump itself is still functioning. 

Since changes in water availability, well recovery rates or overall well performance can affect system operation, contractors should evaluate whether current well conditions match those that existed when the system was originally installed and consider how old the pump is. 

Recent Service or Modifications to the Water System 

Recent service work or system modifications can also contribute to lost-prime issues. Any time the suction side of the system is opened, air can enter the piping and disrupt the continuous water column needed for proper pump operation. Activities such as disconnecting piping, opening the pump housing, servicing a strainer or basket, replacing fittings or draining the suction line can all introduce air into the system. 

If prime-loss issues begin shortly after service work, inspect all recently disturbed components to verify that connections are secure and the suction side of the system is properly sealed.

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