Common water pump failures: why it doesn't pump, loses pressure or overheats

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Why the pump does not pump, loses pressure or overheats: a comprehensive overview of faults and their solutions

A water pump is one of those devices where a fault becomes immediately apparent and usually at the worst possible time – in the middle of irrigation, when pumping out a flooded basement, or when you need a gasoline pump to work on the first try after a long power outage. From experience, I know that the vast majority of faults are not caused by a design flaw, but by incorrect installation, neglected maintenance, or the pump operating outside its parameters. In this article, I will systematically cover the most common faults – why the pump does not start at all, why it does not pump despite running, why it loses pressure during operation, and why it overheats. For each problem, I will provide a specific diagnostic procedure and solution.

If you are still looking for the right type of pump for your situation, I recommend first reading the article How to choose a water pump: gasoline, submersible or drainage? in the same Knowledge Center. Here we will focus exclusively on the diagnosis and repair of existing devices.

Basic classification of faults by symptom

Before you start disassembling anything, it is important to correctly identify the symptom. Experience shows that customers often confuse "does not pump" and "has no pressure", while the causes can be completely different. So first, make sure you clearly understand what you are actually seeing:

The motor runs, but no water flows at all – suction problem, air in the system, clogged filter or damaged impeller.
The pump does not pump at maximum capacity, but a little water flows – partial loss of performance, worn seals or incorrect dimensioning.
Pressure fluctuates or drops after a short time – problem with the pressure switch, expansion tank or check valves.
The motor overheats and shuts off (thermal protection) – overload, insufficient cooling or electrical fault.
The motor does not start at all (gasoline) or does not start (electric) – fuel system, ignition, electrical system.

The pump does not pump at all: causes and diagnosis

1. Air in the suction pipe – the most common culprit

This is by far the most common problem I encounter with gasoline engine pumps. Self-priming pumps can push air out of the suction pipe, but they need a certain amount of time and the right conditions to do so. If the suction hose is too long, has a leaky connection, or the check valve (suction basket) is leaking, air re-enters the system and the pump runs "dry" without creating the necessary vacuum.

Practical test: disconnect the suction hose from the pump and fill the pump body (pumping chamber) with clean water. If after reconnecting and starting the water starts to flow, the problem is clearly an air leak on the suction side. Look for cracks in the hose, loose clamps, or damaged sealing at the suction basket.

Specific values: self-priming heights for standard pumps are usually 7–8 meters. If your suction hose is longer than 6 meters and has any leaky spot, the pump will never prime. With higher pressure, as in the model Gasoline Pump H-BZP-30 with a pressure of 95 m, the suction height is still physically limited, and exceeding this value causes cavitation and faults.

2. Clogged suction basket or filter

The suction basket is the first line of defense against impurities. When pumping from a pond, from a sandy well, or from a flooded basement, the filter quickly clogs. The symptom is typical: the pump runs, the engine sound is normal, but the flow drops significantly or is zero. Remove the suction basket and inspect it visually – if it is clogged, clean it under water pressure or with a soft brush. Never run the pump without the suction basket – you will suck in impurities into the body and damage the impeller.

3. Damaged impeller

If the pump has been running "dry" for a long time or has pumped water with coarse impurities, the impeller may be worn or cracked. Symptom: the motor runs at full speed, the sound is slightly duller than usual, but the flow is zero or minimal. Diagnosis requires disassembling the pump body – on portable gasoline pumps, this is usually 4–6 screws on the front of the pump. A cracked or worn impeller must be replaced; repairing it with glue is not viable at operating speeds of 2,800–3,600 rpm.

4. Submersible pump does not pump from the well

With submersible pumps like Submersible Pump SKM 100, the situation is different. These pumps are permanently submerged, so the problem of air in the suction pipe is eliminated. Here are typical causes:

The pump is not sufficiently submerged – if the water level has dropped below the pump's suction inlet, it is sucking air. Check the water level and lower the pump.
The check valve on the discharge pipe is stuck in the closed position – water cannot flow upward, the pump runs, but the pressure gauge shows zero.
The water column in the well has dropped – during prolonged drought or due to a neighbor's drilling, water levels may drop. A deeper well or seasonal intermittent operation are solutions.

Read more about depths and diameters of wells and boreholes in the article Submersible pump for a well or borehole: which diameter and depth suit which pump.

Pressure loss during operation: what causes it

1. Damaged or depleted expansion tank (pressure tank)

If you are using a home water supply system – that is, a pump connected to a pressure tank and an automatic pressure switch – and the system turns the pump on every few seconds instead of minutes, or the pressure fluctuates between very low and high values, the first suspect is always the diaphragm expansion tank. The diaphragm inside can rupture, air from one side escapes into the water side, and the tank ceases to perform its function as a pressure reservoir.

The test is simple: with the pump turned off and the system depressurized, press the negative valve (like a bicycle tire valve) on the tank. If water comes out, the diaphragm is ruptured and the tank needs to be replaced. The correct pre-charge air pressure in the tank (when the tank is empty) should be 0.2–0.3 bar lower than the pump's cut-in pressure. For example, if the switch turns the pump on at 1.5 bar, the tank's pre-charge pressure should be 1.2–1.3 bar.

2. Worn or incorrectly adjusted pressure switch

The pressure switch has two adjustment screws – one for the differential (the difference between the cut-in and cut-out pressure) and one for the base pressure. After years of operation, the springs in the switch fatigue, contacts wear out, or lime deposits get inside. Symptoms: the pump does not turn on during normal water consumption, or on the contrary, it runs continuously without stopping. Before adjusting the switch, clean it and check whether the diaphragm in the switch's inlet is clogged with lime deposits – this is quite common in hard water.

3. Damaged check valve

The check valve prevents water from flowing back from the discharge system into the pump and the water source. If the valve is damaged or dirt gets into it, water flows back after the pump is turned off. The pump has to build pressure from zero every time it is turned on, which prolongs the cycles and shortens the device's lifespan. The check valve is located in two places: at the suction basket and in the discharge pipe. Both need to be checked.

4. Air in the home water supply system

After the first start-up or after a long shutdown, air can accumulate in the pipes and then behave like a compressible cushion – the pressure rises sharply, but the flow is poor. Solution: air vents at the highest point of the system, or slowly open the end outlet and let the air escape.

Why the pump overheats

1. Dry run (operation without water)

This is the most destructive situation for any pump. Electromagnetic submersible pumps are cooled by the water being pumped – if the water level drops below the suction inlet, the motor overheats within a few minutes. Higher quality models have built-in thermal protection that shuts off the motor, but repeated dry running leads to damage of the windings and bearings. Solution: float switch or pressure sensor that disconnects the pump when the water level drops.

With gasoline pumps the situation is a bit different – here it is dry operation in an air space without water flow through the seals. Seals (usually mechanical) overheat and are destroyed, causing water leakage during subsequent operation. Prevention: always ensure water is present on the suction side before starting the engine.

2. Overload – pump operating outside the performance curve

Each pump has a so-called characteristic curve Q-H (flow – head). For example, if you use the Gasoline Pump BZP-20 in a situation where the discharge head is significantly lower than its rated head, the pump operates at maximum flow, which overloads the motor. Conversely, if the discharge head is too high, the motor operates under load and also overheats. Solution: always choose a pump according to real hydraulic conditions. More about calculating parameters can be found in the article Head and pump power: how to calculate what you need for your garden or well.

3. Clogged cooling (air-cooled motor)

In gasoline and some electric pumps, the motor is cooled by air through fins. If the fins are clogged with algae, dust or other material (often happens with long-term storage in wooden sheds), the motor cannot dissipate heat. Solution: clean the cooling fins with compressed air or a soft brush before each season.

4. Electrical problems – low voltage and phase imbalance

In electric pumps, a common cause of overheating is low grid voltage. If the voltage in the grid drops below 200 V (this often happens with long distribution lines or simultaneous operation of other appliances), the motor draws higher current, which leads to overheating of the windings. Three-phase pumps are also sensitive to phase imbalance – if one phase fails, the motor runs on two phases and immediate damage is likely. Solution: voltage meter before installation and protective relay for three-phase motors.

Gasoline pump will not start at all: systematic diagnostics

Gasoline pumps such as BZP-30 or BZP-10 are four-stroke engines similar to those in lawnmowers. Solve starting problems in this order:

Fuel – is there fuel in the tank? Is it fresh? Gasoline stored for more than 30 days loses volatile fractions and is hard to ignite. After winter storage, always drain the old gasoline and fill up with fresh Natural 95.
Fuel cock – is it open? This trivial matter is the cause of every fifth "starting problem" in practice.
Choke (enrichment) – for a cold engine, it must be closed (position CLOSED or CHOKE). For a warm engine, it must be open.
Spark plug – pull it out, clean or replace. The electrode gap should be 0.7–0.8 mm. A black sooty plug indicates a too rich mixture or wet starting.
Air filter – a clogged filter restricts intake and the engine refuses to start or runs unevenly. Clean or replace.
Carburetor – if the pump has been standing for a long time with old gasoline, the carburetor jets are clogged with varnish. The carburetor must be removed, disassembled and cleaned with a carburetor cleaner.

A detailed guide to the first start and installation can be found in the article Installation of a gasoline pump: wiring, suction basket and first start.

Noise and vibrations: hidden signs of an impending failure

Pumps are not silent devices, but every unusual sound is a signal that should be taken seriously. From practice, I can distinguish several types of sounds and their causes:

Clicking, tapping, metallic clattering – damaged or worn bearings. Bearings should be replaced before the entire shaft seizes. The cost of bearings is only a fraction of the price of a new pump.
Crackling and clacking during operation – typical sound of cavitation. It occurs when the pressure at the suction inlets of the impeller drops below the vapor pressure of water, causing microbubbles to form and instantly implode. Cavitation mechanically destroys the impeller and the pump body. Causes: too deep suction, clogged suction hose, water that is too warm (warm water cavitates more easily). Solution: reduce suction height, use a larger suction hose, and cool the source.
Humming upon startup without running – the motor is trying to start, but the capacitor (in single-phase motors) is damaged. Replacing the capacitor is enough – they are cheap and readily available.
Whistling or whirring – leakage on the discharge side or partially clogged discharge pipe.

Cavitation: the silent destroyer of pumps

Cavitation receives less attention than it deserves, yet it can destroy a pump within weeks. Physically, it is a local pressure drop below the vapor pressure of water at a given temperature – at 20 °C, this is about 0.023 bar. On the suction edges of the impeller, vapor bubbles form, which instantly collapse with enormous energy at higher pressure. Result: the surface of the impeller looks like it has been sandblasted, and holes may appear in the pump body.

The simplest measures against cavitation:

Reduce suction height (place the pump closer to the water)
Use a suction hose with a larger diameter (at least as large as specified by the manufacturer – usually 1.5–2 inches)
Eliminate bends and turns in the suction pipe
Do not pump water warmer than 40–50 °C without a special hot water pump

Water leaks and seals

The mechanical seal (also known as a mechanical seal) is a part that prevents water from leaking along the motor shaft. It is a precise ceramic or graphite pair that, under proper operation, lasts 3–5 years. In short – if you see water dripping from the pump body at the shaft exit, the seal is worn and must be replaced. Operating with a damaged seal is short-sighted – water will get into the bearings, and the bearings will then be ruined.

There is no point in saving on seals – original spare parts or verified alternatives are much cheaper than the entire motor. When replacing a mechanical seal, always replace the O-rings on the pump body as well, as they lose elasticity during disassembly.

Preventive maintenance: how to prevent failures

Experience from customer practice speaks clearly: most of the failures customers bring in were predictable, and avoiding them would have taken only 30 minutes a year. Key points of preventive maintenance:

Before the season (spring): Visual inspection of the suction hose (cracks, hardening of the rubber), inspection of the basket seal, oiling of the four-stroke engine, inspection of the spark plug, and air bleeding from the pressure tank.
During the season: Regular inspection of the suction basket (every 2–4 weeks with intensive operation), monitoring of unusual sounds, and checking for dryness at the shaft seal location.
At the end of the season (autumn): Draining water from the pump body, draining fuel or adding fuel stabilizer in gasoline engines, and storing in a dry and frost-free place.

A detailed procedure for winter maintenance can be found in the article Maintenance and winterization of pumps: how to extend the life of gasoline and submersible pumps.

When is it time to repair the pump and when is it time to replace it

This is the question customers come to most often. A simple economic principle: if the repair costs more than 50–60 % of the price of a new equivalent pump, it usually does not pay off. An exception is more expensive submersible pumps for deep wells, where installation and dismantling alone cost a lot – in these cases, repairs can be worth it even at higher costs.

Parts whose replacement usually pays off: mechanical seal, bearings, impeller, check valve, capacitor in electric motors, carburetor or its parts in gasoline engines. Parts that usually do not justify repairs: cracked pump body, damaged motor (shorted winding), corroded shaft.

Most frequently asked questions (FAQ)

Why is my pump running but not pumping any water at all, even though I just installed it?

This is a classic case of an air lock. A self-priming pump must be filled with water before the first start – fill the pump body with water through the inlet, close it, and then start the motor. If the pump still does not pump, check whether the suction hose is properly sealed at both ends and whether the suction basket is below the water level. The self-priming height is physically limited to about 7–8 meters; at greater distances, the pump simply cannot reach the water.

The pressure gauge on my home water system shows the correct pressure, but the tap only has a weak flow. What is wrong?

If the pressure on the manometer is correct but the flow is weak, the problem is likely in the piping after the pressure switch – the pipes are clogged with lime deposits (common in areas with hard water), the pipe diameter is too small, or there is a narrowing at some fitting or bend. Also check the screens on the taps – these get clogged and can be easily cleaned.

The gasoline engine starts but stops after a few seconds. What does this mean?

It is usually a problem with the fuel system – either the fuel filter is clogged (replace it), or the carburetor jets are partially clogged with varnish from old gasoline (clean the carburetor). It can also be a clogged vent hole in the fuel tank cap – if the cap is airtight, a vacuum forms that stops the fuel supply. Open the fuel tank cap and try again – if the engine runs, the cause is the cap.

Why does my submersible well pump make a grinding noise and then stop after a while?

A grinding noise from a submersible pump most often indicates cavitation (the water level in the well has dropped below the suction inlet) or damaged bearings. If thermal protection is added and the pump stops after a while and restarts after cooling, it is almost certainly due to operation without sufficient water – the pump is sucking air, overheating, and the protective thermostat shuts it off. Solution: lower the pump deeper, reduce the frequency of pumping, or install a float level sensor.

The pump is working, but it only pumps water up to 10 meters, although it should handle 30 m. Where should I look for the problem?

Lost head can have several causes: worn impeller (reduces efficiency), leaks in the discharge pipe, too long or narrow discharge pipe (pressure losses due to friction), or partially clogged discharge pipe. Also check whether the motor is reaching the correct RPM – in gasoline pumps, the problem may be a dirty air filter or improperly adjusted carburetor, which reduces engine power and thus the hydraulic performance of the pump.

Is it normal for the garden pump to start a few seconds after each water withdrawal? Can I damage anything by doing this?

Yes, frequent short cycling (also known as short cycling) is harmful to the pump. Every motor start brings an inrush current 5–8 times the rated current, which thermally loads the winding. If the pump starts more than 20–30 times per hour, either the pressure tank is too small or the diaphragm tank has lost its pre-charge. Solution: inflate the pressure tank to the correct pressure or add a larger pressure tank. The entire pressure setting process takes about 10 minutes with a standard bicycle pump with a manometer.

Conclusion: failures are mostly predictable and repairable

Experience from practice clearly confirms that pumps are relatively simple machines that operate without problems for many years when properly operated and with minimal preventive maintenance. Most faults appear gradually – a change in sound, a drop in performance, short-term outages – and those who detect these signals in time can save on repairs and downtime. Those who ignore them usually have to deal with a more complex (and more expensive) fault at an inconvenient time.

If you are unsure about selecting a pump for a new project, I recommend another article in this section: Common questions about water pumps: selection, performance, depth, power consumption and spare parts. If you are dealing with a specific installation into a well or borehole, the article Installation of a submersible pump into a well or borehole: step-by-step guide will help you. The full range of pumps can be found on the Pumps category page.

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