.When a sea water pump does not prime, the problem is not necessarily caused by the impeller. Possible causes include the suction line, air leaks, the strainer, hoses, cam, wear plate, pump cover, shaft seals or direction of rotation.
The fault may be related to the suction line, air leaks, worn internal components or incorrect installation. Before replacing the pump, it is therefore advisable to carry out a series of checks in a logical order.
This guide examines the most common causes that can prevent a flexible impeller pump from drawing water correctly.
What Does Self-Priming Mean?
A self-priming pump is able to create the vacuum required to draw water into the suction hose without having to be completely filled manually before every start.
In a flexible impeller pump, priming is made possible by the change in volume between the impeller blades during rotation. This allows the pump to move air for a short time and then draw in water.
Self-priming does not mean that the pump can draw water under all conditions or run dry without being damaged.
Priming performance depends on several factors:
- the tightness of the suction line;
- the vertical distance between the water level and the pump;
- the length and diameter of the hoses;
- the condition of the impeller;
- the condition of the cam;
- the condition of the wear plate;
- the flatness of the cover;
- wear inside the pump body;
- the speed and direction of rotation.
Water lubricates and cools the impeller. Prolonged dry running can quickly overheat and damage the blades.
Operating Principle of a Flexible Impeller Pump
After passing over the cam, the impeller blades straighten and create an increase in volume, producing suction.
As the impeller rotates, the liquid is carried towards the outlet and is discharged when the blades are compressed again.
The direction of rotation must be checked by viewing the pump from the cover side. The position of the inlet and outlet ports depends on the direction of rotation and the cam configuration.
Operating principle of a flexible impeller sea water pump and identification of the inlet and outlet ports.
The direction of rotation must be checked by viewing the pump from the cover side. The position of the inlet and outlet ports depends on the direction of rotation and the cam configuration.
1. Closed or Obstructed Seacock
The first check should concern the seacock and the sea water intake.
Check that:
- the valve is fully open;
- the intake is not obstructed by vegetation, plastic bags or debris;
- there are no restrictions in the suction hose;
- the suction connection is not damaged.
A partially obstructed sea water intake may allow the pump to draw some water, but not enough to cool the engine correctly, particularly as engine speed increases.
2. Dirty or Incorrectly Sealed Sea Water Strainer
A blocked strainer restricts the flow of water and can considerably reduce pump performance.
The strainer cover must also be checked carefully. A damaged, dirty or incorrectly positioned gasket can allow air to enter without producing a visible water leak.
The pump may therefore be in good condition but still fail to prime because it is drawing air through the strainer cover.
3. Air Leaks in the Suction Line
A leak on the suction side does not always produce a visible water leak.
A loose connection or cracked hose may allow air to enter while the pump is running.
Check in particular:
- hose clamps and fittings;
- seals and gaskets;
- the strainer cover;
- hardened or cracked hoses;
- excessively tight bends;
- collapsed hoses;
- drain plugs, where fitted.
Even a small air leak may prevent the pump from creating the vacuum required for priming.
4. Worn, Damaged or Incorrect Impeller
The impeller must have the correct dimensions, number of blades, material and hub type for the pump.
A worn impeller may no longer provide an adequate seal between the blades and the pump body.
Similarly, an impeller that appears similar but has a slightly different width or diameter may reduce the pump’s ability to prime.
Check for:
- bent, cracked or missing blades;
- hardened rubber;
- a separated or damaged hub;
- the correct width and diameter;
- all required seals and gaskets;
- correct engagement with the shaft.
Replacing the impeller does not necessarily solve every problem. The other internal components must also be in good condition.
5. Worn or Damaged Cam
The cam compresses the impeller blades during rotation and creates the change in volume required to draw in and transfer the water.
If the cam is worn, corroded, deformed or unsuitable for that particular pump model, the blades may not flex sufficiently.
The pump may therefore:
- struggle to prime;
- provide reduced flow;
- lose suction at low speed;
- cause abnormal impeller wear.
Check the cam profile, its contact surface and the correct tightening of the retaining screw.
6. Worn Wear Plate
The wear plate forms one of the side surfaces against which the impeller operates.
If it is scored, grooved, deformed or corroded, internal recirculation between the discharge and suction sides of the pump may increase.
The pump may therefore lose flow and priming capability even after a new impeller has been installed.
Check for:
- deep circular scoring;
- grooves;
- corrosion;
- deformation;
- reduced thickness;
- excessive axial clearance of the impeller.
7. Worn Pump Cover or Pump Body
The pump cover must also provide a flat and regular working surface.
Deep scoring, corrosion or deformation may create excessive side clearance around the impeller and reduce the internal seal.
The same applies to the pump body. If the impeller chamber is worn or corroded, the blades may no longer remain in proper contact with the internal surfaces.
Replacing only the impeller may not solve the problem if the cam, wear plate, cover or pump body is already worn.
8. Pump Installed Too High or Incorrect Suction Hose Arrangement
The suction capability of a pump is not unlimited.
A pump installed well above the waterline must lift the water through a greater vertical distance.
Long hoses, multiple bends, insufficient hose diameter and restrictive fittings further increase pressure losses.
The installation must therefore comply with the limits specified by the pump manufacturer.
9. Incorrect Direction of Rotation or Pump Speed
On some pump models, the direction of rotation affects the position of the inlet and outlet ports or requires a specific cam configuration.
The pump must also reach the correct operating speed.
A speed that is too low may not provide effective priming, while excessive speed may damage the impeller and shorten its service life.
The direction of rotation should always be checked from the side specified by the manufacturer, normally from the pump cover side.
10. Fragments of the Old Impeller Inside the Cooling Circuit
When an impeller loses one or more blades, the fragments may travel through the cooling system and obstruct:
- heat exchangers;
- elbows and fittings;
- hoses;
- manifold inlets;
- internal engine passages.
Installing a new impeller is therefore not sufficient. All missing fragments should be located and removed whenever possible.
11. Damaged Shaft Seals and Drain Holes
Many sea water pumps have one or more small drain or inspection holes in the pump body.
These holes, sometimes called drain holes or weep holes, must not be blocked with sealant, paint or any other material.
Water or oil leaking from these openings may indicate that one of the shaft seals has deteriorated.
A small initial trace should be investigated. A continuous leak requires the pump to be stopped and repaired.
On pumps mounted directly to the engine, ignoring this type of leak may have particularly serious consequences.
Depending on the construction of the pump and engine, sea water may pass beyond the seals and reach the engine side, contaminating the lubricating oil.
Possible warning signs include:
- an unexplained increase in the oil level;
- pale or milky oil;
- emulsion on the filler cap or dipstick;
- water in the lubricating oil;
- internal corrosion;
- reduced oil pressure.
If the oil appears emulsified or its level rises without an obvious reason, the engine should not be operated until the cause has been identified.
Water contamination reduces the lubricating properties of the oil and may cause bearing wear, corrosion and serious engine damage.
Conclusion
When a sea water pump does not prime, the impeller is only one of the possible causes.
A complete diagnosis should include the entire suction line and the main internal components of the pump:
- impeller;
- cam;
- wear plate;
- pump cover;
- pump body;
- shaft seals;
- hoses and strainer.
A thorough inspection can prevent the unnecessary replacement of the pump and help identify leaks before they cause more serious damage, particularly when the pump is mounted directly to the engine.