Why Do I Need to Prime My Water or Trash Pump?

This article explains why priming is required for Generac water and trash pumps and how to prime the pump before starting the engine. Follow the steps to remove trapped air, restore suction, and avoid running the pump dry for reliable pumping performance.

  • 31 Mar 2026
  • Knowledge

Title
Why Do I Need to Prime My Water or Trash Pump?
URL Name
Why-Do-I-Need-to-Prime-My-Water-or-Trash-Pump

Public Answer

Why do I need to prime my water or trash pump?

  • Priming a water pump removes air trapped in the pump so it can begin pumping water. Before starting your pump, it is essential to Add water to remove air from the pump housing and create a partial vacuum.
  • Water also cools and lubricates the mechanical seal until water starts flowing.
  • Always remove the pump’s water priming plug (or priming cap) and fill the pump case completely with water before starting the engine.
Note: Shutting off a pump can allow water to drain from the suction hose, which may require priming again.

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Environment

  • Generac water pumps and trash pumps.

How to prime the pump

Use the steps below any time the pump is first set up, after it has been shut off, or any time the suction hose has lost water.

Before you begin

  • Confirm the suction hose and discharge hose are connected and tightened.
  • Place the suction hose strainer end in the water source, and make sure it stays fully submerged.
  • Have clean water available for priming.

Procedure

1. Make sure the engine switch is set to OFF and the pump is not running.
2. Locate the water priming plug (or priming cap) on the pump housing.
3. Remove the priming plug/cap. If a gasket or O-ring is present, inspect it and make sure it stays in place.
4. Slowly fill the pump case completely with clean water, up to the top of the fill opening.
5. Reinstall the priming plug/cap and tighten it securely (do not overtighten).
6. Start the engine and allow the pump to run. Water flow should begin once the pump clears remaining air from the suction line.
7. If water does not begin flowing, turn the engine OFF, remove the priming plug/cap, refill the pump case completely with water, and repeat steps 4 through 6.
 
Note: Do not run the pump dry. Operating without water in the pump housing can damage the mechanical seal.

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If the pump still will not prime

If the pump is filled with water but still has low or no suction, or weak discharge, check for common issues such as an air leak on the suction side, loose hose clamps, a suction hose that is not fully submerged, or a clogged strainer.

For additional troubleshooting, see Why Is My Pump Experiencing Low/No Suction or Weak Discharge?

See also

If you need to download documentation, visit the product lookup page.

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