Why Raw Water Pumps Lose Prime and How to Fix It

The raw water pump is the heart of your marine engine’s cooling system. If it loses prime — meaning it’s no longer pulling water — your engine is moments away from overheating. Priming problems usually show up after dry storage, hose replacements, or a recent impeller service. Knowing what causes this and how to fix it can save your engine from serious damage.

What It Means to “Lose Prime”

Raw water pumps work by creating suction. But they don’t self-prime like pressurized systems. They rely on a solid column of water in the intake line. If air enters that line or the pump runs dry, it loses its ability to pull water up from the seacock — especially at low RPMs. Once that suction is broken, flow stops completely.

Common Causes of Prime Loss

  • Dry start after haul-out or repair: If the pump runs dry even briefly, it can’t pull water. Impeller damage may occur in seconds.
  • Air leaks on the suction side: Cracked hoses, loose clamps, or leaking strainer lids allow air in, preventing suction.
  • Improper hose routing: Hoses running uphill or creating air pockets can trap bubbles and prevent priming.
  • Debris blocking the intake: Seaweed, plastic bags, or barnacle growth at the thru-hull can completely stop water from entering.
  • Worn or damaged impeller: Missing vanes or hardened rubber can’t create the vacuum needed to start the flow.

Symptoms of a Lost Prime

  • No water from the exhaust outlet
  • Rapid rise in engine temperature within 1 to 2 minutes of startup
  • Dry or noisy raw water pump (squealing or scraping)
  • Strainer remains empty or partially filled
  • Steam or burning rubber smell from the engine room

How to Restore Prime

  1. Turn off the engine immediately to prevent impeller damage.
  2. Check that the seacock is fully open and the intake is clear.
  3. Open the strainer lid and fill it manually with water to push air out of the system.
  4. Inspect all suction-side hoses and clamps for cracks or leaks.
  5. Reinstall the strainer lid tightly, ensuring the gasket is seated correctly.
  6. Restart the engine and monitor raw water flow at the exhaust outlet.

If water still doesn’t flow, remove the intake hose from the pump and pour water directly into the pump housing. This helps displace trapped air and forces the impeller to engage with water immediately.

Preventing Prime Loss in the Future

  • Never run the engine dry after launching
  • Replace impellers every 12 months or sooner in saltwater use
  • Inspect and replace all aging hoses and gaskets in the raw water system
  • Use clear strainer lids to monitor water level during startup
  • Prime the system manually after impeller or hose changes

Conclusion

Raw water pumps are simple, but sensitive. A small air leak or one dry start can kill flow instantly. If your engine starts to run hot right after launch or service, assume the pump has lost prime — and act fast. Find the leak, fill the system, and restore flow before permanent damage occurs. A $2 clamp can sometimes be the difference between a quick fix and a cooked engine.

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