Why Your Engine Loses Prime After Launch or Storage

Raw water pumps aren’t self-sucking. They depend on gravity or pressure from the seacock and strainer to push water to the pump. If air gets into the system, or the pump sits dry for too long, it can lose prime — meaning it spins but doesn’t move water. This is a common problem after launching, hauling, or long periods of storage. And if it isn’t fixed quickly, it can destroy your impeller in under a minute.

What “Losing Prime” Means

When a raw water pump loses prime, it’s spinning with nothing inside to push. That usually happens because the intake side — from the seacock to the pump — has air instead of water. Since most marine impellers rely on the liquid to form a seal against the housing, the pump can’t move air, and cooling stops instantly.

When Loss of Prime Happens

• Immediately after launching the boat
• After winterization or long-term storage
• After service work involving hoses, heat exchanger, or strainer
• When a strainer gasket or fitting leaks air into the intake line

Common Causes of Lost Prime

• Air in the intake line: Leftover from draining the system or from a leaking fitting
• Dry strainer or improperly sealed gasket: Allows air to enter instead of water
• Seacock partially closed or clogged intake: Limits or blocks water flow at the source
• Pump mounted too high: Some setups rely on gravity — if the pump sits above the waterline, it may not prime without assistance
• Dry impeller housing: After storage, the impeller may not have enough lubrication to pull suction until it’s wet again

How to Prime the System

1. Ensure the seacock is fully open.
2. Check the strainer: Fill it completely with water and reseal the lid tightly.
3. Crack a hose fitting near the pump inlet and pour water into the line to force out air.
4. Remove the outlet hose from the pump and pour water directly into the pump body to “wet” the impeller.
5. Start the engine and monitor flow — water should appear at the exhaust within 15–30 seconds.

If the pump doesn’t prime after 30 seconds of runtime, shut it down to avoid burning the impeller. Repeat the process and inspect for leaks or suction loss.

Prevention Tips

• Always fill the strainer before launching — especially after draining the system
• Use silicone grease on strainer gaskets to ensure airtight seals
• Flush and wet the impeller before first start after storage
• Inspect all intake-side hose clamps and fittings for looseness or cracks
• If the pump is above the waterline, consider installing a priming port or service valve

Conclusion

Loss of prime isn’t a mysterious gremlin — it’s just air in the wrong place. If your engine runs dry after launch or layup, don’t start swapping parts. Start with the strainer, seacock, and pump. The solution is usually simple: reintroduce water, re-seal the intake, and restart. Catch it early, and you’ll save your impeller — and avoid an overheating engine five minutes from the dock.