How to Identify and Fix Air Leaks on the Suction Side of a Raw Water Pump

The raw water pump relies on vacuum — not pressure — to pull water from the seacock through the strainer and into the cooling system. That vacuum has to be airtight. If even a small amount of air sneaks in before the pump, the system can lose prime and stop drawing water entirely. These air leaks often leave no drips, no stains, and no obvious signs — just no flow, steam from the exhaust, or a burned-out impeller.

What Is the Suction Side?

The suction side includes everything between the water intake (seacock) and the inlet of the raw water pump:

  • Seacock and thru-hull fitting
  • Raw water strainer and lid
  • All hose clamps and fittings
  • Intake hose up to the pump body

Leaks anywhere along this section let air in — which prevents the pump from drawing water, even if it’s spinning perfectly.

Symptoms of an Air Leak on the Suction Side

  • No water coming from the exhaust at startup
  • Intermittent flow — good at idle, weak at speed
  • Dry or partially melted impeller even after recent replacement
  • Burning rubber smell or rising engine temp despite clean strainer
  • Engine primes only after removing hose or filling pump manually

Where to Look First

  1. Strainer lid and gasket: The most common source. A poorly sealed lid or cracked gasket lets air in without dripping water.
  2. Loose hose clamps: Even if tight, clamps on old hose can leak air under suction.
  3. Cracked intake hose: Internal cracks or dry rot may not leak water — but they let in air under vacuum.
  4. Threaded fittings: Any NPT fittings at the seacock or strainer that aren’t sealed properly can pull air when under suction.

How to Find the Leak

  • Grease test: Apply marine grease around strainer lid and hose joints. If the system suddenly primes, you’ve located the leak.
  • Soapy water: Pressurize lightly from the outlet side and spray soapy water on fittings — bubbles will appear where air gets pulled in.
  • Manual prime check: Fill the intake hose with water manually. If the engine starts pulling flow only after filling, air may have been trapped or leaking in.

How to Fix the Problem

  • Replace old or brittle intake hose with marine-grade wire-reinforced hose
  • Use double stainless clamps on all fittings — offset slightly and tightened evenly
  • Replace strainer lid gasket and apply silicone grease for better sealing
  • Seal threaded fittings with PTFE paste, not just tape, for a tighter air seal
  • If the pump is mounted above the waterline, install a priming valve or service port to remove air more easily

Conclusion

Air leaks on the suction side won’t show up as drips — but they’ll starve your cooling system and destroy impellers before you even know they’re there. If your pump won’t prime, your engine overheats at startup, or water flow disappears after sitting, suspect an air leak. With a few simple checks and proper sealing, you can stop the air from getting in and keep the water moving where it belongs.

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