How to Bleed a Closed Cooling System on a Boat

Anytime you drain or refill a closed-loop cooling system, air gets in. If you don’t get it out, it stays trapped in the highest points — the thermostat housing, cylinder heads, or heat exchanger. That air stops coolant from circulating, causes localized overheating, and keeps the system from working the way it should. Bleeding the system is a must. Do it wrong, and your boat overheats even with all new parts in place.

When Bleeding Is Required

  • After replacing or refilling coolant
  • After installing a new thermostat, heat exchanger, or hoses
  • After overheating caused by low coolant levels
  • When you suspect air pockets are affecting flow

Tools You’ll Need

  • Funnel that seals tightly in the reservoir or filler neck
  • Shop towels or rags
  • Coolant (mixed to correct spec)
  • Infrared thermometer (optional but useful)

Step-by-Step: How to Bleed the System

  1. Start with a cold engine: Never open a pressurized system when hot.
  2. Fill the reservoir: Use a funnel and pour slowly to prevent bubbles during the fill. Fill to just below the top of the neck.
  3. Open bleed screws if equipped: Some engines have bleed valves on the thermostat housing or heat exchanger. Open them until coolant flows without bubbles.
  4. Run the engine at idle: Start the engine and let it warm up. Keep the reservoir cap off and monitor the coolant level. As the thermostat opens, you’ll see bubbles escape and the level drop.
  5. Top off slowly as the engine runs: Continue adding coolant to maintain the correct level while air escapes.
  6. Watch for steady flow: Once the bubbling stops and coolant circulates smoothly, the system is mostly purged.
  7. Close the cap: Once stable and hot, shut the engine down, let it cool fully, and top off the reservoir again.

Tips for a Better Bleed

  • Park the boat slightly bow-up if possible — it helps air migrate to the reservoir
  • Squeeze upper hoses gently to move trapped air along
  • Use a clear spill-free funnel to monitor bubbling more easily
  • If the engine has a coolant overflow tank, make sure it’s connected and sealed properly

What Happens If You Don’t Bleed the System

  • Overheating within minutes of startup
  • Coolant boiling in the reservoir even at normal temperatures
  • Thermostat fails to open because it’s surrounded by air
  • Uneven temperatures between risers or manifolds
  • Potential internal damage from heat cycling and localized hotspots

Conclusion

Bleeding a closed cooling system is not optional — it’s essential. Any trapped air limits coolant flow and leads to fast, avoidable overheating. Do it right, and your system runs cooler, longer, and more reliably. Next time you service your cooling loop, take a few extra minutes and bleed it properly. Your engine will thank you with lower temps and fewer surprises.

Subscribe
Privacy Policy

If you would like to receive text messages from Marine Squadron, text START, YES, to: (888) 373-7755  You will be opting-in to text messages. Message frequency varies and may include replying to customer questions and inquiries. Message and data rates may apply. You may opt out at any time by replying STOP or CANCEL to (888) 373-7755 at any time to end or unsubscribe. See our Privacy Policy for details on how we handle your information.

Tiden LLC DBA Marine Squadron respects your privacy. By opting into our SMS messaging service, you agree to the following terms regarding how we handle your data:
  • Data Collection: We will collect your name, email address, mailing address, and mobile phone number when you sign up for SMS updates. The information will be collected via SMS once the customer provides opt in consent by texting START, YES to the phone number (888) 373-7755.
  • Data Usage: We use your data solely for replying to customer questions and inquiries.
  • Data Security: We protect your data with secure storage measures to prevent unauthorized access.
  • Data Retention: We retain your information as long as you subscribed to our SMS service. You may request deletion at any time.
  • Contact Tiden LLC DBA Marine Squadron at (888) 373-7755 or mail@marinesquadron.com for HELP or to STOP receiving messages.
  • MESSAGES AND DATA RATES MAY APPLY: Your mobile carrier may charge fees for sending or receiving text messages, especially if you do not have an unlimited texting or data plan.
  • Messages are recurring, and message frequency varies.
  • Opt-Out: You can opt out of the SMS list at anytime by texting, emailing, or replying STOP or CANCEL to mail@marinesquadron.com or (888) 373-7755. After unsubscribing, you will receive a final SMS to confirm you have unsubscribed and we will remove your number from our list within 24 hours.
  • You can send HELP for additional assistance, and you will receive a text including our Phone number, email and website. We are here to help you.
  • Non-Sharing Clause: We do not share your data with third parties for marketing purposes. Tiden LLC DBA Marine Squadron will not sell, rent, or share the collected mobile numbers.
Whatsapp Text Message Rapid Response Duty Officer