MerCruiser 864509 Cooling Hose – Thermostat Housing to Lower Manifold
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MerCruiser 864509 – Molded Cooling Hose | Thermostat Housing to Lower Exhaust Manifold
Part Number: 864509 | Reference: 32-864509
Brand: Mercury Marine
Type: Molded Engine Cooling Hose
System: Closed Cooling
Position: Thermostat Housing to Lower Exhaust Manifold
GTIN: 745061554573
Genuine Mercury Marine replacement hose for the closed-cooling system on selected MerCruiser gasoline engines. This molded hose connects the thermostat housing to the lower exhaust manifold, forming a critical part of the closed-loop coolant circuit. Designed to maintain proper coolant flow and resist the heat and pressure demands of marine engine service.
Compatible Applications
- 5.0L / 305 CID V8 MerCruiser – selected closed-cooling configurations
- 5.7L / 350 CID V8 MerCruiser – selected closed-cooling configurations
- 6.2L / 377 CID V8 MerCruiser – selected closed-cooling configurations
- 350 MAG / Horizon / 383 MAG Stroker – selected configurations
- Alpha, Bravo, Axius, inboard, and V-drive closed-cooling layouts where 864509 is specified
Serial Number Notice: Confirm your engine serial number and closed-cooling diagram before ordering. Mercury uses several similar molded hoses in the same assembly, and exact hose selection varies by engine family and cooling-system configuration.
When to Replace
Inspect this hose at every service and replace if you notice any of the following:
- Cracking, hardening, or surface checking
- Swelling or soft spots along the hose body
- Chafing or wear near clamp areas
- Seeping or active coolant leak at either connection
Also inspect whenever servicing the thermostat housing, exhaust manifolds, or heat-exchanger system — these hoses are easiest to replace with the surrounding components already removed.
Installation Notes
- Allow the engine to cool completely before beginning any cooling-system work
- Note original hose routing and clamp positions before removal
- Inspect both connection points for corrosion or damage
- Replace deteriorated clamps or related sealing components at the same time
- Refill and bleed the closed-cooling system per the applicable MerCruiser service procedure
- Run the engine and inspect carefully for leaks before returning to service
Part Numbers & Cross-Reference
- Mercury Marine: 864509
- Mercury Reference: 32-864509
- GTIN: 745061554573
Related Cooling Hoses
- Related lower-manifold hose: 864508 (not currently in store)
- Upper-manifold hose: Mercury MerCruiser Cooling Hose — 32-864552
- Starboard manifold hose: MerCruiser 864539 Cooling Hose – Thermostat Housing to Starboard Manifold
Parts Desk Insights
The defining identifier for 864509 is its lower manifold position in the thermostat-housing circuit. Several hoses in this assembly look nearly identical, so always cross-check against the closed-cooling diagram for your specific engine before ordering. The most common customer mistake is confusing this lower-manifold hose with the upper-manifold or heat-exchanger hoses nearby. If you're replacing this hose, it's a good opportunity to inspect the surrounding hoses — particularly the 32-864552 and 864539 — since access is much easier with the assembly partially disassembled.
Shipping information
In-stock and OEM parts: ship same or next business day.
Locally warehoused items: ship within 1-3 business days.
Special-order items: timeline confirmed after order.
Returns & warranty
Returns & Satisfaction Guarantee
We stand firmly behind the quality of every product we sell. Your confidence matters to us — and we back it with a straightforward 30-Day Satisfaction Guarantee.
If you are not fully satisfied, you may request a return within 30 days of delivery. All returns must receive pre-approval prior to being sent back. Unauthorized returns will not be accepted.
To qualify for a return:
Item must be in new, unused condition
Product must be in its original packaging
All parts, components, hardware, manuals, and accessories must be included
Packaging must be undamaged and suitable for resale
Important:
Electronics are strictly inspected. No returns on open-box electronics. Exceptions may apply only with prior written approval. Any electronic unit that has been installed, powered, activated, or altered may not be eligible for return.
We are serious about product integrity — and equally serious about taking care of our customers. If there is ever a product issue, contact us immediately so we can make it right.
For return authorization, please contact our support team before shipping anything back.