MerCruiser Closed Cooling Thermostat Kit 807252T4
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Closed Cooling Thermostat Kit — Part 807252T4
| Part Number | 807252T4 |
|---|---|
| Brand | Mercury Marine / MerCruiser / Quicksilver |
| Official Product Type | Thermostat Kit — Closed Cooling System |
| System | Cooling — Closed / Freshwater Cooling Circuit |
| Temperature Rating | 170°F – 180°F (closed cooling specification) |
| Supersedes | 807252T3, 807252Q4 |
| Kit Contents | Thermostat, sealing gaskets, and installation hardware |
| Aftermarket Cross-Reference | Sierra 18-3648 |
This is the genuine OEM thermostat kit for MerCruiser GM-based engines equipped with closed cooling (freshwater cooling) systems. The kit is rated at the higher 170°F–180°F range required for closed cooling operation — not the 143°F or 160°F thermostats used in raw water cooled engines. Running the wrong temperature thermostat in a closed cooling engine causes improper warm-up, reduced efficiency, and can shorten engine life.
The kit includes everything needed for a complete thermostat service: the thermostat, sealing gaskets, and installation hardware, so no separate gasket sourcing is required.
Supersedes / Part Number Reference
- Current OEM number: 807252T4
- Supersedes: 807252T3, 807252Q4
- Aftermarket cross-reference: Sierra 18-3648
Compatible Applications
For closed cooling (freshwater / heat exchanger) equipped engines only. Not for standard raw water cooled engines. Confirm by engine serial number and cooling system configuration.
- 4.3L / 262 CID V6 — Closed cooling models with heat exchanger
- 5.0L / 305 CID V8 — Closed cooling models with heat exchanger
- 5.7L / 350 CID V8 — Closed cooling models, including Bravo sterndrive and MIE inboard configurations
- 7.4L / 454 CID V8 — Closed cooling models with heat exchanger
- 8.2L / 502 CID V8 — Closed cooling models with heat exchanger
Serial Number Notice
Fitment varies by engine serial number and cooling system configuration. Engines converted from raw water to closed cooling may require a different thermostat kit. Confirm your cooling system type and engine serial number before ordering.
When This Part Needs Replacement
- Engine is running too hot or overheating
- Engine is not reaching or maintaining consistent operating temperature
- Thermostat is sticking open or closed
- Corrosion is present on the thermostat or housing
- Cooling system service or rebuild is underway
- Scheduled preventative maintenance on a closed cooling system
Installation Notes
- Use all included gaskets and seals — do not reuse old ones
- Install the thermostat with the spring side facing toward the engine (inward)
- Clean the thermostat housing and all sealing surfaces before installation
- Inspect the heat exchanger and coolant passages for blockage or scale buildup
- Check coolant condition and replace if contaminated or overdue
- Follow the applicable MerCruiser service procedure for torque specifications
Parts Desk Insights
Most common customer issue: Installing a raw water thermostat (typically 143°F or 160°F) into a closed cooling engine. Closed cooling systems require the higher 170°F–180°F thermostat — using the wrong one causes the engine to run too cool, which increases fuel consumption, accelerates wear, and can prevent the engine from reaching proper operating efficiency. If you’re unsure which cooling system your engine has, check for a heat exchanger mounted to the engine — if it has one, you need this kit.
Best use case: Ideal for scheduled thermostat replacement on closed cooling MerCruiser V6 and V8 engines, cooling system overhauls, or any time the thermostat housing is opened for service. The kit format means no separate gasket hunting — everything for a complete installation is included.
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.