143°F Thermostat for Mercury Marine 75, 90 & 115 HP DFI 2-Stroke Outboards — Replaces 833072002 and 833072004
143°F Thermostat for Mercury Marine 75, 90 & 115 HP DFI 2-Stroke Outboards — Replaces 833072002 and 833072004 is backordered and will ship as soon as it is back in stock.
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Delivery and Shipping
Delivery and Shipping
Orders are typically processed within 1–2 business days.
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143°F Thermostat for Mercury Marine 75, 90 & 115 HP DFI 2-Strok...
143°F Thermostat for Mercury Marine 75, 90 & 115 HP DFI 2-Stroke Outboards — Replaces 833072002 and 833072004 is backordered and will ship as soon as it is back in stock.
Product Details
This 143°F thermostat is a direct-replacement thermostat for selected Mercury Marine DFI 2-stroke outboard motors. It is listed for 1.5L 3-cylinder DFI engines rated at 75, 90, and 115 HP, subject to the model and serial-number conditions shown below.
This thermostat replaces Mercury Marine part numbers 833072002 and 833072004. The 143°F operating specification and the listed applications correspond to Sierra reference 18-3537.
- Direct replacement for the applicable original thermostat.
- Designed to open at 143°F (61.6°C).
- Thermostat seal 27-887978 is required for installation and is sold separately.
- Verify the complete engine model and serial-number range before ordering.
Part Number Matrix
Use this matrix to compare the applicable manufacturer and aftermarket references.
| Brand | Part Number | Reference |
|---|---|---|
| Mercury Marine | 833072002 | Replacement reference |
| Mercury Marine | 833072004 | Replacement reference |
| Sierra | 18-3537 | 143°F thermostat reference |
Engine & Model Compatibility
The catalog application information below applies to the listed Mercury DFI 2-stroke engines. Model, year, and serial-number conditions must be considered together.
| Brand | Engine Model | Rated HP | Stroke | Years | Serial Number | Replacement References |
|---|---|---|---|---|---|---|
| Mercury | 1.5L 3-cylinder DFI | 75 | 2-Stroke | 2006 and newer | OT980000 and newer | 833072002, 833072004 |
| Mercury | 1.5L 3-cylinder DFI | 90 | 2-Stroke | 2004 and newer | OT801000 and newer | 833072002, 833072004 |
| Mercury | 1.5L 3-cylinder DFI | 115 | 2-Stroke | 2004 and newer | OT801000 and newer | 833072002, 833072004 |
Important: This application should not be extended to Mercury 135–175 HP 2.5L DFI engines. Those applications use a different thermostat reference.
Technical Specs
- Product Type
- Engine thermostat
- Application
- Selected Mercury Marine 1.5L 3-cylinder DFI 2-stroke outboards
- Installation
- Direct replacement
- Operating Temperature
- 143°F (61.6°C)
- Required Seal
- 27-887978, sold separately
What’s in the Box
- 1 x 143°F engine thermostat
The thermostat seal is not included.
Installation Notes
- Disconnect the battery before beginning work.
- Allow the engine to cool completely before servicing the cooling system.
- Locate the thermostat housing on the engine.
- Remove the bolts securing the housing cover.
- Remove the old thermostat and seal, noting the thermostat orientation.
- Thoroughly clean the mating surfaces of the housing and cover.
- Install the new thermostat in the correct orientation.
- Install a new thermostat seal, part number 27-887978, sold separately.
- Reinstall the housing cover and tighten the bolts to the manufacturer's specified torque.
- Reconnect the battery, start the engine, and check for leaks and proper water flow as the engine reaches operating temperature.
Troubleshooting & FAQ
How do I know if my thermostat has failed?
Common signs include overheating, running unusually cool, failure to reach normal operating temperature, or erratic temperature readings. A thermostat stuck closed can cause rapid overheating, while one stuck open can prevent the engine from warming properly.
Is the thermostat seal included?
No. The required thermostat seal, Mercury part number 27-887978, is sold separately and should be replaced during installation to help ensure a proper seal.
Will this fit my Optimax engine?
This thermostat is listed for selected 75, 90, and 115 HP 1.5L 3-cylinder DFI 2-stroke engines with the model-year and serial-number conditions shown above. Do not assume fitment for 135–200 HP Optimax engines; verify the exact OEM thermostat reference and serial number for your engine.
What is the correct operating temperature?
This thermostat is designed to open at 143°F (61.6°C).
How often should I replace an outboard thermostat?
Inspect the thermostat during scheduled cooling-system maintenance and replace it if it is corroded, stuck, damaged, or not regulating temperature correctly. Follow the service guidance for your specific engine and consider more frequent inspection when operating in saltwater.
OEM vs. aftermarket?
This listing is for an aftermarket replacement thermostat, not a Mercury-branded OEM component. Confirm the dimensions, reference number, and engine serial-number range before installation.
Shipping speed & cost?
We ship worldwide quickly—typical transit is 5–15 days. Standard shipping is free; expedited options appear at checkout where available.
Warranty & returns?
Covered by a 1-year warranty against manufacturing defects under normal use. If issues arise, contact support with your order details. Returns are accepted within our standard window when the item is unused and in its original condition. See the applicable policy for complete details.
Safety & Important Info
- Confirm the engine model, horsepower, stroke, year, and serial-number range before purchase.
- Compare the replacement thermostat with the original part before installation.
- This is an aftermarket replacement, not a Mercury OEM component.
- Allow the engine to cool completely before opening the thermostat housing.
- Professional installation is recommended when the required service procedure or torque specifications are unknown.
About Sea Sierra
- Pro-Grade Standards — We supply marine parts engineered to OEM-level tolerances and tested for real-world conditions.
- Fast Global Fulfillment — Free standard delivery to most regions in 5–15 days.
- Manufacturer-Direct Value — Fair pricing without compromising on materials or quality control.
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