Thermostat Kit (160°F/71°C) – Fits Mercruiser 3.7L 4-Cylinder Engines (Replaces 99155T1, 99155T2)
Thermostat Kit (160°F/71°C) – Fits Mercruiser 3.7L 4-Cylinder Engines (Replaces 99155T1, 99155T2) is backordered and will ship as soon as it is back in stock.
Couldn't load pickup availability
Worry-free returns
Worry-free returns
You may return most new, unopened items within 7 days of delivery for a full refund. We'll also pay the return shipping costs if the return is a result of our error (you received an incorrect or defective item, etc.).
You should expect to receive your refund within four weeks of giving your package to the return shipper, however, in many cases you will receive a refund more quickly. This time period includes the transit time for us to receive your return from the shipper (5 to 10 business days), the time it takes us to process your return once we receive it (3 to 5 business days), and the time it takes your bank to process our refund request (5 to 10 business days).
If you need to return an item, simply login to your account, view the order using the "Complete Orders" link under the My Account menu and click the Return Item(s) button. We'll notify you via e-mail of your refund once we've received and processed the returned item.
Delivery and Shipping
Delivery and Shipping
Free standard shipping on all orders under 1 KG
Thermostat Kit (160°F/71°C) – Fits Mercruiser 3.7L 4-Cylinder Engin...
Product Details
This aftermarket Thermostat Kit (160°F/71°C) is designed for Mercruiser 3.7L 4-cylinder (224 CID) engines, including the 165, 170, 180, 190, 470, 485, and 488 series models manufactured between 1976–1989.
It serves as a direct replacement for Mercruiser part numbers 99155T1 and 99155T2, and provides an interchange for Sierra parts 18-3555, 18-3568, and 18-3667.
- Designed for simple, direct OEM-replacement installation.
- Regulates engine temperature to 160°F (71°C) for optimal performance in closed cooling systems.
- Includes the thermostat and required gasket/seal for a complete installation.
Part Number Matrix
Use this matrix to confirm equivalent references across OEM and aftermarket.
| Brand | Part Number | Notes |
|---|---|---|
| Mercruiser | 99155T1 | Interchangeable OEM part. |
| Mercruiser | 99155T2 | Interchangeable OEM part, may be a later revision. |
| Sierra | 18-3555 | Aftermarket equivalent. |
| Sierra | 18-3568 | Aftermarket equivalent. |
| Sierra | 18-3667 | Aftermarket equivalent. |
Engine & Model Compatibility
Confirm your engine model/year in the table below.
| Brand | Model | Rated HP | Stroke | Years | OEM Part No. |
|---|---|---|---|---|---|
| Mercruiser | 165 (2BBL) | 165 HP | 4-Stroke | 1987 | 99155T1, 99155T2 |
| Mercruiser | 170 (2BBL) | 170 HP | 4-Stroke | 1985–1986 | 99155T1, 99155T2 |
| Mercruiser | 180 (4BBL) | 180 HP | 4-Stroke | 1987 | 99155T1, 99155T2 |
| Mercruiser | 190 (4BBL) | 190 HP | 4-Stroke | 1985–1986 | 99155T1, 99155T2 |
| Mercruiser | 3.7L (2BBL) | 165 HP | 4-Stroke | 1988–1989 | 99155T1, 99155T2 |
| Mercruiser | 3.7LX (4BBL) | 180 HP | 4-Stroke | 1988–1989 | 99155T1, 99155T2 |
| Mercruiser | 470 (2BBL) | 170 HP | 4-Stroke | 1976–1984 | 99155T1, 99155T2 |
| Mercruiser | 470 MIE | 170 HP | 4-Stroke | 1980–1981 | 99155T1, 99155T2 |
| Mercruiser | 485 (2BBL) | 185 HP | 4-Stroke | 1980–1982 | 99155T1, 99155T2 |
| Mercruiser | 488 (4BBL) | 190 HP | 4-Stroke | 1983–1984 | 99155T1, 99155T2 |
Technical Specs
- Product Type
- Marine Engine Thermostat Kit
- Installation
- OEM direct replacement
- Thermostat Rating
- 160°F (71°C)
- Operating Temperature
- 160°F (71°C) opening temperature
- Application
- Closed cooling system
What’s in the Box
- 1 x Thermostat (99155A1)
- 1 x Gasket/Seal (25-807134-1)
Installation Notes
- Ensure the engine is completely cool before starting work.
- Drain the coolant from the system to a level below the thermostat housing.
- Carefully remove the thermostat housing, noting the position of hoses and bolts.
- Remove the old thermostat and gasket. Thoroughly clean the mating surfaces on the housing and manifold.
- Install the new thermostat in the correct orientation, followed by the new gasket.
- Re-install the housing cover and tighten bolts to the manufacturer's specified torque.
- Refill the cooling system with the appropriate coolant type.
- Start the engine, allow it to warm up, and check carefully for any leaks around the housing.
Troubleshooting & FAQ
How do I know if this thermostat fits my engine?
This kit is specifically for Mercruiser 3.7L 4-cylinder engines from 1976–1989. Use the 'Engine & Model Compatibility' table on this page to verify your exact model and year. Always compare the photos to your old part before installation to ensure a match.
What are the signs of a failing thermostat?
Common symptoms include engine overheating (if stuck closed) or the engine taking too long to reach operating temperature (if stuck open). You might also notice erratic temperature gauge readings or poor engine performance.
Why is the 160°F temperature rating important?
The 160°F (71°C) rating is the OEM-specified opening temperature for this engine series. Using the correct temperature thermostat is critical for achieving proper engine performance, optimal fuel efficiency, and preventing long-term engine damage.
What is the difference between Mercruiser parts 99155T1 and 99155T2?
Both part numbers are listed as direct replacements for the same range of engines. The 'T2' designation likely represents a minor revision or a superseded part number from the manufacturer, but they are functionally interchangeable. This kit replaces both versions.
OEM vs. Aftermarket?
This listing is an aftermarket replacement (not OEM). Built to OEM-level specifications with comparable materials. Manufactured in an ISO 9002 facility, batch-inspected for consistency.
Shipping speed & cost?
We ship worldwide quickly—typical transit 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 (unused & in original condition). See our policy page or reach out for instructions.
Safety & Important Info
- Verify images/specifications vs your original part. If anything differs—even if your model appears compatible—do not purchase.
- This is an aftermarket replacement, not an OEM component.
- Professional installation is recommended for safety and performance.
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 QC.
Payment & Security
Payment methods
Your payment information is processed securely. We do not store credit card details nor have access to your credit card information.
