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

This thermostat kit is engineered for Mercruiser 3.7L 4-cylinder (224 CID) engines, including the 165, 170, 180, 190, 470, 485, and 488 series models manufactured between 1976 and 1989.

It serves as a direct replacement for Mercruiser part numbers 99155T1 and 99155T2, as well as Sierra parts 18-3555, 18-3568, and 18-3667.

• Operating Temperature: Opens at a factory-set 160°F (71°C).
• Complete Kit: Includes the thermostat and the required gasket/seal for a full replacement.
• Installation Type: Designed for an OEM direct replacement fit in closed cooling systems.

Part Number Cross-Reference

Use this table to confirm equivalent part numbers across OEM and aftermarket brands.

Compatible Engine Models

Confirm your engine model/year in the table below.

Technical Specifications

Package Contents

• 1 x Thermostat (99155A1)
• 1 x Gasket/Seal (25-807134-1)

Installation Guide

1. Ensure the engine has completely cooled before starting.
2. Drain the coolant from the engine's closed cooling system.
3. Locate and carefully unbolt the thermostat housing.
4. Remove the old thermostat and gasket/seal, noting their orientation.
5. Thoroughly clean the mating surfaces of the housing and the engine manifold.
6. Install the new thermostat and gasket in the correct position.
7. Re-attach the thermostat housing and tighten the bolts to the manufacturer's specified torque.
8. Refill the cooling system with the correct type and amount of coolant.
9. Start the engine, check the temperature gauge for normal operation, and carefully inspect for any leaks.

Troubleshooting & FAQ

How do I confirm this fits my engine?

This kit is designed for Mercruiser 3.7L 4-cylinder (224 CID) engines from 1976–1989, including the 165, 170, 180, 190, 470, 485, and 488 series. Please verify your engine model and year against the compatibility chart and cross-reference the OEM part numbers (99155T1, 99155T2) to ensure a perfect match.

What are the signs of a failing thermostat?

Common symptoms include the engine overheating or running too cold (not reaching the standard 160°F operating temperature). You may also notice fluctuating temperature gauge readings or coolant leaks around the thermostat housing.

What is the difference between part numbers 99155T1 and 99155T2?

Both 99155T1 and 99155T2 are used for the same application. The different numbers likely represent minor revisions by the manufacturer over time, but they are functionally interchangeable. This kit replaces both versions.

OEM vs. Aftermarket?

This listing is an aftermarket replacement part (not an OEM component). We build to OEM specifications using comparable-grade materials. Production is done in an ISO9002-certified facility, and each batch is inspected for consistent performance.

How fast do you ship and how much does it cost?

We offer fast global shipping—typically 5–15 days worldwide—and standard shipping is free. Expedited options are available at checkout depending on your location.

What is your warranty and return policy?

This product includes a 1-year warranty against manufacturing defects under normal use. If something goes wrong, contact our support team with your order details. Returns are accepted within our standard return window (unused & in original condition); please see our policy page or reach out for instructions.

Important Notes

• Verify that the product images and specifications match your original part. If discrepancies exist—even if your model is listed—do not proceed with the purchase.
• This product is an aftermarket replacement part and is not an OEM component.
• Professional installation is recommended to ensure safety and optimal performance.

Why Choose JLM MARINE?

• Quality Assured: As a trusted manufacturer for over 20 years, we supply top marine brands globally.
• Fast Global Shipping: Free delivery worldwide in 5–15 days.
• Direct from Manufacturer: Competitive pricing without compromising quality.