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 designed for Mercruiser 165-190 HP, 4-Stroke, 3.7L inline 4-cylinder (224 CID) engines manufactured between 1976 and 1989. It provides a reliable, direct replacement for original equipment to maintain optimal engine operating temperature.

This kit is a direct replacement for Mercruiser part numbers 99155T1, 99155T2, 991551, 99155A1; Sierra 18-3555, 18-3568, 18-3667; GLM 13040, 13420; and Mallory 9-43001, 9-43016.

• Engine Application: Fits Mercruiser 3.7L (224 CID) 4-cylinder series.
• Operating Temperature: Rated to open at 160°F (71°C).
• Installation Type: Direct OEM-spec replacement for 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. For Mercury owners, here’s a guide to help you identify your engine’s serial/model: Mercury Serial Number Guide. You can also review exploded diagrams to verify part numbers: Mercury Parts Diagrams & Catalog.

Technical Specifications

Package Contents

This kit includes all necessary components for a standard thermostat replacement:

• 1 x Thermostat (Replaces 991551)
• 1 x Thermostat housing gasket (Replaces 33918)
• 1 x Thermostat seal

Installation Guide

1. Disconnect the Battery: Ensure the engine's negative battery cable is disconnected for safety.
2. Drain Coolant: Drain the engine coolant from the closed cooling system to a level below the thermostat housing.
3. Locate and Remove Housing: Identify the thermostat housing on the engine and carefully remove the bolts securing it.
4. Remove Old Components: Take out the old thermostat, gasket, and seal. Note the thermostat's orientation.
5. Clean Surfaces: Thoroughly clean the mating surfaces of the housing and engine block to remove any old gasket material or corrosion.
6. Install New Kit: Place the new thermostat, seal, and gasket into the housing, ensuring correct alignment.
7. Re-install Housing: Secure the thermostat housing back onto the engine and tighten the bolts to the manufacturer's recommended torque.
8. Refill Coolant: Refill the cooling system with the appropriate coolant.
9. Reconnect Battery: Reconnect the negative battery cable.
10. Final Check: Start the engine, allow it to warm up, and carefully check for any coolant leaks around the housing. Monitor the temperature gauge to ensure it stabilizes at the correct operating temperature.

Troubleshooting & FAQ

Q: How do I confirm this thermostat kit fits my specific engine?

A: Start by checking your engine model and year against our Compatible Engine Models table. Next, use the Part Number Cross-Reference table to match your existing part number with the numbers listed for Mercruiser, Sierra, GLM, or Mallory. Visual comparison with our product images is also recommended.

Q: What are the common signs of a failing thermostat?

A: The most common indicators are engine overheating or, conversely, running too cold and never reaching its proper operating temperature. You may also notice erratic readings on your temperature gauge or see coolant leaking from the thermostat housing area.

Q: How is this different from the original OEM part?

A: 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.

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

A: 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.

Q: What is your warranty and return policy?

A: 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.