Sierra 18-3668 Thermostat Kit for MerCruiser 5.0LX and 898 305 cid V-8 Engines

The Sierra 18-3668 Thermostat Kit is a marine engine thermostat built for MerCruiser stern drive and inboard applications, replacing the factory thermostat as part of routine cooling system maintenance or repair. It is a Design II configuration with a check ball, which allows trapped air to bleed from the cooling system as the engine warms up, helping the engine reach and hold proper operating temperature. The housing and internals are finished in white and operate automatically using a mechanical wax-pellet style control, requiring no electrical connection or manual adjustment.

This thermostat kit is built to marine specifications, since automotive thermostats are not designed to handle the raw water or closed cooling demands of a boat engine. Sierra manufactures marine parts to meet or exceed the performance of the original equipment they replace, making the 18-3668 a straightforward drop-in option for the engine applications it covers. Each kit is packaged as a single unit count, ready to install during a cooling system service or when replacing a thermostat that is stuck open, stuck closed, or no longer regulating temperature correctly.

The 18-3668 is specified for MerCruiser 5.0LX 305 cid V-8 engines built from 1988 to 1995, and for MerCruiser 898 305 cid V-8 engines built from 1983 to 1984. Confirm your engine model and production years against these ranges before ordering to be sure you get the correct thermostat design for your application.

Specifications

• Brand: Sierra
• Manufacturer Part Number: 18-3668
• Product Type: Thermostat Kit
• Design Type: Design II, with check ball
• Color: White
• Temperature Control Type: Automatic
• Power Source: Mechanical
• Weight: 0.17 pounds
• UPC: 030999316318
• Unit Count: 1 count

Fitment

• MerCruiser 5.0LX 305 cid V-8, 1988-1995
• MerCruiser 898 305 cid V-8, 1983-1984

Frequently Asked Questions

What engines is the Sierra 18-3668 thermostat kit designed for? The 18-3668 is specified for MerCruiser 5.0LX 305 cid V-8 engines built from 1988 to 1995, and for MerCruiser 898 305 cid V-8 engines built from 1983 to 1984.

Does the 18-3668 have a check ball? Yes, the Sierra 18-3668 is a Design II thermostat that includes a check ball, which helps bleed trapped air from the cooling system as the engine reaches operating temperature.

How much does the 18-3668 thermostat kit weigh? The Sierra 18-3668 Thermostat Kit weighs 0.17 pounds.

What color is the Sierra 18-3668 thermostat? This thermostat kit is finished in white.

Is the 18-3668 an automatic or manually adjusted thermostat? It operates automatically using a mechanical, wax-pellet style control, so no manual adjustment or electrical connection is needed.

Can I use an automotive thermostat instead of the 18-3668 on my MerCruiser engine? No, the 18-3668 is built to marine specifications for MerCruiser 5.0LX and 898 305 cid V-8 engines, since automotive thermostats are not designed for a boat's raw water or closed cooling system demands.

What is included with the Sierra 18-3668 kit? The Sierra 18-3668 Thermostat Kit ships as a single unit count containing the thermostat for the listed MerCruiser applications.

When should I replace the thermostat on my MerCruiser 305 V-8? A thermostat like the Sierra 18-3668 should be replaced when it sticks open or closed, causing the MerCruiser engine to run too cool or overheat, or as part of scheduled cooling system maintenance.

Will the 18-3668 fit a MerCruiser 898 built in 1985? The Sierra 18-3668 is specified for the MerCruiser 898 305 cid V-8 for the 1983 to 1984 model years, so confirm your engine's build year falls in that range before ordering.

Is the Sierra 18-3668 a genuine Sierra part? Yes, 18-3668 is Sierra's own manufacturer part number for this thermostat kit, not a generic or third-party substitute.