Sierra Thermostat 18-3643 for Johnson/Evinrude 6 HP and 8 HP 1990 Outboards

The Sierra 18-3643 is a thermostat built for the cooling system of Johnson/Evinrude (OMC/BRP) outboard motors. It is the correct thermostat for the 1990 model year 6 HP and 8 HP outboard engines in this engine family, where it regulates coolant flow through the powerhead to keep operating temperature within the engine's designed range. Maintaining the correct thermostat opening temperature protects the engine from running too cold (which reduces efficiency and promotes internal corrosion and carbon buildup) or too hot (which risks overheating damage), so a worn, stuck, or corroded thermostat is a common cause of erratic temperature gauge readings and cooling problems on these outboards.

As a direct-fit replacement part, the Sierra 18-3643 is sized and calibrated to install in place of the original thermostat used on the applicable Johnson/Evinrude 6 HP and 8 HP 1990 powerheads, restoring proper coolant regulation without modification to the cooling system. It is a compact, lightweight component at 0.11 pounds, intended to be swapped in during routine cooling system maintenance or when troubleshooting overheating or overcooling symptoms.

Specifications

• Brand: Sierra
• Manufacturer Part Number: 18-3643
• Product Type: Thermostat
• Category: Evinrude Outboard Thermostats and Gaskets
• Fitment: Johnson/Evinrude (OMC/BRP) 6 HP, 1990
• Fitment: Johnson/Evinrude (OMC/BRP) 8 HP, 1990
• Weight: 0.11 pounds
• UPC: 808282375508

Fitment

• Johnson/Evinrude (OMC/BRP) 6 HP outboard, 1990
• Johnson/Evinrude (OMC/BRP) 8 HP outboard, 1990

Frequently Asked Questions

What engines does the Sierra 18-3643 thermostat fit? The Sierra 18-3643 fits Johnson/Evinrude (OMC/BRP) 6 HP and 8 HP outboards from the 1990 model year.

How much does this thermostat weigh? The Sierra 18-3643 thermostat weighs 0.11 pounds.

What is the part number I should order? The correct Sierra part number for this Johnson/Evinrude 6 HP and 8 HP 1990 thermostat is 18-3643.

What does a thermostat do in an outboard cooling system? In the Johnson/Evinrude 6 HP and 8 HP 1990 outboards this part fits, the thermostat regulates coolant flow through the powerhead so the engine reaches and holds its designed operating temperature.

What are signs the thermostat needs to be replaced? Erratic temperature gauge readings, the engine running too hot, or the engine running too cold can indicate the thermostat has failed and needs replacement, such as with the Sierra 18-3643 on applicable Johnson/Evinrude 6 HP and 8 HP 1990 models.

Is the Sierra 18-3643 a genuine Sierra part? Yes, 18-3643 is a Sierra branded thermostat manufactured for Johnson/Evinrude (OMC/BRP) 6 HP and 8 HP 1990 outboard applications.

Does this thermostat require any modification to install? No, the Sierra 18-3643 is sized to install as a direct replacement in the cooling system of the applicable Johnson/Evinrude 6 HP and 8 HP 1990 outboard without modification.

What is the UPC for this product? The UPC for the Sierra 18-3643 thermostat is 808282375508.

Will this thermostat work on other horsepower outboards outside 6 HP and 8 HP? The Sierra 18-3643 is designated for the 1990 model year Johnson/Evinrude 6 HP and 8 HP outboards; other horsepower or model years are not confirmed fits for this part.

Can a bad thermostat cause overheating on a Johnson/Evinrude outboard? Yes, a stuck or corroded thermostat is a common cause of overheating on Johnson/Evinrude 6 HP and 8 HP 1990 outboards, which is why replacing it with the Sierra 18-3643 is a routine cooling system fix.