How do I Replace Thermostat Mercury 200Hp?

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When you need to replace the thermostat on a Mercury 200 horsepower motor, remember that there's a thermostat for each cylinder. The air and fuel rail hoses may partially block access to the bolts you must remove to gain access, and may hinder the removal of the cover, the gasket, the thermostat and the seal somewhat. Even so, there is sufficient room to remove and replace these parts without disconnecting the hose.

Items you will need

  • Paint stick

  • 14 mm thin-walled socket

  • Socket extension

  • Toothbrush

  • Standard slotted screwdriver

  • Thermostat seal

  • Thermostat gasket

  • 2-cycle motor oil

  • Torque wrench

Thermostat Removal

Press down on the top cover lock. Lift the top cover up, forward and away from the motor. Make the boat safe to work on by removing the battery cables from the battery.

Locate the thermostat cover on the top of each cylinder head; it's a small oval-shaped cover, secured by two bolts. Draw a small match-mark between the cylinder head and the cover with a paint stick. Remove the bolts with a 14 mm thin-walled socket and extension and pull the cover from the cylinder head.

Lift the thermostat out of its compartment, taking note of the direction in which the flange side facing the cover is installed. On most Mercury 200s, thermostats are installed with the flange side facing the cover but there are variations.

Remove the old gasket and any gasket material from the middle of the thermostat cover if it remained attached to the cover as you removed it. Clear any foreign matter from the compartment with a toothbrush and scrape the gasket material loose with a standard slotted screwdriver.

Thermostat Installation

Install the new thermostat seal in the compartment. Slide the thermostat through the seal.

Set the gasket on top of the thermostat and return the gasket cover to its position, using the match-mark to determine its orientation. Cover the threads of the cover retaining bolts with two-stroke motor oil and thread the bolts into place.

Tighten the bolts alternately, and in stages, to 20 foot-lbs plus a quarter turn of the wrench.


  • Do not use any gasket sealing compound on the thermostat housing gasket.
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