How to Replace the Thermostat on a Merc Cruiser

Written by norm rogers
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How to Replace the Thermostat on a Merc Cruiser
If your thermostat malfunctions, you'll need to replace it. (motor boat image by Wimbledon from

Although the thermostat in your Mercruiser does not require replacement all too often, it is a good maintenance project that the do-it-yourselfer should know how to perform. The thermostat regulates the operating temperature of the Mercruiser motor for optimum performance. Should it malfunction and cause an overheating condition, it will need to be replaced.

Skill level:

Things you need

  • Socket set and wrench
  • Pliers
  • Flat blade screwdriver
  • Replacement thermostat
  • Replacement thermostat housing gasket
  • Gasket sealer
  • Razor blade scraper
  • Replacement hoses and clamps (if needed)

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  1. 1

    Drain the engine block of water by opening the petcocks on the side of the block. Close the petcocks when the water stops draining.

  2. 2

    Locate the thermostat housing, which you can find by checking a repair manual that pertains to your particular Mercruiser year and model.

  3. 3

    Remove all the hoses and water lines attached to the thermostat housing. It may be tempting to replace the thermostat without removing all the hoses, but don't do it. Take this opportunity to check the hoses and clamps for wear. Replace them if necessary. In addition removing the hoses makes the following steps much easier.

  4. 4

    Remove the bolts that secure the housing to the engine block. Remove the thermostat housing. If the housing is adhered to the block by old gasket sealer, carefully insert a small flatblade screwdriver between the two parts to pry them apart. Do not pound or hammer on the housing as it can be easily damaged.

  5. 5

    Set the housing aside, and remove the thermostat. Take care to note how it is placed on the engine block. It is absolutely critical that the new thermostat be installed exactly the same way,

  6. 6

    Remove all traces of the old gasket material using the razor blade scraper. Insert the new thermostat into position. Apply gasket sealer to both surfaces and install the new gasket.

  7. 7

    Place the thermostat housing into position and carefully hand tighten the bolts. Take care not to force the bolts. They should thread easily into position. Lightly tighten the bolts with a socket wrench. There's no need to over tighten. The housing bolts on a V-8 only require 15 foot lbs. of torque.

  8. 8

    Attach the water hoses to the housing and tighten the clamps. Circulate water through the engine, and check for leaks. Make sure the temperature gauge shows proper engine temperature.

Tips and warnings

  • It is good advice to purchase a repair manual for your particular year and model Mercruiser. Repair manuals contain a wealth of knowledge for the boat owner.
  • Never run the engine without water circulating through the water intake. Doing so, even for a short time, will cause damage to the engine.

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