Gone Outdoors

How to Remove a Mercruiser Thermostat

by David Curtis

The thermostat monitors and maintains the operating temperature of a Mercruiser engine. It performs this function by opening and closing to regulate the intake water flow while the engine is running. Most marine thermostats operate between 160 and 180 degrees Fahrenheit. Operating temperatures outside of this range are abnormal and should be investigated to find the problem. Obtain the proper marine thermostat from your Mercruiser dealer. Regular replacement of the thermostat is something you can do yourself with some common garage tools.

Open the engine compartment and locate the thermostat housing. It will be mounted on the front center portion of the engine block with four hoses connected to it.

Remove the hose clamps connecting the cooling hoses to the housing with a large flat screwdriver. Remove the hoses and inspect them for cracking and softness. Replace them if necessary.

Remove the two bolts holding the thermostat housing to the block with a socket wrench. Carefully pry the housing loose from the block with a small flat screwdriver. Be careful not to damage the housing or mounting surfaces.

Examine how the old thermostat is mounted into the housing. The new thermostat will need to be placed in the same order to operate properly. Some models are equipped with a plastic spacer holding the thermostat in place that will need to be removed first. Remove the old thermostat with a set of pliers.

Remove the old gasket material from the mounting surfaces with a razor blade scraper. Place the new thermostat into the housing in the proper direction. Apply gasket sealer to the mating surfaces and place the thermostat gasket over the opening on the engine.

Align the thermostat housing on top of the mounting surface and reinstall the bolts. Snug them securely but do not over tighten. Tighten them to 30 foot-pounds using a torque wrench. Reattach the cooling hoses to the housing.

Connect a Mercruiser starting tool to the water intake ports located just above the propeller. Connect it to a garden hose and turn on the water. Start the engine and look for any leaks. Allow the engine to reach operating temperature to ensure the thermostat is operating properly.

Items you will need
  • Flat screwdriver
  • Socket set
  • Torque wrench
  • Razor blade scraper
  • Gasket sealer
  • Gasket
  • Replacement thermostat
  • Pliers
  • Mercruiser flush tool
  • Garden hose

Warning

  • Never run the engine without water circulating properly through the engine. This will result in damage to the water impeller and engine.

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