Mercury Verado – How to Change a Thermostat

Does your Mercury Verado need a new thermostat? This critical question should be answered as part of 100-hour, 300-hour, and other interval service or if your engine is not performing up to par.

In the video embedded within this article, the thermostat change was performed on a Mercury Verado 300. However, the video and the step-by-step instructions included within this article are applicable to:

• Mercury Verado 4-cylinder outboards (135 – 150 – 175 – 200hp)
• Verado 6-cylinder outboards (200 – 225 – 250 – 275 – 300 – 350 – 400hp)

What You’ll Need for the Job

What Can Go Wrong with Your Mercury Verado Thermostat?

If your outboard thermostat is not working properly, a broken spring, corrosion, or debris are the most likely sources of the problem.

Broken Spring

The thermostat in a Mercury Verado outboard engine has a spring that helps to control the opening and closing of the thermostat valve, which regulates the flow of water through the engine. When the engine is cold, the thermostat valve remains closed, which allows the engine to warm up quickly.

If the spring in the thermostat becomes weak or breaks, it can affect the performance of the engine’s cooling system, which may lead to overheating or other issues. Specifically, if the thermostat is stuck in an open position, the motor may never warm up properly, which can hamper efficiency and cause long-term durability issues. If it’s stuck in a closed position, the engine could overheat.

Corrosion & Debris

Over time, the brass in your Mercury outboard thermostat will become corroded, and debris like shell fragments, dirt, and grime can accumulate. Both corrosion and debris will impact your Mercury outboard thermostat’s performance.

Signs Your Mercury Verado’s Thermostat May Need Replacing

Here are a couple of signs that may indicate that your thermostat needs to be replaced:

• Overheating—if your engine is running hotter than usual or the temperature gauge shows that the engine is overheating, it could be a sign that the thermostat is malfunctioning.
• Poor performance—a faulty thermostat can cause your engine to run poorly, reducing power or acceleration.

How to Change Your Mercury Verado Thermostat

The following details the parts and processes to change your Mercury Verado’s thermostat.

You’ll need the following:

• Mercury Verado thermostat with housing, SKU# 892864T04 (superseded by SKU# 8M0174561)
• Mercury clamp – 54-888988001 – SKU #54-888988001
• Screwdriver or drill will with screwdriver bit

Step-by-Step Instructions for Changing a Mercury Verado Thermostat

1. Turn off the engine and allow it to cool down completely.
2. Locate the thermostat housing. It is typically located near the top of the engine block and is connected to the engine’s cooling system by a hose.
3. Remove the hose from the thermostat housing by loosening the clamp.
4. Remove the bolts that hold the thermostat housing in place.
5. Lift the thermostat housing off the engine and remove the old thermostat.
6. Clean the inside of the thermostat housing and the mating surface on the engine block. It is common for shell fragments and other debris to accumulate in the thermostat housing.
7. Install the new thermostat, making sure that the spring end of the thermostat is facing into the engine block.
8. Grease the edges of the thermostat housing.
9. Replace the thermostat housing and tighten the bolts.
10. Reconnect the hose to the thermostat housing and tighten the clamp.

**Important Note**

Always consult your engine’s owner’s manual for instructions and information specific to your engine. If you need clarification on any part of changing your Mercury Verado’s thermostat, consult a Mercury Marine-certified mechanic.

PartsVu and Platinum Mobile Marine Partnership

Huge thanks go to Frank Cutri, the owner and operator of Platinum Mobile Marine, a mobile mechanic service serving the Lee County, Florida area for partnering with PartsVu on the Boat Maintenance Matters series.

Together, we are working hard to build a library of step-by-step instructions on common outboard motor repair and maintenance tasks.