Mercury Mercruiser Cooling System

MerCruiser Cooling System Parts - Resources & Information

PartsVu stocks a wide selection of certified Mercury MerCruiser MerCruiser Water Pump Repair Kits, MerCruiser Thermostat Kits, Oil Coolers, Hoses, and everything else you need to fit the exact specifications of your MerCruiser motor. Your Mercury outboard’s cooling system prevents your engine from overheating, enabling a smoother operation and preventing the failure of critical engine components. Don’t wait until it’s too late to replace these crucial components. 

Not sure which MerCruiser parts or MerCruiser sterndrive parts you need? No problem! Check out some of our helpful MerCruiser resources to help you find the right Mercury MerCruiser cooling system parts and how to troubleshoot your MerCruiser engine:

• MerCruiser Inboard Maintenance
• MerCruiser Maintenance Parts Library
• Mercury Outboard and Mercruiser Resources Collection

MerCruiser Thermostats, Water Pump Repair, and Cooling FAQs

How do I know if my MerCruiser water pump is bad?

One of the sure signs that a water pump is acting up is if the motor temperature starts to climb during idle or while on plane.

Typically, the normal operating range for your Mercury MerCruiser motor at idle should fall within the 120 – 140°F range, most commonly right around 125 – 132°F. On plane, your outboard’s temperature will vary widely based on the temperature of the water (colder in cold water, warmer in warmer temperatures).

If you’ve noticed temperatures beginning to consistently rise above this range at idle, it’s very important to not use your outboard until the problem is completely resolved. Also, if your overheating alarm goes off, you can be sure of some issue is causing the overheating issue temperature up. 

If you’ve experienced how your outboard doesn’t throttle up as normal and bogs down, you may have a malfunctioning water pump. Essentially, the water pump doesn’t provide a consistent stream of cooling water through the lower gearcase via the inlets on the side of the lower unit. Because of this irregularity, your engine will intermittently heat up and cool down, creating an intermittent engine bog during high RPMs as the engine compensates through reduced performance (as part of its built-in safety features). While your boat may be operational, this effect masks the issue until the water pump fails completely.

What makes this condition hard to diagnose is that at lower temperatures—during the winter months or when the water is cooler—the engine may cool off due to the environment and perform normally. Then, the bog will reappear in warmer months, in warmer water, or during extended usage. 

Your MerCruiser also features a telltale hole, which emits a small, visible stream of water that passes through the powerhead. As the name implies, the tell tale helps indicate when cooling water is flowing to prevent overheating.

In most cases, a blockage often prevents water pressure from flowing efficiently from the hole. If you notice less-than-normal water falling from the hole or no water at all, insert a thin wire into the outlet to determine if it is blocked. Normal blockages include accumulated debris and salt (scaling), but insects often burrow into this hole. That is especially likley during periods when your outboard motor is being stored for the winter Read our guide How to Winterize a MerCruiser Sterndrive for more info about proper winterization procedures (most winterization tasks are the same for Mercury outboard motors, too).

If the tell tale hole is clear and the lack of flow continues, you might have a damaged pump housing instead. In this case, you may notice water flowing from another place on your outboard, such as a crack in the water pump housing.

How often should I change the water pump on my Mercury outboard engine?

In most cases, you won’t need to replace your MerCruiser water pump unless the housing is damaged. 

Instead, your water pump impeller is often the component most susceptible to failure - and is a much less expensive and time-consuming repair. As part of your regularly scheduled maintenance, you should check your impeller if you’ve noticed overheating issues or uneven performance. Most experts agree that checking every three years or sooner should avoid most water-pump-related issues and keep your outboard running in excellent condition.

However, if you’ve noticed persistent problems keeping your engine cool, you may need to replace the water pump completely. Bear in mind that replacing the entire water pump is a significant repair for your Mercury outboard motor. 

How do I replace my Mercury outboard engine water pump impeller? 

• Step 1: Begin by removing the gearcase/lower unit from the outboard.
• Step 2: Next, remove the rubber water tube seal from the top of the water pump assembly. 
• Step 3: Remove the four bolts that hold the water pump assembly in place (Note: there may be a fifth bolt located above the propeller, depending on your outboard model). 
• Step 4: Remove the water pump housing. 
• Step 5: Remove the impeller, including the two (2) nylon washers, located above and below the impeller. 
• Step 6: Remove the woodruff key from the base of the driveshaft. 
• Step 7: Remove the upper gasket, the wear plate, and the lower gasket from the gearcase.
• Step 8: Inspect the shift shaft boot for cracks or splits. If it is worn, replace it with a new one. 
• Step 9: Install the new lower gasket, wear plate, and upper gasket with the neoprene side of the upper gasket facing upward. 
• Step 10: Install the new impeller with the nylon washers above and below it. Make sure the impeller sits over the woodruff key for a proper installation. 
• Step 11: Grease the inside of the water pump housing and the top of the wear plate with marine grease. 
• Step 12: Reinstall the water pump housing while rotating the driveshaft clockwise. This will ensure that the impeller blades are turned in the correct direction within the water pump housing. 
• Step 13: Reinstall each of the four water pump assembly bolts. 
• Step 14: Install the new rubber water tube seal onto the top of the water pump assembly. 

How much does it cost to replace a water pump impeller on my Mercury outboard?

As you saw in the last FAQ section, replacing the impeller is a relatively straightforward repair. Depending on your model of Mercury MerCruiser, you can expect to pay around $25 - 35 per impeller - which is a bargain when you consider how much a complete overhaul of your water pump costs.

Of course, it’s more efficient to replace other components while you’re working on the fuel pump, so PartsVu recommends that you purchase water impeller kit to have everything you need on hand to perform a complete job. 

How do I select the right parts for my MerCruiser engines?

PartsVu’s Mercury MerCruiser Maintenance Parts Library is the perfect reference when you need to replace common parts. That’s because these parts charts make it simple to find the right parts for your particular engine to perform regularly-scheduled maintenance. Additionally, find your MerCruiser cooling system diagram in your MerCruiser Owner’s Manual - an invaluable resource when choosing the right parts and repairing your outboard. 

For MerCruiser motors, the first maintenance interval comes at 20 hours, then you’ll perform maintenance after 100 hours of service or one year of operation (whichever comes first). More extensive service is recommended after reaching 300 hours of service or three years, so the 100 hours addresses components that receive more wear and tear (such as fuel filters and oil filters).

After this period, maintenance is often more regular as the engine ages and parts wear out. For example, 1,000 hours generally requires an in-depth assessment by a qualified mechanic. Also, if you’re usually running your Mercury outboard in a more demanding manner, the manufacturer recommends more frequent servicing.

When selecting the correct Mercury Outboard Parts, there are some essential things every boater must keep in mind before purchasing and installing parts:

• Choosing MerCruiser OEM parts (original equipment manufacturer) over after-market parts often yields better long-term results for your engine as these components and products are specifically designed to meet the precise tolerances of Mercury engines.

• Check (and recheck) to ensure that you have the exact part number that matches your Mercury outboard engine. It’s important to do this because Mercury has many MerCruiser engine models that look very similar but contain slight differences in performance. Therefore, unmatched parts may not fit uniformly, leading to malfunctions, underperformance, and severe engine damage.

• Proper maintenance is essential when it comes to extending the life of your Mercury outboard motor and should always be mandatory to retain the value of your outboard motor. While Mercury outboards are built to last, hazards like salt water, humidity, excessive heat, electrolysis, and demanding usage eventually take their toll on your motor.

• Proper storage for your Mercury outboard is required to preserve your motor for the long term. Failure to properly winterize your Mercury outboard can cause components to degrade while the engine sits and is exposed to the elements. Therefore, properly store your outboard when it is not in use.