How to Troubleshoot an Overheating Mercruiser 5.0L

How to Troubleshoot an Overheating Mercruiser 5.0L

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The Mercruiser stern drive is one of the most popular marine propulsion systems on the water. It is reliable and usually the annual maintenance performed during winter layup will prevent overheating problems. But when things do go wrong with the cooling system, the Mercruiser is easy to troubleshoot and repair. This article assumes that your boat has a raw water cooling system. That is, one that draws water from the lake, circulates it through the motor, and then discharges it back into the lake.

Items you will need

  • Garden hose

  • Cooling system ear muffs

  • Standard tool set that includes wrenches and screwdrivers

  • Replacement raw water impeller

  • Replacement thermostat

Depending on the age of the boat and the maintenance history, the most likely cause of an overheat condition is the impeller located in the lower unit of the drive. Newer boats have a warning siren that alerts the pilot that water is not being pumped up from the lower unit. Older boats may not be equipped with the warning siren. To troubleshoot, install a section of clear water hose at the input to the thermostat housing. If the boat is on a trailer, attach a garden hose and ear muffs to the raw water intake on the lower unit. Start the engine. If the impeller is not working, no water movement will be seen through the clear hose. Replace the impeller and re-check for water movement through the clear hose.

If the overheat condition only exists when the boat is run at high speeds, the cause may be clogged exhaust manifolds or risers. The Mercruiser has inspection ports on the font of the exhaust manifolds. Remove the ports and visually inspect them for deposits that would restrict the water flow. Remove the manifolds if they appear clogged. Use a metal rod to knock rust and deposits loose from the interior of the manifolds and flush with water. Perform the same maintenance on the risers.

A stuck thermostat can also cause an overheat condition. Remove the thermostat from under the thermostat housing and replace it with a new one. Make sure to install it in the direction indicated on the thermostat body. Never use a thermostat designed for use in an automobile. They are different and using a car thermostat can cause damage to the boat engine.


  • Warning: Never run the engine without water circulating through the lower unit. Even running the engine for a moment will cause damage to the impeller. Also, never run the engine faster than an idle while on a garden hose and ear muffs. A garden hose cannot deliver the amount of water required to prevent damage to the engine at high rpm.


  • Performing annual maintenance, such as replacing the impeller and thermostat, will prevent overheating problems before they start. Also inspect all hoses and belts during annual inspection.
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