How to Inspect and Replace a Mercury Verado Thermostat

The thermostat on most Mercury outboards should be inspected annually or every 100 hours. It’s a task that usually can be done along with the 100-hour oil-and-filter and gearcase lube change, or when offseason layup is performed.

Most do-it-yourself owners understand the importance of the oil and lube change, but the thermostat is often overlooked. The function of the thermostat is to control the flow of cooling water through the engine. When the engine is cold, the thermostat stays closed or restricts water flow, which allows the engine to warm up to its optimal operating temperature. 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.

When inspecting the thermostat, check for a broken spring, for corrosion that may be hampering its operation or for debris stuck in the thermostat, which could prevent it from functioning properly. This is especially important for motors used in saltwater. The thermostat on a motor used in freshwater may last many seasons, but should still be inspected.

In this how-to video, Mercury Pro Team member Scott Glorvigen of Wired2Fish demonstrates how to inspect and change the thermostat on a Mercury 350hp Verado outboard model, and the procedure is similar for other Mercury FourStroke, Verado and Pro XS outboards. Replacement of the thermostat requires a 5/16-inch socket and ratchet wrench plus a tool for removing and crimping Oetiker hose clamps. This tool can be purchased at auto parts and tool stores, and online.

For specific instructions on locating and changing the filter on other Mercury outboard models, consult a Mercury owner’s manual for your model outboard. The Quicksilver thermostat for Verado models (part No. 892864T04) and the appropriate one-time-use hose clamp (part No. 850896) to connect the thermostat to the hose are available through Quicksilver dealers, retailers and online.