A defective thermostat can result in engine overheating and potential internal damage. Improve the reliability of your 2003 Toyota 4Runner 4.0L cooling system with a Mishimoto Low - Temperature Racing Thermostat and housing This thermostat is designed as a direct - fit replacement for your aged stock unit. The Mishimoto thermostat has a lower activation temperature (71C160F) compared to the factory thermostat (82C180F). The lower activation temperature allows coolant to flow through the engine earlier, which reduces and maintains lower engine temperatures. This thermostat is ideal for use in warmer climates, on vehicles making more power than stock, or under extreme towingdriving conditions. The stock - style housing will provide a proper seal and installs easily. Like all our Toyota Performance parts, this thermostat and housing includes the Mishimoto Lifetime Warranty.