Why Do Diesels Have Two Thermostats?

By: Ray Bohacz

Most, if not all, large diesel engines found in trucks and farm equipment employ a dual thermostat system. For the large part, the dual thermostat has replaced grille shutters from old days. Though a very effective system, unlike the grille shutters, you need to understand why dual thermostats are used and how to diagnose the cooling system.

When the thermostat is closed (single or dual system), the coolant stays in the engine block and does not go to the radiator to be reduced in temperature. Most engines have a radiator bypass circuit that allows coolant movement in the engine when the thermostat is closed (engine cold) to hasten warm-up and to eliminate localized hot spots. Once the thermostat opens, coolant flows from the engine to the radiator, where heat is rejected into the air.