Symptoms, Diagnose a Bad Thermostat

Symptoms of a Bad Thermostat


Is a bad thermostat causing the engine to overheat? The engineering requirement on system critical components takes into account failure analysis. In modern times, the thermostat fails in the open flow position. The intention is to design the cooling system to be resilient to partial failures.

The typical thermostat functions by using a heat sensitive spring. This spring holds a diaphragm closed and prevents coolant from circulating. At high temperature the spring weakens and compresses to allow for coolant to flow to the radiator.

Most references to a stuck thermostat are misleading. They’re mostly a term of the past and only apply to some classic cars. All in all, a stuck thermostat implies that the valve is rusted shut and unable to open. But this is unlikely in modern cars as the coolant contains effective anti-corrosion components. Also, modern materials are far less rust prone, and nearly all thermostats are zinc coated.

Identifying the forces that act on the thermostat can help figure out its function. There is a spring that acts against the coolant flow to hold the valve closed. The coolant flow pushes back on the spring. These 2 opposing forces decide if the thermostat allows flow.

Can I just remove it then?


The purpose of the thermostat is to regulate the ratio of coolant going to the radiator versus that directly re-entering the engine. The benefit to closing the valve is a faster warm up and also a partially open state. Regulating the opening size, and thus the flow rate through the radiator helps maintain steady engine temperature. As such most failures will leave the thermostat at least partially open. The spring weakens and allows for flow through the radiator.

The main symptom of a failing thermostat is the inability to reach normal operating temperature. It’s most noticeable sign is a lukewarm cabin heater. But engine overheating can happen for many reasons, and an overheating alarm can prevent extensive damage.

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