When a car thermostat fails

Your car’s thermostat is a really simple device. But if it fails, your engine can overheat and self-destruct. Here’s how thermostats work and how they fail. The manufacturers inject a mixture of ground-up brass and wax into a copper cup (called a pellet). Then they slip a highly polished metal rod through a rubber “O-ring” gasket and into the wax. The entire pellet is sealed with a crimped ring. Next, they weld a metal “skirt” around the pellet. When the engine is cold, a spring forces the skirt up against a seat (just like a closed faucet), stopping the flow of coolant. As the engine heats up, the wax melts and expands. The expansion pressure builds to the point where the wax literally tries to “spit out” the metal rod. But the rod can’t go anywhere. It’s attached to a “bridge” on the other side of the thermostat. Eventually the pellet itself moves, overcoming the spring’s pressure. So the thermostat opens and allows coolant to flow. The entire system works well until the metal rod corrodes. That corroded rod damages the rubber seal and the wax leaks out. Once that happens, the thermostat stops opening, coolant stops flowing and your engine overheats. The result can be catastrophic engine failure costing several thousand dollars.

So, if your thermostat fails, don’t just replace it and top off the coolant reservoir. It’s sending you a message of a more serious problem—the corrosion inhibitors in your coolant have failed. So flush the system and add fresh coolant any time you replace the thermostat. And don’t try to save a few dollars by buying a cheap thermostat (about $5). Premium thermostats (about $15) are built to much higher quality standards to resist corrosion. The difference in price is small compared with the potential engine damage. Some manufacturers even offer “fail-safe” models that fail in the fully open position to prevent engine overheating.

Fail-Safe Thermostats

Rick Muscoplat, Automotive Editor

To learn more about thermostat construction, click here.


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