Review on MotoRad 7333-198 Fail-Safe Thermostat: Optimal Temperature Control for Enhanced Engine Performance by Matt Simpsons

I have a 1999 Ford Ranger with a 4.0 OHV engine. 198 degrees is the optimum temperature for this engine. The thermostat fitted as it should and solved the problem with the previously stuck open thermostat, but after 6 months this thermostat remained open. I understand this is a fail safe thermostat and should have failed in the open position but it should not have failed. I monitor the engine coolant temp with an OBD2 tool and the engine temp is usually between 180 and 203 degrees. When I pulled out this faulty thermostat, I saw that it was stuck open, but only on one side. There are small protrusions sticking out where the thermostat presumably should the engine overheat open to the protrusion part which in turn "hooks" on the valve and keeps it open. Not sure why it failed but replacing the thermostat after 6 months I have no intention of doing it again.