Overheat of the engine core???
Fire/overheat have nothing to do with the temperature of the engine core. What a load of nonsense. Please remember this: The engine is always on fire, unless you got a flameout!
Fire/overheat in inside the engine cowling, and has nothing to do with what takes place inside the engine itself.
It appears it does...from the B738 MRG
Each engine has eight overheat/fire detectors, four in loop A and four in loop B. The detector consists of a 3-level pressure switch. Each detector is connected to an individual stainless steel, gas charged sense tube.
The detectors monitor four sections of the engine ; the upper and lower fan case and the left and right core section.
In each section, two detectors (one of each loop) are mounted around the engine and make an assembly.
Gas in the sense tube is charged with a minimum pressure to keep the FAULT pressure switch closed. A leak in the sense tube causes the gas pressure to decrease and opens the FAULT pressure switch. Except for the OVHT/FIRE test, there is no flight deck indication of single loop failure. However the overheat/fire detection system will take the fault into account.
If both loops in one engine sense a fault in one of its detectors, the FAULT light on the flight deck illuminates and the system is inoperative.
A temperature rise inside and around the engine will cause the gas in the tube to expand. The increasing gas pressure will close the OVERHEAT pressure switch and further on the FIRE pressure switch.
Upper Fan Case
174 °C / 345 °F Overheat limit
304 °C / 580 °F Fire limit
Lower Fan Case
174 °C / 345 °F
304 °C / 580 °F
Left Core Section
343 °C / 650 °F
454 °C / 850 °F
Right Core Section
343 °C / 650 °F
454 °C / 850 °F