See
here, and
here,.
The theory being that P4 CPU will never burn out, even without a heatsink, but will just run run at an increasingly slower speed. However developers papers show that the max temperature before they start slowing down is around 80°C. And see
here:
Regardless of enabling of the automatic or On-Demand modes, in the event of a catastrophic cooling failure, the processor will automatically shut down when the silicon has reached a temperature of approximately 135 °C.