The thermostat is often out of ajustment or completely shot. The thing is very simple. A switch (on/off) is activate by the push an release action of a bi-metal spring. As the temperature increases, the pig tail spring wind up until the little lifter hit the micro-switch arm. It needs just a gentle upward toutch to the micro-switch to open the circuit. The micro-switch is normaly closed what means in the ON position and that the 12 volts current is going to the fuel pump.
The gap at the lower position
The spring can be removed for reajustement
Installing the spring back to the unit