Mike wrote:
>
> "Mike Walsh" <spam_sucks@bellsouth.net> wrote in message
> news:47F6750C.D65AF225@bellsouth.net...
> >
> > An idiot light might not come on soon enough to prevent engine damage. On
> > some vehicles if you loose coolant pressure because of a leak the boiling
> > temperature will be too low to cause the idiot light to come on.
>
> I don't believe that is true. The coolant does not have to boil for the
> light to come on. The temp sensor only senses the temperature of the water, it
> has no way to determine if it is boiling or not. What may happen with a sudden
> leak is the coolant level will drop low enough that the temp sensor is no
> longer immersed in coolant. Being the sensor is no longer in contact with the
> coolant it will not turn the light on until it is too late.

If there is no pressure e.g. because of a loose cap the coolant can slowly boil away without the idiot light coming on. The same is true if you use only water with no antifreeze. The boiling point of water at sea level pressure is 212 degF. With 50% antifreeze and a high pressure radiator cap the boiling point will be about 270 degF. An idiot light might not come on until 240 degF.
You are correct about the light not coming on if the coolant is low. I have seen engines that became so hot that the the head gasket blows and the driver swears that the idiot light never came on.

--
Mike Walsh