Hello,

I cannot guarantee the accuracy of this information, but this is the
workings of a pressurized cooling system as it was explained to me.

First consider the system as one large container.  If the container and its
content get heated, does the pressure increase?  Yes, if the contents
expands at a greater rate than the container.  No, if the container expands
faster than the contents.  If we consider the first case, how much pressure
increase can be expected?  Well, much of the container is fairly flexible,
like the hoses and the thin tubes in the radiator and heater cores.  So,
even if the coolant expands faster than the container, the pressure
increase would be small.

So then, where does all the pressure come from?

Head gasket leaks!

As it was explained to me, even the best head gaskets leak ... a little.
 So, as the engine runs, the cooling system pressure gradually builds until
the pressure cap relieves the pressure.  But, the gasses leaked from the
head gaskets are supposed to accumulate at the pressure cap, so coolant
doesn't leak, rather these gases leak.

So why then is the overflow of the coolant recovery tank an indicator of
bad head gaskets?  Well, if the head gasket leaks badly enough, instead of
just gasses at the pressure cap there gets to be foam.  So, as the pressure
gets relieved, coolant flows to the coolant recovery tank along with the
gases.

So, the bottom line is that a little leak is OK, a big leak is not.

If the pressure cap opens at too low a pressure, and that's often how they
fail, the coolant boils in the hot spots of the engine even at normal
operating temperatures.  The resulting foam carries coolant out the
pressure cap just like a bad head gasket leak.

So, I'd try the inexpensive pressure cap first before the expensive engine
repair.

Thanks for listening,

Mike  --  Phoenix Arizona (something like Egypt)