The intake manifolds are known to fail and are basically considered a normal maintenance item every 150,000 or so miles. If it is just that, then replace it and keep on driving that Lincoln. I suggest taking it to Ford and use only Ford intake manifolds as the aftermarket ones like Dorman have been giving people problems with vacuum leaks. They tend to start leaking around the thermostat, where the heater hoses are, or at the cylinders.  Get the car checked by Ford first then decide.

Yes, I had a couple grandmarquis motors that the coolant went through the plenum chamber, and if the gasket wore out, that would happen.. is the oil milky? change the oil.. immediately if it is .and have the car checked out