The Villager has a V-6 engine that has two head gaskets. Depending on which head/gasket is bad, it may be easier to remove the engine from the vehicle than try to work on it in place, since the rear head and exhaust manifold is very hard to get at. If you're going to all the trouble of doing that, it pays to have the heads carefully inspected and a valve job done, as well as replacing the timing belt and water pump while it's all apart, as this could save a lot of trouble later.
You don't, you replace or rebuild the engine.
How do you change the thermostat in 1999 Mercury Villager
Yes.
Probably around $200 for the tank, and $200 for labor.
According to the 1999 Mercury Villager Owner Guide : ( 20.0 US. gallons )
The most likely cause is an intake manifold gasket leak (that is if all the hoses and water pump have been ruled out)
This is as complex procedure. I suggest you buy a repair manual if it is not obvious to you, and you don't have experience.
In the engine.
134-A
No, no room.
In the fuel tank.
There are two.