Depending on how bad it is, if there is no obvious things like leaks or an old fuel filter, I'd guess the secondaries are constantly open. The cars run an inlet manifold runner controller, this is to save on fuel under 3.5k revs, when you go full throttle over 3.5 revs the secondaries open, so the car gets more fuel/power. Due to the absolutely genius place ford located the IMRC, bolted to the front rocker cover which gets very hot, it's very common with age for the PCB in the IMRC to cook itself, which results in the secondaries not opening at 3.5k. Now this, will actually improve petrol consumption, but a loss in power. Instead of replacing the IMRC, a lot of people will manually jam open the secondaries, so your getting full power from idle, this affects petrol consumption, you use more.

But, I've had three V6 mondeo's, and all were pretty bad on fuel, and that was with fully working and relocated IMRC's