I recently had a similar problem. My fuel economy has been gradually decreasing over the last year...no codes though. Last week, I started to notice a gradual decline in engine power. I changed the fuel filter then took it for a ride. It seemed to do the trick until...POP, HISS, uh oh. I brought the car to my mechanic, and the code for faulty EGR valve popped up. Turns out the actual problem is with my Catalytic Converters...all three of them were plugged solid and one of them split, causing the HISS. I was told by Ford that this is common and is probably to blame for the decreased fuel economy. I am supposed to get the car back on Monday. I can confirm it then.
Oh yeah, the cost for new Cats is $1800 Canadian, not including labour!!! Luckily, Ford has a secret warranty on the Catalyst system for 8 years / 130000 Km, so the repair won't cost me a dime!!
Just one possibility.
1998 Ford Contour GL, 2.5 Duratec
