My F250 has a fuel injected 302 windsor engine. The motor slowly got worse and worse fuel economy. The idling went up and down from 500 to 1500 revs and when it got down to 500 it would nearly die and i would have to rev it to keep it going - this would result in black smoke like a badly running diesel! It eventually clears out though once driving but it still has bad economy. I have replaced the oxygen sensor and got the computer checked out but it is fine. I took some of the fuel injection system off to take a look at it but now it is back on and the injectors don't even open anymore. Please Help me because no-one seems to be able to fix it.