Does it have low power? If so check how much exhaust is coming out of the tailpipes. You might have a plugged catalytic converter. This causes too much exhaust back pressure resulting in blown exh manifold seals, poor power, and poor fuel economy. Hope this helps.

From what you've said in your question here. First thing that comes to mine is a stopped up catalytic converter. That could be the cause of both the bad fuel mileage & the cause of a bad donut in the exhaust system. I know I've had this problem with one of my old 84 fords. It only had a 300 6 in it. And was giving about 18 miles per gallon until this happen & then it gave only about 4 to 6. Being I had another truck to take its place. I just transfered the Lic: & never having plans on putting that 84 back out on the road again. I pulled the muffler off and drove a rod back into & through that catalytic converter a few times, until I broke the honeycomb louse in it. Then started it back up a let it blow all that junk out. Then I put the muffler back on it & it runs well now. Even tho I never plan on using it off the farm any more. It will always be used haling hay and pulling other stuff around here on the farm. Anyway your problem could be many things. But I think I would check on that catalytic converter first. And if you replace it with a new one. Don't throw the old one away. Because these scrap iron places now days will give up to $40.00 or more for your old ones. Depending on what they where made of. And that is as long as you don't knock out all that stuff in them that they want.....Good Luck my FRIEND Hope you get better gas mileage soon, with these gas prices...WOW.........Plus 5 for your question.............M.C.S.

maybe you should take it to a mechanic