I have a 1998 F-150 5.4 with just over 140,000 miles. The engine has been flawless, however over the last few weeks I have noticed a reduction in fuel economy of about 50 or 60 miles per tank or about 3mpg. Now the "Check Engine" light has come on and is staying on. The engine sounds and feels fine. I plan on taking it in asap but was looking for any ideas before I do as I don't want to get taken for a ride. My only idea is that it may be the plugs as they haven't been changed yet (I know). All I've done to the engine is changed the oil regularly and replaced a few vacuum hoses that were deteriorating. Thanks for any help.

 

plugs are a good possibility.

But in any case, you can go to an autozone near you and they can scan your truck for the CEL codes. Write them down and post them back here (or you can search for the code).