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I have the 5.4L in my '97 and have seen performance increases by adding the following: K&N air filter with cold air intake, Gibson split rear high flow exhaust, B&M Shift improver kit, new dual platinum plugs. My next mod will be the Super Tuner programmer to reprogram the ECM. In the past I have always started with these mods before moving onto the motor, and as you have probably already figured out, the 5.4L is a to work on. I lucked out by adding a 3" body lift so the motor is easier to get to. Try more air, let it breathe better and you will start to notice the difference. High octane gasoline always helps. Then, when you feel adventurous, clean the intake, i.e. throttle body, idle control, etc and it will run better. Hope this helps!!
P.S. you can get teh cold air intake without buying the whole system, it's a pretty easy modification to your intake with a K&N filter. Let me know if you're interested and I will e-mail you the link on how-to.
So far I have added a cold air intake and a hypertech programmer Supertuner. It has given increased performance and better shift points. As an added suprise my fuel milage actually went up by 2mpg. Next will be adding an exhaust system.