Originally Posted by Joe Styles
Changed out all plugs & two coils in my 1999 5.4 and now it's only missing when in fourth gear at speed's from 40-50 mph.
Before that it was missing in all gears.
It runs smoth at speeds over 55mph,around town I have to keep it in over drive so it will not buck.
I pulled the super chip and cleaned both computer and the chip,I thought I had it fixed but only lasted 20 miles.
I had the chip since new,do they go bad.
What else can I check.
You have a low grade misfire Joe. Keep those two COP you purchased as spares - Go to Global Automotive , purchase 8 COP's (DG 508'S) from Kevins store. You'll only pay $96 for all 8 together.
Install these at the same time to keep your 5fours integrity intact and you problem will go away .
It happens quite a bit and you can search this forum if you like . When the COPS get weak , there favorite spot to act up is 45mph (in the 5.4L) just before the shift into OD.
Heres the COP deal link -
Make sure you cover all the bases and use Motorcraft Plugs (there OEM ) for best results.
You can check out Matts directions on a sparkplug change here -
I'll add that you need a tube of dielectric grease and apply liberally to the insides of all COP boots (very important) Spark goes nuts inside the boots without grease and will carve a some carbon trails with a new COP system.
She'll feel like a different truck when your done