On my 98 f-150, 4.6, The boot on number 4 would pull off and trip the CEL, misfire on 4. I would reach back there and sure enough it had popped up about an inch. I would push it on and all would be good for a while, would do it again. Then it also started to miss, missfire on 3, found the same issue. After countless times getting out and pushing them on, I finally pulled the wires off and stuck a extra old plug on them. What I found was when the plug snapped into the wire the boot was compressed about 3/8". I suppose it sat there under pressure until it popped off. Money is too tight right now for new quality wires, so i just snipped off about a 1/2" of the boot at the bottom. Problem solved. Anyone else have this issue?

 

You will often have issues like this when non-OEM wires are used. Us only genuine Motorcraft or Autolite brand wires on Fords. It makes a difference. If you are using OE wires, then there must be a manufacturing defect or the wires are intended for a different application.

 

They are the OEM wires that came with the truck in 98. All has been good till this winter. I think age pulls the inside of the wire up so in reality the boot becomes longer. One of the reasons I do not want to replace them is just as you say. If you don't buy good top of the line wires you will have nothing but problems. The wires are fine otherwise and it runs and starts perfect in our harsh Minnesota climate.

 

Are you sure your plug isn't leaking? The hot bypass air could be pushing the boot up?
Be very careful to use a torque wrench and a gentle touch on the plug!!!!