So my girlfriend was driving my truck (1998 F150 5.4L V8 w 145k miles) on the interstate and noticed that the truck was starting to shake over 50mph and that the check engine light would switch on and off. She brought it into the Sears center and had them check and after doing a test drive themselves they think its Cylinder 4 misfiring.

I believe this is a common issue with the Triton engines since the Coil Pack sits right below some hoses that are coming from the heater. Has anyone else had this issue and how hard is it to replace the Coil Packs especially #4 as its in the far back? Spark plugs were just replaced by the dealership last summer so don't believe it would be the spark plugs at this time. Maybe the lead wires from pack to the spark plug? How would I test?

Let me know what you guys think or if you think its some other issue I might be experiencing. Thanks!

 

You're correct in making sure that NO water or coolant is getting to or into the recessed areas of the plugs this can cause misfires as well. yes it common for that area

If it is dry then just replace that COP (Coil On Plug), and boot. That said, some have pretty good success just changing the one showing miss fire...others say to do them all....eventual other misfires to come... this kinda depends on how many miles are on your unit and the level of care given to this point. Which ever route you take, Just the 1 or all, be sure to use Dielectric grease or compound on the boot to the plug...this helps seal moisture out. or... if you have the Coil pack, then replace the plug wires.... all of them.

IF there is no corrosion on the plugs top, you're probably OK to leave them in...but if there is, then I'd replace the plugs too. Unless you drive like 60k mile a year..then change them anyway, but if you're like most who only put on 10-15k miles per year you're probably OK.

Considering the location....#4 cyl....might not hurt to have a new plug on hand in case the misfire doesn't clear up....there is a possibility that the misfire ruined the plug

 

when I had my 97 F150 I would remove the front tires and sit truck on Jack stands just high enough to get the jack in and out

Really makes plug 4 much easier to access