I had a tuneup done on my '98 4.6L on Saturday. I asked the dealer to replace the wires and believe it or not he talked me out of it. He said they would test them first and if they needed to be replaced they would replace them, but at $120 for just the wires not including labor he recommended not to.
The wires tested fine and I didn't get new wires. On Monday it felt like it was skipping, so I brought it back in and they said the number 8 wire came loose from the plug and I was good to go again.
Last night, it felt like I lost almost all power, and it sounded it too. I actually thought I lost a valve. The noise could be heard from inside the cab and it was making a very loud tapping noise.
The check engine light was flashing constantly. I drove it straight to the dealers last night for them to look at it today. They called me and said I needed new plug wires because they weren't holding onto the new plugs. They did give me the employee price and did not charge me labor. The truck runs again.

Is this common? Anyone else ran into this? I usually do my own preventive maintenance and I have never had a problem with plug wires falling off for no reason.

 

Did you replace the plugs with the exact ones that came out of the truck? If so, everything should have been the same. I could see it if differnt brand plugs had smaller diameter porcelains, but if they were the same plugs, they should have held just fine. Something isn't adding up here.

I replaced all my plugs with Bosch Plus 4s (did it myself), and had not cable issues at all.

 

A few other members here have reported this problem when going from the original wires and new plugs to new plugs only. What causes it? Who knows... Just curious - how many miles on the plugs and wires?

 

I had the same problem as u.

 

I had 93k miles on the original wires. In my opinion it was time to replace anyway, and I was shocked that the service manager suggested he test the old ones first. He made the decision not to replace them. It still doesn't make sense to me though, I could see if they were different plugs with different diameters, but it's the same plug.

Oh well, problem solved.

 

Well, with that mileage it isn't a big deal they needed replacement. I did mine at 112k. Glad they took care of you after the fact, though!