Help! I have a 99 F150 4.6 5sp with 179,000 on the clock. It started missing about a week ago. It would miss at low RPM under laod and over 3000 rpm. I took it to the shop that did the plugs and wires a year ago and so far they have replaced 2 plugs that they say were bad, DPFE sensor(cleared the CEL) flushed the injectors, checked for vacuum leaks, compression test, new fuel filter, checked fuel pressure. So far the low speed miss is cleared up but over 3k under load it missed and sputters terribly and I am $550 deep and they have thrown their hands up.
HELP!!

 

By the way I drove it a little while and the low speed mis is still there, it only misses under a load. It starts, idles, and cruises as if it were running like a top.

 

Need to check the fuel pressure with a gauge. I would also do a compression test to verify that you don't have a weak cylinder.

 

They did a compression test and checked fuel pressure. It starts to miss under load at low speed (RPM) and high speed as well. They say it might be something internal such as a valve or valve spring problem. The frustrating thing is that this exact same thing has happened at least four times since I bought the truck new with 2mi on it and every time plugs and wires cured it, so I have a hard time believing it could be anything internal this time. The same shop did plugs and wires august 07 approx 17k mi ago. I have spent $550 so far for nothing. HELP Should I take it to the dealer?? This place I have taken it to has a stellar reputation and I am baffled. I was snooping under the hood last night and did find a plug wire that was laying against the thermostat housing and the wire had worn into the aluminum housing. I moved it away but still had the miss.

 

I had a 1998 that had the same problem. It sounds if you have spark plug wires and not the coil on plug. The Spark plug boot on I believe it is number eight cylinder would vibrate loose after a lot of hard acceleration. It is the cylinder next to the fire wall on the driver side.
I fixed this problem weekly. I bought motorcraft plug wires and it did stop the problem.

 

I visually inspected all the wires last night and checked to see that all were snapped onto the plugs. I took it back this morning to the place that has been working on it and told them to call me when they figured out what is wrong. I refuse to believe there is anything wrong internally because without a load it runs like a sewing machine.

 

I also replaced the two coil packs at 120K and that was also done when I installed the motorcraft wires. I did use motorcraft coil packs.

 

I had a 97' 4.2l V6 that I was going crazy with the similar problem after taking it to the dealer several times I decided to do my own troubleshooting. With the truck running I took a pair of plastic pliers and pulled spark plug wires off one at a time and discovered that the pack was bad. I know you said that you replaced the packs 60k ago but figure it is worth a shot. I find that sometimes it is best to forget what has been done and start fresh on stubborn problems.

 

Well after the third trip in it turned out to be plugs and wires just like I thought from the beginning.