My dad has a 2000 expedition with a 5.4 and 164000 miles. when the truck hits 1400 to 1550 rpm, there is an off beat jerk in the truck. when it is put in 3rd gear, it dissapears even if I am traveling at the same speed. when the truck is in neutral and I move slowly through the rpm range, there is no jerk.

The man who we bought the truck from said he thought it was a COP gone bad, but I'm not so sure.

Any thoughts?

 

Sounds like a low grade misfire, which would make him right. Most likely a coil. The problem is they are hard to hunt down which cylinder when they are intermittent like that. You can buy a known good one and swap with the number 1, then drive it for a few miles and see if it goes away. If not then swap back and try number 2 and so on. But that takes forever. Your best best because its probably due for new cops is to pick a set up at a recommended source like global automotive or uneek on ebay. Look for their stores, they are reputable (since these 5.4's are picky) and cheap, around $100 for the whole set of 8. Considering they are $40 and up each anywhere else, the math is simple. Plus they have a lifetime warranty. Keep your old ones for spares down the road or for future diagnostic reasons.

 

The Last 2 coils I had crap out it missed under any acceleration. If I had that problem I would go ahead and swap out for a full msd coil set. Mine just hit 200k and has 3 or 4 stock coils still in it. but the time is near!

 

Yup, bad or on the way out coil