My daughter has an 05 Ram 1500 with a 4.7 l. It has thrown the code for misfire cylinder 7 numerous times. When she drives it, going from 0 to 40mph is no problem, going from 40 to 50 mph it shakes, anything above 50 it runs fine.
She had a mechanic look at it, and he suggested that it was the number 7 valve spring was bad. But I've never heard of an engine running rough only through certain speeds.
I thought if it was a valve spring it would run rough the whole time.
Any suggestions?

 

It's possible..... it may be running rough all the time, it is just more noticeable in higher gears/lower RPM, when the engine has a greater load, and you have a more direct mechanical connection from engine, to the rest of the drivetrain. Could also be a failing lifter, or valve seat. Best bet would be to pull the valve cover, and have a look.

 

It could be a lifter too. If they stick, it causes the valves to not open and close properly. Eventually, you lose a rocker... like I did.

https://dodgeforum.com/forum/2nd-gen...ine-issue.html

More reasons can be failing injector, bad spark plug or bad coil pack. The latter can be swapped around to see if the problem moves. If you're going to pull valve covers, might as well do them all, along with lifters and valve stem seals.

 

I had a very similar problem. The truck was fine at anything under 40mph but around 50mph it would start to shake. After 3 months, multiple shops and a few thousand one genius mechanic figured it out. The problem was different voltage being sent to both sides of the engine and they traced it back to the computer. They got a computer from the junkyard, plugged it in and reprogrammed the key. That solved the shaking issue and also my multiple cylinder missfire. My driveshaft also contributed a bit to the shake also. The previous owner welded in the u joints and screwed it up. I did the driveshaft first and it made the shake a lot less violent but it still stuck around until the computer was switched out. I also had another mechanic tell me it was a problem with a valve. Look at the computer before you pay someone to dig into the engine.