Hi there. So I had the notorious miss between 45-55mph, changed all the COP's and plugs, which cured the problem. But now I have this weird shake at 40mph, regardless of whether my foot is on the gas or I'm coasting. It's definitely not the same feel as the previous miss, and it doesn't really feel like a tire out of balance issue. It's almost like when you're driving a stick shift and you make your car kinda buck when changing gears (not that I've ever done that...). But it only happens between 38-40mph. Any ideas what this could be?

Thanks, as always, for anyone's help.

 

My Lightning is doing the exact same thing. Set the cruise at 40 and it will bounce a little and shake. It's in the shop right now so hopefully I can share whatever the fix is.

 

Found this TSB thought I share.
http://www.ford-trucks.com/tsb/fullt...php?tsb=04-1-6

 

Most likely the rear end friction modifier has broken down. Just service it .

 

My 2WD does this. Like you said only from 38-40 mph. It even happens after I had a rotation and balance. Its weird.

 

my old truck did this and it turns out it was my U joints and my driveshaft eventually broke and fell out

 

Well that is not a promising outlook.

 

You expected promising ?

Did you hit your head ?

 

Well I had the rear differential fluid changed, and that didn't really help this problem. Probably a good thing I did it though, because the mechanic said the fluid looked horrible. But, still back at the same problem

 

So what exactly did they use? All rear end mixtures aren't the same. You have to get this one right, - then it may take around 75 miles for the vib/shake to quit. But yea, these rear gears need the good stuff + Modifier or the problem may persist.

 

Hmm, well that's a good question. I'm quite the uneducated newbie when it comes to rear diff stuff, so I just assumed they would know the correct stuff to put in there. I'm out of town for a couple weeks, so further testing is gonna have to wait, I suppose. Sheesh

 

mine does the same thing, someone told me it was the torque converter going bad. 1999 2wd f150 with 134k

 

Hey Jbrew, would this whine have anything to do with the backend? I can't really tell where it's coming from, but I've had it for a while, and it's pretty much 45mph and over, but only when my foot is on the gas pedal. In the video, it goes away when I pull my foot off, but comes back when I step back on it. I've just heard people say their rear diff is bad and causing a whine, but no explanation or example of the whine.

http://tinypic.com/r/23wpug2/7

 

Sounds like a u joint problem. If the whine is like mine, it might be normal, but if the vibration is a foot on gas only deal, u joint is a likely culprit. Alot of Ford tranny's make a hellova whine which to me is normal.

 

Ah. Well it's good to know that the whine is normal. But the shuddering and shaking is both accelerating and braking... and coasting. Between 39-42, I get it, regardless of speeding up or slowing down.