Hello all, first post for me. Just bought a 91 f150 5.0 , seems like a real clean truck but has a few kinks to work out. First off when I take off from a stop there is a clunk that sounds like maybe a bad ujoint. Secondly when in 2wd (but not in 4wd) I get bad driveline vibes around 45mph. And finally (and most annoying) when at idle it shakes rather badly(worse in 2wd than in 4wd). I think maybe these problems are related, but I appreciate any suggestions.

-tyler

 

I would say thats a bad U joint on the drive shaft. I had to replace on my 2000 already with only 60k!

Mike

 

Being that it is a '91 they have probably taken the driveshaft out once or twice already. If they don't put it back exactly how it came from factory it will vibrate because it is no longer balanced. Check where the driveshaft meets the transfer case and rear differential, you should see aligment marks painted on both ends of the shaft(s), the transfer case yoke, and the rear differential yoke. If you look and see that the marks don't line up, take the drive shaft off and put it back on with the markings aligned. By the way ford usually paints yellow aligment markings on the shaft and yokes. If you don't see those than you could rotate the shaft one position and test drive it, keep doing it until yu don't feel it any longer and then puting reference marks on the shaft and yokes for next time. Of course, change all your u joints at the same time, they are cheap and you don't want to be under the truck taking the driveshaft off more than you nedd to. You could always take it to the shop that specializes in driveline problems. I warn you its messy! depending on your truck have plenty of transmission fluid and transfer case fluid handy soemtimes they leak! Good time to do lub changes on trannys also.

 

josser is right.. I would check the drive shaft first. If you take the weight off the rear wheels, slide under the truck and check if there is play in both the front or rear U-joints. If there is..Replace both at the same time.. If not..Take a look at the flywheel/disk.. If it is an automatic, you could have a cracked or broken disk. But by your discription it more than likely is a drive shaft that has lost it's weight or a U-Joint. The other thing is a bad "youk" (sp) what I mean is the part that the drive shaft fits into at the transmission or the rear-end. It could even be something as simple as a loose drive shaft. Carl