Howdy. My 93 d150 w/ 5.9l has some pretty severe vibrations while driving above 40mph at low RPMs. With acceleration it goes away. I can switch the overdrive off and the vibration goes away as well. It is not the tires/wheels. It has new balanced tires on it. I believe it might be my U Joints but I am not entirely sure.

 

Engine misfire..... Above 40, TCC is locked, and the trans will shift into O/D.... and any engine misfire is amplified DRAMATICALLY.

 

I have considered that. I just had the 5.9 put in it. (Swapped out the original 5.2) and the engine itself is running quite well. This issue has come up after a little over 500mi put on the engine (engine already had 114k when put in)

 

Did you change the flexplate (driveplate....) as well? Or the torque converter? 5.2 is internally balanced, 5.9 is external. There are weights on the torque converter (pre-96) or the driveplate (96 and newer)

Try this.... Run the truck up to 45 mph, is it shaking? If yes, turn off O/D. (maintain the same speed) If it stops shaking, it's an engine miss. If it doesn't, something else in the driveline.

 

Yes, the torque converter was changed out. As well as the intake manifold and fuel injectors and rails.

 

What spark plugs ya have in it?

 

DDDrive down the road and when you get the vibration, slip it into neutral. Be careful to NOT put it in reverse. Bad mojo there if you do. While coasting, does the vibration go away? If so, doe it reoccur when you add some throttle? Also, while parked, put it in drive. Then go back and forth from drive to reverse a few times. If it's your joints you'll get a solid THUNK if there is slack in the joints.