hello, I recently bought an 88 mustang 5.0 with a 351w swap. it has the 5.0 transmission. there is a very bad vibration through the shifter when acceleration under power, becoming more violent as the rpms increase. there is also a vibration when above about 70mph on the highway, which also gets worse with increase. when above 70mph, if I shift into neutral, the vibration becomes way worse. if i am sitting still in neutral and rev the car it also experiences the vibration. i havent done any work to the car myself, as i said i just bought it. i could see that the fly wheel is new beings that they left the cover off and it is visible. im not sure if they did something improperly which could be causing my issue, or if it could be a harmonic balancer? i just replaced both ujoints on the driveshaft, because i also have a clunk noise sometimes when shifting into gear. this did not fix the problem. Any ideas would be very much appreciated!

 

You can isolate the vibration to the engine , if at idle with car in neutral the vibration is present. If its in the engine I would start by looking at the harmonic balancer at idle and see if you can see it wobbling around . If its bad enough for you feel it in the shifter youll see it at idle . However if you are traveling at 70 let off the gas and shift to neutral and the vibration remains bad and slows with time as the car coasts slower then its in the drive line , bent drive shaft , or a bad ujoint maybe one side of the ujoint is partially froze up. I had that same problem with a ujoint , it wasn't loose or anything but when I put the car up on a jack and turned the wheels the drive shaft would bind up on the one side and the tranny would lift up and down at the tail.

 

ok thanks i just replaced ujoints but i will recheck. what do you think about possibly being the flywheel? i havent much experience with those but from what I'm told that could be the issue as well? its more to do with rpms, i realized about 70mph is just those rpms in fifth gear (about 3,000 rpms) but if the vibration starts in fifth gear and i go into neutral, the vibration is still there and slowly decreases with speed.

 

If your in neutral and running 70 mph with engine at idle coasting and it continues to vibrate but slowly decrease with speed it sounds like in the drive train behind tranny someplace. Jack the car up and put it on jack stands spin the wheels and look for any kind of movement of the drive shaft up and down also the tail of the trans up and down side to side.
If your brave like me you can put it on jack stands and start it up and put it in gear and let it idle in gear and go back an watch every thing to see if you can catch something moving around. You could have a bend drive shaft its not uncommon.
Ive heard of pple having a vibration after they did a gear swap in the rear diff , if its not done correctly.