I've got a '93 5.0 LX Auto. I just replaced the motor and tranny mounts and she rides smoothly most of the time. However, between 1100 and 1600 RPMs, I feel a vibration in the steering wheel and some in the seats. The spark plugs are new, wires are new, ignition is new. Any ideas?

 

First you need to identify if this is a rpm or speed related problem.

Drive at your vibrating speed again. Now downshift to change the rpm but cruise at the same speed. Or try driving at that speed and slip it into nuetral.
If it's rpm related, it will vibrate in the driveway reving it to 1100--1600rpm but since you feel it in the wheel I suspect it's a tire problem.

 

My buddy and I drove it around the block a few times and it's definitely an RPM problem. It behaves exactly the same in neutral as it does in all 4 gears.

 

get a nice bright light and look on the back of the harmonic balancer. should be a rubber ring going around the backside of it. if its sticking out anywhere, its junk.

 

Will I see it sticking out while the engine is off or only while its running?

 

while off. take a pic if u can.

 

On my way to do it right now.

 

Just figured out that to get a good view and a nice pic, I'd have to get underneath the car and since I just came home from the gym and freshly showered, I don't feel like getting dirty again tonight. I'll be working on the car tomorrow so I'll check that and snap a pic.

 

well lets just put it this way. that rubber ring shouldn't be sticking out, at all. it should be flush with the balancer, all the way around.
if not, replace.