infiniphonic

This is my first post here so bare with me. Got my Mustang about ten months ago. Ran and drove perfectly when i bought it. After about 4months it developed this problem. Discovered that the diff was low on oil. Filled it. problem went away (mostly). Now it's back. In any gear, at any speed (especially going slower) there is a heavy vibration at exactly 1100 rpms. I can feel it in the seat (my butt), in the floorboard (the gas pedal), and in the steering wheel. if i accelerate faster it moves through the rpms faster and the vibration is not nearly as noticeable.

houtex

I'd suggest putting it on a lift or at least jacking up the rear axle on jack stands (WITH appropriate 'no roll' mechanisms on the front wheels, mind, or jack up the front on jackstands too) and running the car that way to see if it's road caused or drive train caused. In the air the car might work differently, and/or give you the exact thing. Someone can see the out of balance that's happening. Or, if it's a suspension problem you can check all the bushings and see that.

If it's not the axle, then...

Center bearing of split driveshaft, if you still have that.

Universal Joint at transmission.

CV joint at pinion.

Tire(s) out of balance

Tire(s) flat spotted

Brake problem (perhaps not backing off like they ought, either the return spring is busted (yes, they're a thing on brakes) or the piston(s) in the caliper(s) are hanging up.)

Alignment problem.

Something *really* odd like a big ol' chunk of mud in the wheel..?

:shrug: If it's none of those, well... "I got nuthin'." - Col. Jack O'Neill. Two 'L's.

Hope that helps maybe. And welcome to the forums!

infiniphonic

I am going to put the rear on jackstands today and try to replicate the issue. As far as any of the other issues i have checked most of them and everything is really good under there. I will check the rest today. I cannot believe you quoted Col. O'Neill. I am a Stargate fan as far back as it goes!

infiniphonic

I put the rear on jackstands and ran it through idle to 1500k rpms. the vibration is still there but very faint. Almost like a hint of what i experience when i'm driving down the road. Also, the rear wheels remain slightly driven when the car is in neutral. That last part was a surprise to me as i thought being out of gear completely de-coupled the engine from the transmission.

houtex

Nah, the transmission in neutral still has whirring shafts, which have gears, which makes fluids move around, which makes other gears move around... not unnormal per se. Without resistance, the wheels move sort of as an 'induction' from all that in the transmission. Hydraulics is powerful stuff sometimes...

So if the rear's on jackstands and the vibration is still there, just reduced, sounds like the wheels ostensibly are out of balance. Just go get a balance and see what happens. It could also still be the rear suspension, CV joint, center bearing, U Joint, and hopefully not, but the axle gears/pinion, axle gear/pinion bearings, or wheel bearings.

Maybe go get a mechanics stethoscope... or a long screwdriver... and in either case, *be careful*... stick it back up on the jacks and see if you can hear grinding noises in the axle wheel bearings or at the pinion. NOT on the whirry bits, that'll get you maybe chewed up, yeah? And won't get you anything because scraping of probe on the spinning thing just makes noise.

Past that... Shop for diagnosis?

infiniphonic

Lime GT

Check the run-out of your driveshaft. Mine was out at the 0.012" mark. This is not my pic. I just used it to label the specs I got on my dial gauge run out.

infiniphonic

well now that is an idea. I don’t have a dial guage yet but i might have to pick one up very soon. What were the symptoms you were experiencing with that part of the DS out of tolerance?

VGMStudios

would it be as simple as warped rotors?
My car was shaking and I recently replaced my front rotors and it went away.
Just an idea.