Driveline Vibration ‘20 PP2

[ramairgt1]

I recently purchased a ‘20 mustang GT PP2 with 8k on it.
There is a high frequency driveline vibration that starts around 40 mph and gets worse the faster I go. At 80mph the rear view mirror looks like it wants to rattle off.
I replaced the tires and it’s still there.
Put the car on Jack stands, removed the rear wheels, put the lugs back on and ran it to 60. Vibration was still there. Jacks are on the frame, not sure if they should be on the rear suspension or not.
Car has a QA1 CF drive shaft that had hose clamps on it. I removed them and the vibration stayed the same.

Just looking for advice at this point.

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[NGOT8R]

Sounds like they put those clamps on the da to try and balance it. I’ll bet that’s where your problem lies. There’s a company that makes a mercury balancer that bolts to driveshafts. I believe the concept was initially intended for trucks, but I can’t see why it won’t work for cars as well.

https://floridashorestruckcenter.com/store/product.php?d=584

 

[agmattocks]

I'd check the rubber on the drive shaft where it connect in the middle. My 18 had it have it replaced.

 

[Hadelson]

QA Carbon Fiber DS is one piece.

 

[MrMike]

Go back to the factory two piece driveshaft.

 

[Cobra Jet]

If you have a 1-piece DS, check the U-joints…. They do not last forever.

Another source of a RPM/MPH specific driveline vibe would be a bad rear pinion bearing in the rear diff, which results in a rear diff rebuild. If needing to rebuild the diff or buying a complete new loaded diff, now is the time to change rear hearing if that was a past thought…

If your rear axles or any 1 of the wheel hub assemblies were bad, the vibration or harshness would not be RPM/MPH specific; the feeling or noise wouid be present at all RPM/MPH ranges. With bad wheel hubs, when traveling and the car leans into a curve, the noise would be more present and louder on the side with the failing wheel hub or axle.

Usually with a driveline vibe, the noise/feeling will always be RPM/MPH specific. When traveling, at the peak of the noise/feeling let off the gas, the noise or feeling should start to subside or almost disappear. This is because you’re taking the load off the driveline. The same is true if you were to carefully put the vehicle into a neutral coast while traveling, the load will be taken off the driveline and the noise/feeling would subside. Same is also true if you have a manual trans and depress the clutch, the load is removed from the driveline and the NVH should disappear.

The reason it is RPM/MPH specific is because while at that certain RPM/MPH the rotating mass of the driveline hits a certain frequency or resonance due to the mass becoming unbalanced which causes that NVH - and as it either gets slower or faster it will level out as if it just “disappears”. This will not happen with a bad axle or bad wheel hubs.

Before digging deep, check the front trans flange and rear pinion flange for any damages, seepage, etc. I believe there is a runout measurement for each in the Ford shop manuals. The other thing to check is loose DS bolts at both the front and rear flanges. These have to be torqued to factory specs, can’t just be “hand tight”.

To check a bad rear diff - get the car up on stands or a lift where the rear wheels are free. Remove the wheels and pop out brake pads so there isn’t any other interference with sound. By hand manually turn/spin the rear axle and listen for any sounds coming from the diff pumpkin - like crunching or a feeling of not smooth operation. If you have 2 people, you get an auto stethoscope and put it to the rear pumpkin while the other person slowly turns/spins the axle, listening for anything abnormal. You can also drain the rear diff and look for metal particles in the pan (a sure sign something is or has failed in the diff).

Tires:
Make sure all logs are torqued to spec. Check air pressure when cold and match it to PSI spec on driver door jamb. Check tire balance by looking for missing weights OR abnormal tire tread wear. Any of the above tire concerns could cause a vibration feeling BUT again it won’t be RPM/MPH specific; it would be present at all RPM/MPH ranges AND won’t dissipate when letting off the gas, throwing the car into a neutral coast or depressing the clutch pedal. I mention this because many times Ford Service Centers want to do a “force road balance” with the wheels - it isn’t going to make one damn bit of difference IF the problem is RPM/MPH specific so don’t let them take you down that rabbit hole…

 

[ramairgt1]

It ended up being the driveshaft.
Swapped in a stock DS and the vibration is gone.

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