Intermittent Vibration

remman · Sep 17, 2009

Hi everyone,

I have been having a problem since I first bought my car a couple months ago. I will try to make this as clear and concise as possible.

1997 Mustang GT Automatic; 119,000 miles. CAI, SCT Tune, Magnaflow Catbacks

Things done since purchase are plugs, wires, coils; PCV; tranny flush; rear axle bearings, aluminum drive shaft, differential rebuild with FRPP 4.10 gears and bearings; fuel filter change; oil change; front and rear shocks, struts, front swaybar bushings, endlink bushings.

What I have is a pulsating/intermittent vibration that gets faster and louder/more noticeable at higher speeds. It is related to speed if I had to take a guess. It continues if I let off the gas while moving but will stop if I am moving at highway speeds and put the car in neutral. The local performance shop diagnosed it as the rear axle bearings. Those were replaced, then it was diagnosed as the ring and pinion. The diff was rebuilt with a trac-lok kit and motive bearing kit as well as FRPP gears by a reputable shop. I elected to replace the driveshaft which was also done professionally with the yoke changed to match the spline on the auto tranny.

The tech called it a "growl". None of the repairs I have done have made any difference. The diff fluids have been drained twice now. The only thing I can think it might be is that the rear upper control arms need replaced which is causing the pinion angle to be off.

Sorry for the long read. Thanks for any help.

Win · Sep 17, 2009

Could possibly be the Pilot Bearing.

remman · Sep 17, 2009

In an automatic trans?

2slo95fiveoh · Sep 18, 2009

Hmm.. those kind of vibrations are going to kick your ass... sorry to say but it's true..
The Fact that the vibration increases with speed, until you put the car in neutral makes me think it would be narrowed down
To the Transmission or possibly driveshaft. You said you had the driveshaft replaced, as well as the yoke, did you have the
U-joints replaced as well? Is the Driveshaft a factory Iron or aftermarket Aluminum?

remman · Sep 18, 2009

It is the FRPP aluminum driveshaft. It had new u-joints with it when it wasx put in.

Win · Sep 18, 2009

remman said:
In an automatic trans?

Sorry i was tired i have been workign 5-2's lately i didn't see Auto tranny.

Win · Sep 18, 2009

remman said:
It is the FRPP aluminum driveshaft. It had new u-joints with it when it wasx put in.

Try rotating it 180deg and see if that helps.

GDTrumbo · Sep 18, 2009

With a 4-bolt flange at the rear of the driveshaft, it could be rotated in 90* increments. But I'm sure there is a "sweet spot" there somewhere.

With the vib stopping when put in neutural, it kinda rules out a twisted trans output shaft since it spins at DS speed in neutural or drive. Some owners think neutural slams are cool.

Ummmm.... There's many things spinning that could contribute to a driveline vib.

You might:
Do a quick inspect of the harmonic dampener. Look for rubber sticking out/seperation. Does it vibrate sitting still in neutural at 2500 or 3000 rpm?
Pull the DS and check the U-joints for good articulation. Especially the front one where the jolk was installed.
Your idea on the rear UCA's is possible. Look for bushing damage. OEM replacements are no longer avalable for '97's from Ford as of 5/08. Would have to go with FRPP replacements.

Gearsets anywhere below 3.73:1 & 3.90:1 tend to roar a little and gearset installs are no less than an artform. They can be the difference between a 30K lifespan and an indefinate lifespan. There's a chance they are not set-up quite right.

See what you can find.

GT

remman · Sep 18, 2009

It doesnlt vibrate in neutral at higher rpms.. The tech who did the axle bearings had also said that the UCAs needed replaced. Alot of the bushings are very worn in the suspension.

The gear install was done by what is known as the best install shop in the area. I had 4.10s put in as well as the trac-lok rebuild and motive bearings. The old gears were torn up pretty bad as well as the diff being loose by almost a quarter inch. I suppose anything is possible and the gearset install may have been incorrect. What leads me to believe it was done correctly is that the vibration/growl has stayed the same before and after all the driveline work was done.

Are the UCAs something I can do myself?

GDTrumbo · Sep 18, 2009

You can replace the UCA's yourself. You will need 4 jack stands and a jack. They are attached with 18mm bolts/nuts.

2 of their bushings are mounted in the axle housing. Tool/instructions are available from Max. Motorsports website for ~$25. Measure carefully.

For the street. MM recommends the FRPP UCA pieces to prevent binding.

Do one side at a time. After swap have the pinion angle checked.

GT

remman · Oct 8, 2009

UPDATE:

I had the UCA's installed about a month ago. It did not help with the vibration whatsoever, it felt like it made it worse. The car drove much much better after.

Just replaced the motor mounts and the transmission mount and the vibration is gone. The motor mounts were basically shredded. I got a really good look at them when it was on the lift at my local performance shop.