Vibration Problem
 

I have an '85 Gt with a vibration and am trying to narrow down where it's coming from. It has over 147k miles on the T-5 and it is getting very balky. It doesn't like going into 1st at lights without a fuss and it refuses to powershift any more. My car starts vibrating at around 70MPH. I don't believe the vibration started until after replacing the 2.73 geared 7.5" with a FMS 3.73 8.8". I make decent power (14.6 @ 96, w/2.73s, street tires, short shifting 3rd and 4th gears, 100+ degrees).
Does the t-5 vibrate when worn out? Could it be that the vibration dampner from the 7.5 not being large enough to absorb the vibration? I installed Megabite Jrs and police UCA's when replacing the RE. The added bite hit the drivetrain unlike ever before, hastening the T-5's woes. I just upgraded the brakes to '99-newer fronts and Mark VII rears and 99 17" wheels. I had hoped replacing those components would fix things. I have also had the driveshaft balanced since the vibration started. Are there any other possibilities I may be overlooking? Thanks in advance for the help.

Does it do it on deceleration? acceleration? who installed your clutch? i had a bad vibe for a while, i replace u-joints and everything, i finally found the bellhousing bolts loose? i guess it's somthing to consider.
I wouldnt worry about that vibration block on the rearend, i know people that took their's off, i took mine off too, but put it back on, felt no difference really.
sorry to ask a dumb question, but are your tires balanced?

Thanks for the info. I hope the tires are balanced, they are new car take-offs from MPS. The clutch has been in the car for a while. Longer than I have had the vibration. The vibration was there before swapping wheels/brakes also. The bolts had come loose on my bellhousing, causing the trans to not go into gear at all while the engine was running. I took it to have it checked out and the mechanics found the problem and tightened them down. I had parked my car for four years because of the bolts. I am slowly bringing it back to life. I guess I'll check those bolts again and plan on replacing the trans soon. Oh yeah, it vibrates on acceleration and while cruising at or above 70, even with the clutch in and coasting.

hmm, alot of people seem to have these problems in these foxbody's, hope you find it, my bellhouse was loose for a long time, it finally started scraping my flywheel making noise, thats how i found that.
It still seems like it has to be a tire problem though, because you say it does it with the clutch in, that tells that the t-5 ha snothing to do with it, well unless your input shaft is bent, but you would feel that at all speeds, hope someone else can help you.

I had my old tires checked for balance and they were fine. I guess I'll have the new ones double checked. I'll have to take it for a spin tomorrow after work. It'll be a relief if that is my problem. Thanks again.

I get the same problem @ around 100MPH. My guess is drive shaft out of balance or U-joints. I haven't fixed it yet because I rarely go above 100.

I've found this on three t-5's so far and all had the same sort of vibration you describe. Try this, jack it up and check the lateral play on the slip yoke where it goes into the transmission. The tailshaft bushing is prone to extreme wear, especially if it ever ran low on fluid. When it does, it eats the slip yoke up and causes a lot of play between the yoke shaft and the bushing. Just a few weeks before we took the tranny out of the green thing, I found the bushing completely out of the transmission riding on the yoke. Once the shaft begins to eat at the bushing and the gap developes it will vibrate like mad, getting worse as you go faster. Try to wiggle the yoke only. If there's any play, you need to look into replacing the bushing (easy to do) and maybe the slip yoke as well. The bushing is really easy to install and costs about 4 bucks. The slip yoke is about 60 bucks from Denny's Driveshaft (the cheapest I found it) and 100 bucks from Ford and most other places that handle them.
FYI. . .this bushing is vital to keeping the output shaft centered in the transmission. If the problem persists, it can damage or destroy the output shaft bearings and the fifth gears.

Thanks, I'll check that endplay when I get the chance to climb under the car. It is a high mileage T-5 so I am relatively sure it is my problem. Powershifting it sure was fun after installing the new RE and LCAs. I got a little carried away!

my 88 gt had a similar prob when i went to 3.73's
evidently the driveshaft was'nt in orig. loc.
i rotated it 90 degrees & prob solved, also i heard
the rearend damper does no good after a gear change?
my 2cent.

Thanks for the tip. I had considered that. I'll try the other 3 possible locations when I am checking the tailshaft bushing this week-end.

I checked for slop in the tailshaft and there was none. I also tried reclocking the driveshaft in each of the other three locations and could not tell any difference. There are eight different ways to faze the axle with the differential. I guess I'll have to check the other four after I spray the holes with some penetrating oil and they can soak. I did notice that one of my muffler hangers has broken and my exhaust is slightly touching the UCA on one side and the frame on the other. I am going to try and get a new rubber hanger and some urethane from work and place it between these points.