87 base coupe with 5 spd 2.5

The car tracks straight and I can take my hands off the wheel at any speed and it stll goes straight. The problem is that as I speed up from 65 to 70 plus the car begins to shake. The shaking gets more intense as the speed goes up untill you hit 75 then it starts to go away. If you lift off the gas the car shakes really hard untill you slow to 65. However if you gently slow down the shaking is mild.

You can feel the shaking in the steering wheel but it is the whole car that feels like it is shaking.

It has new shocks and struts with allignment which didn't have any effect on the problem. The problem has gotten worse over the last year.

I have been told that it's bad bushing by one mechanic then told by another that the bushings are good.
Then told it a wheel balance problem. Then told it's a tire failure. But none of these have helped.

I'll listen to you folks over the mechanics I've had look at it. I just don't want to throw mony at it when I can't seam to get any type of agreement to what the problem is.

Here are some possibilities:
Loose lug nuts
Tire Balance is off
Defective tire- (belts can occasionally shift)
Loose ball joint and/or control arm bushings.
Totally worn shocks or struts.
Pre-load on front wheel bearings not to spec or the nut is loose.
mud on inside of rim- (no joke this happened to me once)

Jack car up support on jack stands (or lift) Let control arm to sit without support. Grab the tire with both hands and try o wiggle the tire right to left and top to bottom This should detect any looseness

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87GT - with 3800SC Series III engine, 4T65eHD
87GT - 3.4L Turbocharged engine, modified TH125H
" I'M ON THE LOOSE WITHOUT THE JUICE "

Recently, I had the same indications with mine. At 65 or so, the shaking would start. At 75 to 80, it was the most pronounced. After about 85 to 90, it would reduce but not go away completely. At 100+, it was hard to notice, but was still there.
I looked at everything. Bushings, ball joints, tie rods, had tires balanced, and had alignment done.
I took the car to sears automotive for the alignment. They aligned it, and unknown to me, pierced my CVD boot in the process, (that is another story).
It still had the shake / vibration.
A week later I was fixing the boot and discovered the rear bearing was just starting to get sloppy. It had enough play that the wheel would change toe. the bearing had very little movement, but it was enough.
I was not worried about the bearing itself, as it was fine at 100+ Mph, but I changed it out and the vibration / shake went away.

Sorry for the long winded story, but check your bearing. It might have just enough play to cause the vibration, but not fail.

sounds like a drivetrain vibration. I'd look at the play in the axle shafts at the transmission bearings (at the inboard cv joint). Also, check the engine mounts.
It is hard to diagnose a car that isn't in front of you, but I'd start looking there.
Dave

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1987 GT (my toy-see above), 1987 GT (wife's toy), 1986 SE soon to be VR6, certified master technician/shop owner
www.njautobahn.com

every fiero I have ever ridden in has had this shake that you are talking about... its usually just an alignment deal I assume because every car I have been in that did that had little to no alignment.

Neither of my 2 Fieros ever had such a problem.

I had this problem with my old tires. It would shake at 65 MPH so I replaced the tires and it was gone for good.

quote Originally posted by bmwguru: sounds like a drivetrain vibration. I'd look at the play in the axle shafts at the transmission bearings (at the inboard cv joint). Also, check the engine mounts. It is hard to diagnose a car that isn't in front of you, but I'd start looking there. Dave

I'd have to acknowledge Daves suggestion that it could also be drivetrain related. Best way to start the diagnostic is to detmine where the vibration is coming from-front or back. Then try to isolate the problem by doing the checks. The only times that I had this problem was first with a tire failure ( where a bubble actually appeared in the tread) and another time where an excessive amount of dried clay mud was trapped on the inside of the rim. Those two things threw off the balance big time. Many factors could cause the vibration from a bad CV axle to tire damage. Go over each possibility one at a time and you'll find the problem.

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87GT - with 3800SC Series III engine, 4T65eHD
87GT - 3.4L Turbocharged engine, modified TH125H
" I'M ON THE LOOSE WITHOUT THE JUICE "

I also had such a shake, the prevoius owner used a can of "fix a flat" and that stuff unbalances a tire fast.

cheapest check is go and have your tires balanced. I'm betting that is your problem.

For years I had the same problem on many of the fieros. Eventually, I found out that most places that use spin balancers for the wheels/tires only balance to 55MPH. I went back about 50 years and pulled out the antique bubble balancer and started doing it myself. No more problems and now I can do over 120mph without any shake. Maybe something else to take into consideration.

Tire ballance is my next fix.

While I have tried to figure out where the source of the shake is coming from I have never been able to pin it down. Most of the time I would say it had to be the back since the car drives straight even when shaking.

Thanks for the ideas. I will work on them one by one till I find my answer (or run out of money).

bent rim? that can cause that to happen to. if u want to check and see if u have a slipped belt in your tire and don't want to pay someone to do it, take yellow tire marker and spin ur tires up in the air put the tire marker on and let it make a line, should be a straight line all the way around the tire, ie.. if you mark dead center between your treads then it should stay that way all the way around, if it gets close to one side or the other then u have a slipped belt in your tire and i recommend replacing. when u do a wheel balance try to find a shop that dose a higher speed balance, I use to work at walmart and they balance tires about 60-80 mph i think. try to find a shop that dose 100+ mph balance and see if that helps.

Steven

I got a brand new set of Pirellis from Sears that were complete trash and the car shook so bad that I lost a rear wheel bearing. My mechanic spun one of the tires with it off the ground and it looked like an egg.

quote Originally posted by bmwguru: sounds like a drivetrain vibration. I'd look at the play in the axle shafts at the transmission bearings (at the inboard cv joint). Also, check the engine mounts. It is hard to diagnose a car that isn't in front of you, but I'd start looking there. Dave

also look to see if it has a bent cv axle, u can also do this by lifting the rear end of the car up off the ground, wheel chock the front 2 start it up and put it in first gear and have someone watch the axles. look for wobbling kinda like jump rope. Good Luck.

Steven

quote Originally posted by ARKaiser: Whole body shakes at 70.

I've heard this also. Not looking forward to it at all. Hehe.