very bad vibrations

here is the question i have been bugged about on one of my cars. i have a bad vibration in the drive train, i checked the rear end and all is well in that department, i put new tires and wheels on it still have the vibration, i replaced the drive shaft and still a vibrations, motor at idle does not have vibration but when going down the road at around 1800- 3500 rpm there is a very bad vibration, once i got the vibration to quit by getting in and out of the throttle. but eventually it started back. do anyone have any suggestions on what it might be?

here is the question i have been bugged about on one of my cars. i have a bad vibration in the drive train, i checked the rear end and all is well in that department, i put new tires and wheels on it still have the vibration, i replaced the drive shaft and still a vibrations, motor at idle does not have vibration but when going down the road at around 1800- 3500 rpm there is a very bad vibration, once i got the vibration to quit by getting in and out of the throttle. but eventually it started back. do anyone have any suggestions on what it might be?

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I'm having the same problem with my 71. It happens when I'm slowing down. I have a c4 transmission. It's probably the tranny but I'm not really sure either. Anyone have any ideas?

Maybe harmonic balancer is slipping changing the balance on the crankshaft.

if this is so wouldnt the timing mark be out? because i thought i looked at that....and the timeing had not changed.

the only other things i could think of that would cause a vibration would be flywheel, or torque converter. unless your crank cracked. There was one guy that had a bad vibration and after looking high and low for it decided to drop the oil pan and look into the bottem end, and tried to move the crank around and noticed something didn't look right, so he pulled one of the bearing caps and the crank had snapped right behind the cap.

the only other things i could think of that would cause a vibration would be flywheel, or torque converter.

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+1 to these and the tranny. Are the vibrations bad when in gear or just at that rpm range?

here is the question i have been bugged about on one of my cars. i have a bad vibration in the drive train, i checked the rear end and all is well in that department, i put new tires and wheels on it still have the vibration, i replaced the drive shaft and still a vibrations, motor at idle does not have vibration but when going down the road at around 1800- 3500 rpm there is a very bad vibration, once i got the vibration to quit by getting in and out of the throttle. but eventually it started back. do anyone have any suggestions on what it might be?

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You may also want to check your trans mount.I chased a vibration many years ago on my Mach and did most of what you are talking about. Finally narrowed it down to the trans rubber mount.The mount looked good but the rubber had become soft.Just an idea.

My trans guy changed the trans mount and adjusted the pinion angle.

Got rid of 90 percent of my vibation. After changing the mount my fan rubbed the shroud on both of my cars. MB

iv narrowed it down as far as i know and it has got to be the torque convertor or the the fly wheel.....iv checked all the above and all is good there.. maybe i can get some time soon and pull the tranny to check convertor and or the flywheel.. thanks for all the advise guys......oh by the way ill let everyone know what i find when i find it to close out the question for future users on here

akherber to answer your question it does the vibration in gear or out does not matter......but always at the same rpm range

My trans guy changed the trans mount and adjusted the pinion angle.

Got rid of 90 percent of my vibation. After changing the mount my fan rubbed the shroud on both of my cars. MB

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Possibly wrong mounts? Never had any shroud issues.Why the pinion angle change? Were / are you using different housing / shock pads? Just curious.

I just used autozone trans mount on my mach1. The new mount must have raised the back of the engine just enough to make the bottom of the fan get into the shroud by a 1/4 to 1/2 inch. My fastback has been lowered 1 inch all the way around and it had the same problem. The aftermarket mounts may not be quite like factory. MB[/font]

im sure it isnt the mount as it is almost new.

the car has a 90's model engine in it, which being a 50.oz balance i had to use a late model flexplate. the tranny is a fmx. im wondering if the fmx converter is not matching up to late model flexplate.the converter bolts are same , but now im not sure that the pilot is in the crankshaft. is a late model 50 oz flexplate more dished than the fmx flexplate?

or vicea versa.or the same. im also suspecting that the converter may be going bad, it isnt slipping or any thing but it may be the viberation.when it gets warmer and we get some other projects out of way going to pull tranny and check again on everything. its just been a real pain tho not being able to drive because of the viberation. thanks to all for responding.

and to all have a very merry christmas

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The FMX converter pilot is the same as the newer trannys. It could still be the tranny or converter going bad, do a tranny oil change and check the pan, Also check if you have a factory flex fan for missing pieces or bent blades. Those fans are likely to fall apart, mine exploded once at high rpms and took out the shrould, luckily no other damage.

The FMX converter pilot is the same as the newer trannys. It could still be the tranny or converter going bad, do a tranny oil change and check the pan, Also check if you have a factory flex fan for missing pieces or bent blades. Those fans are likely to fall apart, mine exploded once at high rpms and took out the shrould, luckily no other damage.

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thanks for info on pilots being the same , but do you know if the flex plate itself is same dish? ive got several plates and some are flat and some dished. but made mistake of not marking what they came off of and its been several years since ive took em off . and my memorie is about as long as ..well its short lol.

again thanks for replying and help

FMX and AOD flexplates are the same except the later ones for the 5.0 and such are 50oz imbalance

They have a dish, my guess is about 1/2" deep and approx halfway up the flex if that makes sense

FMX and AOD flexplates are the same except the later ones for the 5.0 and such are 50oz imbalance

They have a dish, my guess is about 1/2" deep and approx halfway up the flex if that makes sense

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thanks again . yes i know about the balance of 28 oz and 50oz.i could have had a brain fart and put a 28 oz on tho. but im pretty sure it was a 50. will just wait till it warms and i can get to it. thanks again to all who replyed.

If you have an earlier engine you want the 28oz flexplate. If you installed the 50 thats your problem. The flex imbalance must match the engine.