Crazy Vibration

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so me and my little brother went to go pick up a 93' LS400 for him yesterday, the body and paint is still good conditions, interior is ok and engine seems good. the previous owner did told us it needed some work, basic tune up and motor mounts. if you drive the car under 30 mph it'll be smooth but once you go over that speed limit it starts to vibrate and if you go around 50-60 mph it'll vibrate like crazy. does this sounds like a bad motor mounts or something else? one of my buddy test drove it and says it's not the motor mounts but maybe the tires, brakes or something else instead.

 

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New Update: ok so i took it back to the shop and they think they have found the problem. on the drive shaft rubber bushing it has some cracks around it and one of the bolt is missing. they say that missing bolt can cause it to be unbalanced and will cause vibrations, so from what they explained to me it makes sense. i'm gonna try to find a bolt that is the same size and thread to see if it'll fix the problem. i'll update again once i put on the new bolt.

 

[lill daddy]

they are right!! as soon as i started reading your post i was going to reply "check that all the bolts are fitted to the shaft" its amazing how out of balance one bolt can make it.... i bought a van once with the same problem....

 

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Update:
i got the vibration fixed, it was b/c of the missing bolt on the drive shaft bushing. i just happened to have a bolt in my bolt bucket that was about the same size and length. went for a test drive up and almost no more vibration...but here's the thing, when i got back home, i parked it and turn the car off, checked out everything making sure it's all good turn the car back on to go for another test drive but right when i was going down the road it felt like it doesn't want to go any faster. i'll give it almost WOT and it barely moves. from a stand still if i give it gas the rpm's will go up to 1500 than the car starts to move, sounds to me like the transmission is going out (the previous owner just got the transmission rebuilt two years ago).

so i drove it back home and parked it and popped the hood open and smokes starts to come out, i looked under and it seems like it has an exhaust leak from the headers to the exhaust. try revving it but it feels like it doesn't want to and will drop down to 300 rpms once i let off. maybe it needs a good tune up? this car has been nothing but trouble since my brother got it two weeks ago and i'm trying to avoid having to get a new engine for it....

 

[stevechumo]

Bad tranny doesn't give smoke. Exhaust leak doesn't give smoke either. And even if exhaust is leaked, the car still accelerates normally. I'm thinking that it may be a blown head gasket. You may check the compression to see if the engine is healthy.

 

[JBrady]

I would check the catalysts.