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708 views 7 replies 4 participants last post by  lernst 
#1 ·
In an attempt to diagnose a vibration in my 97' 9000 auto, I put the car into neutral at 60mph (vibration did not subside) and noticed my tack jumping from 1100-1800 rpm. The car idles perfectly ~900rpm while not moving (in park or drive). Any body ever experienced this before? Is it possible that the transmission is backdriving the engine slightly? Someone please try this and let me know.

Oh, and if anybody knows anything about the vibration problem I'm having, that would be helpfull too! And no its not the tires, just replaced all four.
 
#2 ·
assuming your car is an automatic, 4th gear is direct drive, so yeah, the engine will be driven by the transmission when you let off the gas.

that being said, the tach shouldn't jump like that...

Just because the tires were replaced, doesn't neccessarily mean they were balanced properly...
 
#4 ·
Well....if it's in neutral then the clutches connecting the direct drive shaft and the output shaft are disconnected.....or it wouldnt be neutral now would it :cheesy:?

I second the imbalanced tires.

Also wheel bearings, axles/driveshafts.

You can confirm the imbalanced tire by simply putting the front end up on 2 jackstands so the tires dont touch the ground, removing the suspected tire and "driving the car" at 60 mph. Then you'll know if the vibration stops.
 
#5 · (Edited)
Windexcheese said:
Just because the tires were replaced, doesn't neccessarily mean they were balanced properly...
Exactly, i got new tires and i was furious when i was getting harsh vibrations at highway speed, i thought my eyes would jiggle out. Anyways i took the car back, they balanced them, and now it only vibrates if i go over 85. I guess they didnt do that good of a job because it used to be smooth up to 120 MPH.. Go get them balanced, and get an allignment done just for the hell of it and see how it is.
 
#6 ·
boon94 said:
Exactly, i got new tires and i was furious when i was getting harsh vibrations at highway speed, i thought my eyes would jiggle out. Anyways i took the car back, they balanced them, and now it only vibrates if i go over 85. I guess they didnt do that good of a job because it used to be smooth up to 120 MPH.. Go get them balanced, and get an allignment done just for the hell of it and see how it is.
sometimes, due to lovely new england potholes...the entire wheel deforms a bit because of bump impacts. It will balance out but not entirely.

My mechanic pointed out a huge dent in one of my alloys and I told him to go ahead and try to balance it out anyway and it worked....except for a high speed vibration...
 
#7 ·
Certain sections of highway near where I used to wrench were/are tough on rims, and we would get a steady stream of customers looking to have their wheels balanced.

You can quite effectively "balance" a bent rim/tire combo, but that doesn't mean you are truing the rim as well...:cheesy:

When customers would come back complaining, we would put their car up on a lift and show them the out of round rim, and expain that we could quite easily balance a perfectly square rim, but a dynamic weight balance doesn't neccessarily mean that the rim is round...
 
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