Vibration at 65 MPH+

[Cen-Tex Coupe]

I have had my Coupe for about a week and have noticed that at speeds above 65MPH I get a low frequency vibration or rumble. This only happens when holding a steady speed or decelerating. If I give it some gas, the rumble/vibration goes away. Have any of you heard of this, and if so, what was the cause?

I am going to take it back to the dealership if it continues, just don't want to be without my car for any longer than I have to. Thanks a lot.

 

[Sayantsi]

Does the rumble feel like it comes from the wheels? A new car could need an alignment, a tire could be out of balance, there could be incorrect tire air pressure, or there could be an engine issue.

Look at the engine rpms at 65+ and use a lower gear to gt the engine into that range at lower speeds - does the problem crop up then? At speeds above 65, drop down a gear and see if the problem persists - that will let you know if the shudder is coming from the engine or wheels.

 

[Cen-Tex Coupe]

I tried down-shifting into 5th gear on the way home tonight, and the noise was still there. It is definitely in the drivetrain somewhere. I am thinking that it is either in the rear-end or the alignment. I'll let you know what the tech says on Thursday.

 

[Squirrels]

With speeds over 65 I also get a vibration but its deffinatly in the steering wheel. My service adviser says it is because I have missmatched tires. Just dropped it off at the dealer for them to change out the tires and some cosmetic work. When i get it back tomorrow I will see if the vibration has gone away and keep you updated. This is on a 3.8 GT coupe

 

[mikec]

Cen-Tex Coupe:

What you're describing sounds like a differential (aka "3rd member to some folks) that wasn't assembled quite right: improper drive pinion preload. In a nutshell, the input shaft of a differential is typically supported by two bearings with a spacer sleeve between them. This spacer is a "crush" spacer... when the differential is assembled the factory worker tightens the input nut squeezing the bearings together, crushing the spacer a bit until the amount of torque it takes to turn the input shaft reaches some specified number. The nut is then backed off a little and re-tightened. This pre-loads those input bearings. When not done properly, the input shaft isn't held in place perfectly: it can move in/out a little bit, or side to side, because those two bearings aren't "shoved" into their mounting spots firmly enough by the spacer. Or they can be shoved too hard, leading to binding bearings. When the input shaft isn't in the right position, it doesn't engage the big ring gear of the differential consistently - imagine gear teeth that aren't fully meshed together... there is a lot of slop between them. When you are driving at a constant speed, or decelerating, there isn't a whole lot of load on these gears so they can rattle around in this slop - making noise. Accelerate and the engine torque causes the gears to "stick to one side of the slop" so the noise goes away.

Whatever, it's a dealer/warranty claim. Odds are they'll just install a new differential.

It's possible it could be bad CV joints (axle joints) or bad U-joints in the driveshaft too but I'll bet on a poor differential.

mike c.

 

[motorsportsauthority]

I've seen out of balance tires do this, and we've corrected it with a simple tire balance. different size tires will definitely contribute as a possbility.

 

[Squirrels]

Matched tires now and no vibration. Before they went this far the tires were reset, balance and alligned in which none of fixed the problem.