Car shakes from 70-80 MPH. whats the problem
03-01-2004, 10:04 AM
#21
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I had/have a similar problem with my 88 formula. Originally it happened around 75 mph w/ the stock 2.73 rear. Over the years I had replaced the rear with a 9bolt aussie rear with 3.45 posi and put in a new alum driveshaft at the same time. I thought that the ds would fix the problem but it did not. The vibe moved down in the mph area with the 3.45s though. I have also had several sets of tires on it and these did not help as well.
So id imagine it's either my stock torque arm all along (pinion angle) or the tailshaft of the 700R4 (it's an auto).
When the vibe happens it does shimmy the whole car so it's got to be a driveline problem!
PS. My 02 Z has no problems with vibs
So id imagine it's either my stock torque arm all along (pinion angle) or the tailshaft of the 700R4 (it's an auto).
When the vibe happens it does shimmy the whole car so it's got to be a driveline problem!
PS. My 02 Z has no problems with vibs
03-05-2004, 03:53 PM
#22
I had a similar problem, though slight, at about 2100-2300 rpms in 3rd, particularly. I just swapped my 2.73s for 3.42s and included an aluminum DS in the exchange (with new u-joints at both ends and freshly balanced) hoping that would cure it. It is now much more noticeable with the new set-up. Should an aftermarket torque arm be next??
03-07-2004, 03:07 AM
#25
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I came across this in the October '03 Hot Rod Magazine Pit stop section:
Problem:
"I have an 88 Iroc Z convertible running TPI on a GMZZ4. The trans is a 700-R4 with a B&M 2400 stall converter. I have not been able to eliminate a peculiar vibration since I switched to 3:73 rear gears (from 3:23). The vibration is peculiar in that it happens only in 4th gear (overdrive) and about 2700 RPM (80 MPH). It is a beating type of vibration similar to an airplane with two propeller engines not running at the same RPM. In 3rd the vibration is not present; in 2nd I might hear a faint "beating" at 2700 RPM. I can't figure this on out- I've got a precision balanced driveshaft, but I'm running the stock 8" Ø dampner on the ZZ4. If the car is out of gear with the engine at idle at 80 MPH, the vibration is not present"
Answer:
"...first try different wheels and tires or have yours balanced...
If the problem persists, disconnect the driveshaft from the rearend yoke. Grab the pinion yoke. You should be able to rotate the yoke radially by hand, but there should be no fore-and-aft, or up-and-down play. If there is, the preload is wrong. Go back to your rearend guy.
While the driveshaft is disconnected, inspect the U-joint seat in the yoke. If the shaft was bolted into the yoke crooked, it could have damaged the locating nubs on the yoke seat, causing excessive u-joint play. If there's damage, replace the yoke.
At the trans end, check for excessive looseness at the slip-yoke spline, then remove the driveshaft and inspect the rear bushing in the extension housing for excessive wear. Sometimes when you change operating RPM ranges or rear gears the loads on the bushing change, causing mysterious vibrations. Even if there's no obvious wear, it's relatively easy and simple to replace, and often cures the problem.
If the preceeding all check out, there's no DS damage or missing balance weights, and the shaft isn't bent, try rotating the DS 180 deg. from its installed position in the pinion flange. If this solves the problem, remember in the future to always scribe alignment ref. marks on the DS and pinion yoke so the parts can be reinstalled in the same orientation.
Still have the problem? Grab the shaft and check for side play at each end. If there's more than .005"-.010" of play, and assuming the trans bushing is OK and the yoke isn't damaged (see above) the problem is inferior quality U-joints. Replace with a quality Spicer part.
If none of this helps, the DS itself requires re-balancing."
I've got the same friggin problem (exactly) as what this guy described. I just gotta get the car back together to try all of this stuff. Good luck, and keep us posted if you figure it out.
Problem:
"I have an 88 Iroc Z convertible running TPI on a GMZZ4. The trans is a 700-R4 with a B&M 2400 stall converter. I have not been able to eliminate a peculiar vibration since I switched to 3:73 rear gears (from 3:23). The vibration is peculiar in that it happens only in 4th gear (overdrive) and about 2700 RPM (80 MPH). It is a beating type of vibration similar to an airplane with two propeller engines not running at the same RPM. In 3rd the vibration is not present; in 2nd I might hear a faint "beating" at 2700 RPM. I can't figure this on out- I've got a precision balanced driveshaft, but I'm running the stock 8" Ø dampner on the ZZ4. If the car is out of gear with the engine at idle at 80 MPH, the vibration is not present"
Answer:
"...first try different wheels and tires or have yours balanced...
If the problem persists, disconnect the driveshaft from the rearend yoke. Grab the pinion yoke. You should be able to rotate the yoke radially by hand, but there should be no fore-and-aft, or up-and-down play. If there is, the preload is wrong. Go back to your rearend guy.
While the driveshaft is disconnected, inspect the U-joint seat in the yoke. If the shaft was bolted into the yoke crooked, it could have damaged the locating nubs on the yoke seat, causing excessive u-joint play. If there's damage, replace the yoke.
At the trans end, check for excessive looseness at the slip-yoke spline, then remove the driveshaft and inspect the rear bushing in the extension housing for excessive wear. Sometimes when you change operating RPM ranges or rear gears the loads on the bushing change, causing mysterious vibrations. Even if there's no obvious wear, it's relatively easy and simple to replace, and often cures the problem.
If the preceeding all check out, there's no DS damage or missing balance weights, and the shaft isn't bent, try rotating the DS 180 deg. from its installed position in the pinion flange. If this solves the problem, remember in the future to always scribe alignment ref. marks on the DS and pinion yoke so the parts can be reinstalled in the same orientation.
Still have the problem? Grab the shaft and check for side play at each end. If there's more than .005"-.010" of play, and assuming the trans bushing is OK and the yoke isn't damaged (see above) the problem is inferior quality U-joints. Replace with a quality Spicer part.
If none of this helps, the DS itself requires re-balancing."
I've got the same friggin problem (exactly) as what this guy described. I just gotta get the car back together to try all of this stuff. Good luck, and keep us posted if you figure it out.
