: 84 Fleetwood driveline vibration

05-17-13, 08:17 AM
so, when I let off the throttle at 55mph or higher, i get a vibration. car drives pretty smooth otherwise, driveshaft is perfectly balanced by a reputable shop, u joints are good and mounting is all good. the car is lowered, and my pinion angle has been adjusted to match the transmission angle (both are at 2 degrees opposing each other). any ideas?

05-17-13, 09:26 AM
Rear ends will vibrate when they are getting loose and worn out.

05-17-13, 10:26 AM
Since you say that everything else is good, I'll guess pinion bearing in the rear end.

Cadillac Giovanni
05-17-13, 12:54 PM
Yeah, my '89 did this a few weeks before the diff gave out. I had a whole new rear axle from a mint donor car, so I just drove it 'til it died.

The Ape Man
05-17-13, 09:21 PM
Sometimes the rear brakes will even vibrate a bit when axle bearings are ready to puke oil.

Load sensitive like described usually points to driveline but slack side of fan belts fwapping around or even a worn out radiator fan clutch can vibrate in a way that fools.....

Another strange one is aftermarket exhaust or exhaust system missing mountings.

05-17-13, 10:33 PM
hmmmm, looks like I'm gunna havta take a look at the diff, i suppose. doubt its gunna be anything to do with the belt drive system cuz its all new. replaced everything new when I put the engine into it (350 out of an iroc z LOL) I know my exhaust is resting against the trans cross member, but thats a totally different noise. I'm putting the car on the lift tomorrow morning at work to do some work to it, will post what I find! Thanks for the replies so far!

05-17-13, 10:51 PM
What year iroc did the motor come out of?

05-17-13, 11:03 PM
bout an 88 or 89

05-17-13, 11:19 PM
Nice. That would make it an LT1, right? Nice. You document the swap on the internet anywhere?

05-18-13, 07:55 AM
no unfortunately its not the lt1. but soon I will be bolting vortec heads onto it, give her that extra lil kick in the pants haha. unfortunately I havent had time to put any of the pics or story online yet.

05-18-13, 11:02 AM
I would bet anything that your vibration is caused by the exhaust hitting the crossmember. On my 94 Caprice wagon, the left over the axle pipe is a little too far forward. It touches the rear end. I chased a strange vibe on this car for ages. Three sets of wheels, lots of tires, two sets of shocks, full front end, all brakes. I finally figured it out.

93 was the first year for the LT1 in the F body. If its a TPI motor, it's an L98. Some IROCs of that era were just TBI LG4s, though. Like an LO3/LO5, just less power/torque (hard to believe that is possible)

05-18-13, 11:14 AM
You're right Jay, thats actually what I meant, tuned port. I had a 92 bird, my first real baby. One day I'll get another.

05-18-13, 07:13 PM
ya it was a tpi 350. running tbi now tho. i had the car on the lift today and the pipes are definitely not touching. double checked the driveline angles, went after the pinion, tire balance. was thinking maybe pinion adjustment but normally its followed with a gear whine, so I'm kinda really stumped, but the thing is so annoying to drive on the hiway. thinking on hooking up a switch for the air ride, and run it with no air and see what the vibe is like, and then start increasing the height gradually just to see if its a pinion angle thing still. rather doubt it cuz i got that dialed in pretty good, 3 degrees up on the pinion, and 3 degrees down on the transmission.

05-21-13, 07:25 PM
well, diff checks out ok, tire balance was ok, driveline angles are 2 1/2 to 3 degrees. over 55 it starts vibrating bad, and when I let off the throttle it has a real good shake to it until i put my foot on the throttle a little, then it mostly goes away.thinking possibly pinion angle is too high? gotta get it back in the air when i get a chance to measure the u joint working angle. just about ready to light a match under her lol........

05-21-13, 07:28 PM
You measuring the drive train angle in the air?

05-21-13, 09:57 PM
no, suspension is loaded on a ramp hoist.

05-24-13, 02:08 PM
Torque converter lock up above 55? A mismatched flywheel to torque converter can cause a vibration once the converter locks up just above 55 MPH. Can you disconnect the lock up wiring at the trans and drive it. What trans is in it?

06-18-13, 10:19 PM
hey all, I have an 84 fleetwood and I lowered it 3", and have had a driveline vibration since. I've had the driveshaft balanced, alignment, tire balance and every other possible thing done for vibration. I've figured out that its my u joint working angle causing my vibration. Put adjustable rear upper control arms in, but I can't adjust the pinion up enough to match the transmission angle. the trans is running at around -4 deg and the pinion is running -2 deg. the transmission is at its limit and almost hitting the floor boards so i have to settle for the 4 deg. but the pinion when adjusted up, once it takes any power the yoke starts hitting the pinion snubber frame. anyone else ever come across this, or have any ideas short of cutting the snubber frame out? really want to keep the low stance and not go back to stock height.

06-19-13, 08:15 PM
Is this engine bolted to the original transmission? Above 50 MPH the original trans locks up the torque converter. If the torque converter is unbalanced or the clutch within it is burnt, it will shudder unless the brake pedal is slightly depressed to dis engage it. Try driving at a steady 55 then keeping your foot on the gas pedal, depress the brake pedal with your left foot. If the vibration quits, I would suspect an unbalanced torque converter to flex plate because of the engine swap.

06-19-13, 08:30 PM
This site may help:


06-19-13, 11:09 PM
ya its a rebuilt 700r4 but its definitely not the internals of it because even with the engine off and rolling down a hill its still vibrating. thats the second time i've seen that link tonight so it must be good lol gonna give it a try!