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Discussion Starter #1
Pulled my engine out a while back(400/AT) to fix oil leaks and clean and paint. Installed new motor mounts and took care of a few other items.
Well now I have vibration at around 60 mph on up. I put on new tires and rims(cause I wanted to). Changed u-joints, front shocks, tranny mount too. NO DIFFERENCE. So my passenger side motor mount looks like it is riding on the frame. My mechanic buddy says it looks ok to him. Can someone email me a picture of the front of your passenger side mount so I can see if it mounts like mine. Thanks,,Paul [email protected]
 

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What year car?

High speed balance on the tires/rims? Try installing your old tires and rims to see if this changes anything. Had a buddy buy new American Racing rims and one was incorrectly welded putting it out of round. Have you checked the rims with a dial indicator to make sure they are not out of round or bent?

Out of round or "heavy" tire? This can happen and give you a vibration as well as a shift of the radial belt inside the tire.

Bad bearings at the axle or pinion. Have you checked the axle bearings? How much play in the driveshaft if you grab it and twist it left and right?

Front end aligned? Could be out of alignment or worn bushings/front end causing the alignment to change with higher speeds.

Bad front wheel bearings or too much end play. Check for this.

Might be the pinion/driveshaft angle, worn out rear control arm bushings.

Did you "jack" the car up in the rear or use bigger rims & tires? This will affect driveshaft/pinion angles.

Is the new trans mount centered?

It could still be an engine mount problem, but they should have been the same in appearance other than one being left and one being right.

Many things to check & look at other than engine mounts which could be your issue.

:thumbsup:
 

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Discussion Starter #3
Thanks Jim. The vibration happened with the old wheels, I even swapped front to back, and the new wheels, so I don't think its the wheels.

I had replaced wheel bearings and axle bearings not too long ago.

Hardly any play in the drive shaft.

No I didn't jack up the rear end.

The trans mount was replaced after vibration occurred thinking that may have been the prob.

I don't know how to check the pinion. Rear end bushings are only a year old.

You did mention front end suspension,, it is old and in need of replacement.
That is what I will tackle next but it may be a while till I get to that.
I will post results then.
 

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"Hardly any play in the drive shaft."

PJ: Did you make sure the trans input shaft found on the driveshaft is not butted right into the back of the transmission? You need to have, as I recall 1 - 1 1/2" of space from the end of the trans to the front of the input shaft so it can slide in and out due to changes in the rear suspension as it moves up and down over the road.

Silly question: Have you check all your lug nuts for tightness? Does the rim seat evenly/squarely on the hub ring found on the rear axle and the front axle (at the grease cap). I had a set of aftermarket rims that had a larger center hole in the rim than the hub ring and the rim was only being supported by the lugs and held in place by the nuts. Because the rim was only being supported by the wheel studs and not the hub center as it should have, the weight of the car over time sheared right through the studs dropping off the tire/rim, and the front of my car went down to the ground at 55 MPH and all I could do was hang on as I rode up a guard rail and came to a screeching stop hanging in the air. Had to change my underwear on that one.

Your rim may be good, but off center at the hub, so double check this.

A worn front end can give you vibrations as well. Simply have a shop inspect it and let them tell you what they see. They can put it up on a lift and use bars to put pressure on the parts that you cannot in order to find out if parts are worn.

Pinion angle requires an angle gauge which can be easily purchased at a hardware store like Home Depot or Lowes if you have one nearby. Not very expensive. Takes a little time and some math. You can find more on the internet and YouTube by looking up "how to check pinion angle." I would also suspect that if you have a 4-wheeled drive shop in your area that they could check it as they have to get this correct when they raise those trucks up to the sky. :thumbsup:
 

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Motor mounts do have specific sides, they aren't interchangeable.
Reference this thread:
http://www.gtoforum.com/f178/motor-mounts-31013/

Also the attached photo. That tab's purpose is to help stop that famous Pontiac torque from ripping the rubber out of the mount. Orient the mounts on the engine block such that the weld nuts are on the back side (you insert the long thru-bolt that goes through the frame mounts from the front side).

I tried to get photos of the thru-bolt hole offset but I wasn't coordinated enough to hold everything in the correct orientation so that it shows in the photos, but it looked to me like the thru-bolt hole was 1/16" farther away from the backing plate on one side than it was on the other side. That's not much, but I'm having this feeble fragmented memory of having installed the mounts on the wrong sides once, and the result was what you described - the engine seemed to sit lower on one side.


Bear
 

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Discussion Starter #6
Thanks for the picture Bear. But I was hoping to get one of how is sits in the car. The mounting tabs with the holes rest on the frame in my car. I suppose that would not make for vibration since the rubber should take that out, just wanted to make sure.
 

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Discussion Starter #7
Jim, I will have to recheck that end play, I was going off of memory(which isn't so good).
As for the wheels and hubs.
After pulling and reinstalling the engine, the wheels were never off of the car.
The vibration started without messing with the wheels. I tried different set ups with the wheels hoping that would solve it.

Bear may have hit on something though. When I look at the engine, it does seem to be a little lower on the passenger side.
 

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Aha! I just walked over to my shop and took another look at my car and saw something significant. It turns out that although the motor mounts themselves are very close to being dimensionally the same (and the one with the tab goes on the drivers side I can see), the frame mounts that the thru-bolts go through are NOT the same. The one on the drivers side has a noticeably taller "arch" than the one on the passenger side. So, if you get the frame mounts reversed it will definitely make the engine sit cockeyed in the frame. The bottom sides of the mounts that bolt to the block rest on top of the arches on the frame mounts.

Bear (who obviously needs to spend some quality time with the power washer before I put this thing back together)
 

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Discussion Starter #9
Hmm, well darn. The motor mount with the tab is on the driver side, and the frame mounts are correct.

Back to the drawing board
 

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Val,......go back under and check your flexplate to torque converter mounting bolts, the three little ones. Make sure they are tight to spec.
 

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Checked the 3 flex plate bolts. All tight and to spec.. No loose bolts.

While I had it up on the lift. I did check the drive shaft. There doesn't seem to be any play in it at all, in and out, side to side. I don't know if that is good or not.
 

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Here is a follow up on an old post. I found the vibration that happens over 55 mph to be the driver side axle bearing. When I had the engine out to paint and fix oil leaks I forgot that I had changed the rear axle bearings at the same time. I guess the bearing was bad from the beginning. Finally got tired of the vibration and on a whim I pulled the rear axles and found one was bad. Replaced it and no more vibration.
 

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Here is a follow up on an old post. I found the vibration that happens over 55 mph to be the driver side axle bearing. When I had the engine out to paint and fix oil leaks I forgot that I had changed the rear axle bearings at the same time. I guess the bearing was bad from the beginning. Finally got tired of the vibration and on a whim I pulled the rear axles and found one was bad. Replaced it and no more vibration.

Hmmmm. right from the get go, Post #2 , "PontiacJim: Bad bearings at the axle or pinion."
 
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