i have been reading on harmonic balancers failing after i noticed it wobbling a bit when i started it up the entire motor was shaking like it wasnt on motor mounts. thats when i noticed the balancer wobbling.
it vibrates like hell 3200-3700 rpms and gets worse when i let off the gas or pop it in neutral, till it gets below the rpms. below that it doesnt vibrate and above that it doesnt. mainly 60-70 mph is when it vibrates.
its an old balancer.. so i am thinking thats what it is

does this make sense to anyone else?

 

A bad balancer will cause a vibration and if you can see that its got a wobble in it you need to replace it weather its the cause of your vibration or not. I've also read on here about people with a poly tranny mount getting an odd vibration.

 

I have the same vibration issues at 70 mph.

How is the condition of your air dam?

When i was changing my oil recently it just occured to me that it could play an aerodynamic factor at high speed...must of been mauled from a cement log in a parking lot. It is bent in several places so I can just imagine the turbulent air flow underneath the car may be causing the vibration... Hmmm

Please do tell if you find the culprit... When the glacial period in northern Illinois ends, I will test my theory with a new and badly needed functional air dam.

 

mine isnt really tore up.. i was going to swap the balancer anyways.its either the balancer or torque converter so might as well change both

 

now i have noticed that it does it only in 3rd gear i havent noticed it when i was holding in 2nd at those rpms, i heard i could have a problem with 3rd gear in the trans? then again holding it in 2nd trying to get to highway speed sounds like the trans is going to blow. i work at a dealership and the trans guy was talking to me about it. he said to try the balancer first and rotating the torque converter each bolt to see if it goes away.. i put in my other 350 awhile back and it didnt vibrate, i used the same flexplate and balancer i think.. i have 2 identical balancers. i rotated the tires a couple weeks ago and it didnt change the vibration at all so i know its not them.

 

Is this a 5 spd car?

 

no its the 3speed automatic th350

 

Ok well just take this advice how you want to because i have been through the same thing you are talking about! I have a 1991 TA, I had this crazy vibration at the same points or just around the same gear roughly that you are mentioning. I pulled the transmission twice, changed a harmonic balancer, changed u joints, looked into bent axles etc. Im assuming you have a steel driveshaft in that car, I was curious so i pulled mine and took it to a local driveline shop and had it checked for straightness, guess what it was bent, almost .040 thousandths. I dont know how it occured! I chased this problem FOREVER!!! I almost wanted to cry it was so aggravating! If you dont have a shop like that near by it'd almost be worth just running down to a local junk yard and buying a shaft from another camaro or firebird. I got mine from a 1996 firebird so idk if a 350 turbo's length is different! Let me know how it goes.

 

Think about it? The vibration is coming from something that is rotating! The faster you go the worse it vibrates, the reason it gets worse when you let off is because of the deflection, be careful with that, if it ever vibrates bad enough it can throw the shaft making your problems alot worse! Broken tranny, holes knocked in the floorboard and so on! I hope this helps!

 

i have thought it was a bent driveshaft, i have looked into getting another driveshaft from another car but the th350 is about 2 inch shorter than the trans supposed to be in there. i have a shop i can take it to but i dont have the funds at the moment..
but what was puzzling me was that outside of the 60-70 mph range it doesnt do it. i will try to see if i can get another shaft from my brother hes got quite a bit of car parts laying around.

 

Its worth a try!

 

very true.. but another puzzling thing is when i put my other motor in a couple months ago it didnt vibrate at all.. i had to pull that motor cause it lost compression.
what could have caused it to stop vibrating at that time when it vibrated before that and now with the motor i had in it before?

 

Are you pulling motor and trans all at once or leaving one in there?

 

i already have the old motor back in.. i pulled the motor without the trans. i have done it both ways. i have had both pulled out for different reasons.

 

that was confusing after i read it..
i had a 4 bolt 350 in to start with and the vibration started about 6 months later,
i pulled it out and put in a 2 bolt bored 350 and the vibration stopped.
i pulled the 2 bolt 350 out due to loss of compression and put the 4 bolt in after rebuilding it and the vibration came back.

 

that is weird. Just try the obvious more easy things before pulling your hair out on other things!

 

yea i have been chasing this for awhile.. i can put it on the lift where i work and try the torque converter and rotating the driveshaft and i can run it up to that speed on the lift and see if it is the driveshaft

 

Its gonna be hard to see. I tried that originally with mine but its so hard to tell. You dont wanna be under there if something comes undone!

 

yea i have been in a vehicle when it threw the driveshaft i cant imagine what it would be like under it.

 

Sounds like your 4 bolt 350 is out of balance to me. Does it get worse with RPMs or with speed?

 

X2.

 

As far as balance goes that makes no sense. everything from harmonic balancer to flywheel interchange from 305-350.

 

It depends on the engines. There were changes over the 50 year production run.

 

Its a fresh rebuild and the new pistons(?) may not have the same weight. If he didn't spring for a balance job it may vibrate in certain rpm ranges.
The tolerances on the older motors were not even close at all.
If you got one that doesn't vibrate your lucky. This is another reason the LS stuff is a bit "better", the build tolerances are way tighter.

 

i reused the pistons just cleaned them up.. new rings, bearings and gaskets of course, i cleaned up the entire thing.
i did keep things numbered so i didnt mix them up. i paid strict attention to how i took things apart and put it back together, except for which of the 2 balancers i put back on it. i know the balancers interchange but i had a feeling mine was bad and got ahold of a good one but i think i mixed them up.

 

i put it on the lift at work and came to realize the harmonic balancer bolt was loose.. i retightened it and the vibration went away at 65. then it started back up at about 75.. i think its coming loose after i drive it for a few miles. so i think once i hit it with my impact it shouldnt come loose again, but i did manage to tear up the shift cable and bend the bracket. so i got to get it in the air again and fix that.

 

TTOP350 is spot on when he talks about 'tolerances'.

It's not hard to look at the balancer if it is a factory type balancer and see that the rubber insert is dry rotted and probably already falling apart. Yea them old balancers NEED to be replaced. In general most fbodies vibrate at higher speeds because of the crappy GM 'tolerances'. The driveshafts are notorious for being out of balance and vibrating at about 65-85 mph which is REALLY scarry when deflection occurs as you lift off the gas. Someone could get hurt. Driveshaft balancing is cheap and you will notice the smoothness right away.

I would prefer to use a torque wrench on the balancer bolt rather then an impact. Also if your flywheel was 'impacted' on then it is probably warped. All these driveline bolts should be torque wrenched on not impacted. When removing/reinstalling the driveshaft was it set back in the same clocking with the pinion yoke? It is wise to put a mark on both driveshaft and pinion so that when you do have a functioning setup, you know to reinstall the driveshaft in a known manner so as not to create a vibration. I know mine will vibrate if i don't put it back EXACTLY the way it came out. Have you lowered the car and now changed the pinion angle which will also cause a vibration? All these 'little' things affect your driveline and can cause vibration. I've had my share of these issues and yea it sucks chasing ghost!!!