I noticed an oscillating vibration the other night and figured it something to with wheel balance, U joints or wheel bearings.

Today in the garage with the car in park, I noticed the same vibration between 1500 and 2500 RPMs. It's hard to describe but I'll do my best.
Holding the RPMS at 2000 as an example, the car shudders in an oscillating interval. It steadily increase to it's worst point, then steadily decreases to none and continuously repeats. By steadily I mean over a span of 2 or 3 seconds it goes from none to max back to none. It's a kind of a drone.

It's not noticable at idle.

I took the serp belt off, and vibration was still there. When I look at the crank pully at idle, it almost seems to have a shimmy, but seems to straighten out at higher RPMs.

I rebuilt the tranny a year ago, put on new TC, and flex plate. Maybe there is a problem there. Someone suggested motor mounts broken or bent. Any possibilities there?

This is really starting to worry me. I was better off thinking it was the wheels.

By the way, this is a 92 automatic.

Appreciate the help.

 

check the cats... they will sometimes go bad internally kicking off a vibration.

Just tap them with a rubber mallet... if you hear a rattle, they are toast.

 

Hmmm....do this - check to see if the oscillating vibration is there with A/C both on and off. I have one *HELL* of a similar-sounding vibration that happens only when the A/C is on, and it appeared only after my LT header install (literally on the 1st drive after completion). I have been unable to isolate the problem ever since, though, so if we could get this figured out together, I'd be in heaven.

 

Thanks guys,
I'll try both first thing in the morning. I think I have it with air on or off, but I'll double check tomorrow.

 

Have similar vibration above 3000rpm only with air on, but also appeared just after longtube header install.

 

My guesses:
1) If the damper really has a shimmy, the damper may be defective -- the rubber deteriorating.
2) The original flexplate was part of the balancing (external balance) and now you will have to balance the rotating assembly with the new flexplate.
Unfortunately, you can't put weights in the holes like the dual-mass flywheel.

The torque converter should be neutral balanced.

Tom Piper

 

Remove the 4 10mm bolts that hold on the torque converter cover and remove it. Disconnect the 3 bolts that hold the torque converter to the flexplate and slide the converter towards the rear bumper. I had a pos TCI converter do this to me. This will confirm or deny the engine balance.

 

That would be the 'no excuses' way to ensure that
the R/A is balanced. However, if everything else
remains unchanged, a pragmatic solution to restore
balance is to find someone who will match-balance
the new flexplate to reproduce the weighting of
the OEM one that has been removed.

Balancing is a bit of a compromise under the best of
circumstances. This might not be a perfect solution
but it could make for an improvement and sure beats
pulling the engine for a tear-down or leaving things in
a state of imbalance.

.

 

I think the problem is exhaust resonance, when I dropped me exhaust recently I picked up a vibration. I dropped the exhaust and reinstalled it pushing the rear portion further up on to the headers when I did the rear seal and the vibration got much better. I have LT’s and the problem is the same as several others, only with the A/C on but mine is around 2K. I have a BB resonator I plan on installing and will see if that makes the problem better or worse. If it gets worse I plan on installing a set of dampening cones (The ones Jegs sells) and see what that does. I really feel the problem is just a resonance frequency and it can be solved by tuning the exhaust.

 

Maybe it could be the exhaust. I did the muffler ellim. thing and I guess that could have something to do with it.

I tried it with the air on and off. It's a little worse with the air on.

 

Noticed something today and wondered if this might be the problem.

With the engine running, looking down at the pully (torsional damper or harmonic balancer I think is the correct term) on the crank shaft, it has a wobble on the outer ring. The inner hub that has the bolts going through it and mounts to the crankshaft seems alot more stable. I noticed to that the belt dances around on the tensioner and alternator pullys too. Bearings are all good on all the driven accessories, so the cause of the wobble is the dampner. Is that at all normal? Or should I be looking for a new torsional dampner?

Still hoping it's not because of the flexplate change. I tossed the old one, not realizing it was the balance. Figured it was neutral.

 

Get a new/different dampner. The outer ring on yours is shot.

 

Do you think this enough to be the cause of the engine vibration described above?

Thanks for the help.

 

The damper should not wobble.

Sounds like you need a new one.

Unfortunately, the OEM one is not cheap. I about fell over when I saw the GM list price was about $400.

There may be some weights in some of the holes, you may have to insert weights in the new one.

Tom Piper

 

How about an after market damper? Anyone have any experience with any of those?

 

No need to weight the balancer the weights are to zero balance the balancer as well as the holes drilled in the back of it.It seems to me GM oem balancers were crap the one I got from http://www.ventureproducts.com/pictu...pers%20GM2.htm
for $235 was almost perfectly balanced with no weights and only 1 shallow hole drilled in the back.

The LT1 engine is internally balanced and the balancer is zero balanced the flexplate is what the balance it with. I have been trying to find the source of what may be the same engine vibration you have. The only thing that made a change was putting some Zmax in the oil. It is less pronounced and the rpm range has narrowed since it did that. I do not thik it is exhaust. I considered the TQ converter but the tranny guy said no way.That leaves internal problems my car has high miles so it may be part of the problem. Mine does it sitting still but untill you wind her out it is not terribly noticable till you hit say 70mph with the pedal down then the whole car vibrates. It used to be very noticable but has gotten somewhat better since the Zmax. The balancer on yours may be a problem if the rubber has cracks in it or looks distorted(mine did) you may want to replace it.

 

How about the ATI and Proform balancer they have on Ebay? You can find them new for $150 to $200. I guess I really don't know a whole lot about these things. The service manual says you should put new weights in the exact same location as the old balancer which would lead me to believe these other balancers might not be quite right for the engine. Here are a few. Would these work? I keep getting conflicting info on how the LT1s are ballanced. I've got people telling me internal, others saying external. If it were internal, why would the manual say to install these weights in the new balancer in the same location?

I just found this one at NAPA. Picture is not correct, but it is for a 92 LT1. Only $78. I ordered it. At least I can try it, and return it if it's not right.
http://www.napaonline.com/cgi-bin/nc...rpid=100491272

http://cgi.ebay.com/ebaymotors/ws/eB...MakeTrack=true engine.

http://cgi.ebay.com/ebaymotors/ws/eB...spagename=WDVW

http://cgi.ebay.com/ebaymotors/ws/eB...spagename=WDVW

 

The one from NAPA will not work it is for a camaro or caddy Vettes are one of a kind.Believe me I already did the reasearch you are not going to beat the one I gave you a link to for a bolt up to your hub replacement new.You can get yours refurbed from damper doctor.The after market ATI etc. require the hub removed and a new one installed a lot of work when all that needs done is 3 bolts to fix it.The reduction pulley is a possibilty but needs a new belt to fit it they ain't cheap either.Good luck with what you decide

 

Thanks, that clears up the ATI deal. I really don't want to replace the hub either.

I remembered seeing the Damper Doctor thing a while back. Have you heard good things about his work? If I remember right it was fairly cheap.

Thanks for the good info!

 

Talked to the Damper Doctor today, he said $68 bucks would rebuild mine. Has anyone used him before? Real nice guy, but would like to know if the dampers are good.