vibration, new post---please help

I have an '85 Firebird 2.8 V6 mpfi, I just put in a factory remanufactured engine and a new harmonic dampener.

It has a real bad vibration , rather it's idling in park or while being driven down the road, Its bad enough that its shaking the car apart.


It doesnt act like a dead cylinder, a plugged injecter,bad motor mount, or something power related, it just seems like something is way outta balence. the ignition has been trouble shot, so has the fuel system. Motor and transmission mounts were fine.

the tranny is a 700R4 and I have replaced the torque converter with a new one, it shifts fine.

Could I have the distributor out 360 deg and have it still run? would a cracked flexplate cause this much of an out of balance? I did check it for cracks and can not find any.

Anything else to check ?

Please help.

I'm not sure it they use them on the 2.8 but I know alot of GM V8's use weight's on the the flywheel's and Harmonic balancer pulley to keep the engine in sync but your problem could also because caused by a cylinder not firing or low compression in one cylinder. Was this engine balanced during the rebuild.

Thank you for your comments.

I dont know what was done during the rebuild. It is a crate motor and is a factory reman. so whatever the factory did is what it has. I was thinking it could potentially be the internals (pistons/rods) are way out of weight tolerances. Each cylinder is firing as I trouble shot that and with each wire removed I get the same RPM drop. I havent looked at each cylinders compression yet.

Another thought that use some discussion would be whether the engine is internally balanced or externally balanced. The original engine was externally balanced with weights on the flexplate. I believe GM also did an internally balanced with a straight flexplate. If currently they updated and the engine is internally balanced and Im using the original flex pate from the externally balanced engine, that potentially could be the problem. Does anyone have thoughts on this?

I had a similiar problem with my car when I put in a new engine and this might not work but it also might. How I fixed it was. I got under the car unbolted the flexplate to torque converter bolts you may have 3 or 4. I made a mark with white out on the two to remember where I started and then I turned the torque converter to the next bolt. Started the car up and listened/looked for vibration. Then check the next bolt until finally there was no more vibration on one of them. It couldn't hurt so this is my suggestion but if anyone else has a idea please share.
John

You mentioned a new harmonic balancer-
I remember gm swapped from an externaly balanced to internal or vice versa around 86-87 but I think it was FWD only. Did the engine come with the balancer or you have the wrong one ??
You have the old balancer yet ??

If the converter is 'new' I don't think it should matter where you bolt it up.

I would NOT run it anymore till you make sure about the internal/external issue - and maybe do a compresssion test. (you ever wonder how many engines they rebuild a day ?)

it may not be a broken engine mount but, internal or external, somehting is obviously off balance. i would make sure your engine mounts are tight, 1 of them may be giving enough range of movement to allow the car to vibrate. also, did this happen recently or right after the rebuild? and do u know if there was a preformance cam installed? sometimes the preformance cams can make the engine shake more that the factory intended them to.

I thank all of you for the comments so far.

1) I will check the engine mounts again for worn bushing.

2) This vibration started immediately after the new engine was swapped in, however, the car had been sitting two years prior to the engine swap as I had lost a rod bearing in the old engine, but in the few months prior to me loosing the old engine, I vaguely recall a vibration starting to come in the car thus my reasoning for replacing the torque converter. This is a completely different engine though.

3) I dont think a performance cam was installed. It is a generic crate motor and this vibration is more than the cam bump.

4) The engine didnt come with a new harmonic balancer. I do still have the old one. I purchased a new one though it was an aftermarket unit and not one by GM, trying to save a few dollars.

5) A compression test sounds reasonable to me at this point also.

Thank you all for your comments, any more thoughts would be appreciated.

aero, if u have fuel injection, check your injectors. a few bad injectors can kill half the engine and will cause the car to shake violently.