need help figuring this out; have a 91 2.8 that has a lot of vibration, I have replaced the harmonic balancer, pulled the torque converter away from the engine, replaced the flexplate and I still can't get rid of the vibration. you can see the crank bouncing but it must be a bad crank is all I can figure.
Anyone have any ideas?
Thanks

 

What do you mean by "crank bouncing"?

The crankshaft is not to show any visible movement except that "rotation". The only clearance that may be felt is axial and measured in just 2 or 3 100's, and usually needs a dial gauge to be measured.

What has been done to this engine before that vibration started?
Has it stalled? Modifications or repairs to the moving parts?
Knocking noises? Banging?

Not sure about the I4 engine but as far as I know GM it may be an external balance engine as well as the V6's and V8's.
That makes it imperative to use the correct flexplate and correct balancer.

Another source of such vibration which can make the whole engine shake is misfire.

 

OK, upon further review I have the wrong flexplate in the truck. I put a 1993 (thought it was a 91 until I looked at an old receipt a few days ago) engine in the truck a couple years ago and one brain fart later I put a 87 flexplate back in the truck. I will be putting the new flexplate in this weekend and just hope to much damage hasn't been done already. will update later.