I put a new engine (86 trans am 305 into a 85 Iroc TPI)in and just recently got it running. Now, when I get it up around 2500 rpm, the car starts to shake real bad, but only when in gear. If I put it nuetral, it doesn't shake nearly as bad, but the engine still shakes the car worse than my last one.

I have 2 ideas why it might be doing this 1. I need new motor mounts
2. Something with the way the flexplate is balanced. (I went from a 2 piece to a one piece rear seal, hence, a new flexplate)

Any ideas?

 

Another couple ideas.

If by chance the flexplate is in the wrong position relative to the flywheel, could that do it?

Do harmonic dampeners go bad?

Maybe a bad plug or two (engine runs fine at idle through about 2000 RPMs, and gives no codes)

???????

 

Is this a new motor or a used one?

Some posibilities are the Harmonic dampener, plug wires crossed/swapped. I once had a late 80's Surburban I chased a vibration in for weeks, the guy finally took it into the dealer, and the Service manager said to rotate the torque converter, we did, and the shakes were gone....I'm still scratching my head as to why.

 

This is a used motor. It didn't run like this in the last car it was in though. I'm gonna check the plugs and wires this afternoon, but I think the torque converter to flywheel was bolted on wrong. I'll try that tommorow.