Ok I've got a vibration issue that started up about a month or so ago. I assumed it was the u joint but after getting under the car there doesn't seem to be any play at all. The vibrations starts up at about 2k rpm in 3rd gear(th350) and the faster I go, the faster it pulses. Tranny mount looks good, I've checked and tightened everything I could find loose, and it's still vibrating.

Ideas please, guys. I'm scared to drive this thing.

edit:

So I've been outside messing around and I never noticed it to do this before but it's actually vibrating in park as well. I revved it to around 3500 and let off. As it was coming back down the whole car started shaking at around 2500 rpm. Where does this leave me? Motor mount or flexplate I'm guessing?

 

Been messing the past few hours. It's definitely "clunking" like a cracked flywheel but I spun that sucker 3 times with a shop light right on it. All torque converter bolts are tight. I can't see any problems. Will the converter its self do this?

 

hey does your crank pulley vibrate when you rev up your motor? Or does the whole motor vibrate? A common problem i have with my car is that the tranny to block bolts loosen up after time, thats how i broke my fly wheel one time ha ha.

 

haha nah man I've narrowed it down to the flywheel/torque converter area. The 3 bolts that connect the two had gotten a little loose(barely) but I tightened them and it's still making noise and the car still vibrates. I've spun the flywheel several times looking for a crack and it appears to be fine.

 

last year i was helping a freind repair a 406smallblock had to change lifters

did all this and we go to start it it keep kicking back messed with timing finaly figured out it got to run then started it again engine sounded like it was knocking

turns out it was a cracked flywheel so we pulled the engine for nothing

it was cracked but you could only tell once the engine was out as the fly wheel looked fine in the car the crack was a vertical one
instead of the normal crack around the crank bolts

if you remove all the tq converter bolts and push the converter in and start the engine then watch to see if the flywheel flexes if its cracked it will

 

I'm an idiot and only checked the flywheel to TC bolts. Laying in bed a few nights ago I realized I didn't check the ones to the block. They were loose as could be. I tightened them up and the knocking is gone but it's still vibrating.

 

ur driveshaft may need to be rebalanced. Shouldnt cost more than 50 bucks. That cured my issue with vibration.