I purchased a used Stage two preformabuilt 4l60 with a 3200 FTI stall from a buddy of a buddy. Claims to have had less than 50 miles on the unit. Reason for selling is because motor blew and he just parted out the whole car. Came out of a GTO. I’ve put a couple hundred miles on it with no issues. It started having issues unlocking when I came to a hard stop causing motor to bog down until it unlocks. Now at higher rpms the whole car vibrates. Higher rpm; harder vibrations. In third under 25% throttle at highway speeds you can hear it lock up and then go right back out. Question is, is the stall just going out or could this be related to the stall not being seated all the way on the input shaft on the trans. Keep in mind it was fine for a couple hundred miles. I’d really appreciate some help!

 

If the converter was not all the way seated it would have went zero miles due to cracking the pump when you tightened the bell housing to the block.

Between the converter pads and flex plate you want about .080" - .200"
So if you have more than about 1/4 in. (.250") then you will want to shim it to get within specs.

What do you mean you did a hard stop?
First I would recheck my converter bolts and make sure they are tight.
Next time you feel the vibration maintain your speed with the gas and just slightly press the break with your other foot, maintaining the same speed. This will unlock the converter. See if the vibration goes away.

Are you going from a stock converter to the 3200 without a tune?

 

Hard stop as in I’m going a decent speed then I lay on the breaks pretty hard to a complete stop. I also did what you suggest and it made the vibrations worse. It’s definitely rpm based. Higher the rpms the faster and harder the vibrations. I was at a stop light in driver and the motor would surge because it’s like it was trying to engage and then stop. The vibrations are like the surging when stopped just a lot more fine and not as spread out. Not sure what the gap was but I’ll have to check on that.

EDIT: correct. Stock to 3200 no tune. I’ve done a host of other things and still have a few more things to add before I tune it all together.

 

My car used to do this and it was indeed the converter.