New transmission - vibration problem

[Geesie]

Installed a Bowtie OD-built 200-4R this past week. So far, it's really done wonders for my fuel economy, which was a big reason I got the thing.
Made a big mistake, though... accidently ordered the 2200 stall converter instead of the 1800 stall, so that cuts a little into efficiency and makes for a weird city driving experience.

Anyway, I noticed a really odd and unpleasant problem. Once I push past 60-65 MPH, I get horrible vibration when I take my foot off the throttle. As in, I'm going 70, come to a downhill, take my foot off to coast/brake, and the care shakes badly enough that I can't see out the rearview mirror because it's too blurry. If I give it gas again it will stop vibrating (but of course, this isn't a good idea when I want to slow down).

Any idea on what is causing this problem? Is there something wrong with the converter lockup circuit or what?

 

[jrm81bu]

Do you know for sure if the convertor is unlocking when you take your foot off the gas?

 

[Geesie]

I don't know for sure but I disconnected the lockup wiring and took it on the highway for a few miles and the vibration was the same.

 

[85 Cutlass Brougham]

Make sure the engine to transmission bolts are still tight. It sounds to me like that may be the issue, or the flexplate may have come loose. I would also check the U joints and the U joint strap bolts at the rear axle.

Also, at 70 there would be no issue with the converter clutch staying locked. I do it all the time in my 5 speed truck and that is essentially the same thing.

 

[pontiacgp]

I had a vibration similar to that and it turned out to be the torque converter overheated and in turn damaged the stator......

 

[Geesie]

The converter is brand new so I really hope it's not burned up! How could I check it?

 

[Mudweizer]

Any slack in the u-joints?

 

[pontiacgp]

The converter is brand new so I really hope it's not burned up! How could I check it?

take off the inspection cover and if the converter overheated it'll show on the torque converter

 

[megaladon6]

also check your trans mount and x-member bolts. i've seen similar vibrations that came from the engine/trans being just a hair loose. with power it's all pushed to one side and locked. with out power it's centered and loose.