Background of the car: 2001 z28, stock tranny, stock rear end, mostly stock engine. Headers, exhaust, SLP lid, and that's it. The car will go into gear just fine, it'll take off just fine, and shift through all the gears no problem, EXCEPT....when it goes into 4th, after about a minute or less, it'll come back out, but it feels like only half way. The RPMs will rise, giving it gas will only rev if farther, but it will not go all the way back into gear, it won't lose speed, and if you mash on it, the car will pick up speed, but the RPMs shoot really high. Once I stop, and take off again, the very same thing happens. It feels like if you're driving a stick shift, and press the clutch halfway in. The fluid level is fine, and doesn't look or smell burnt; it's a nice red color. When it slips, I can drop it out of overdrive into drive for a few seconds, and put it back into OD, it'll be fine for a minute, then slip back out. Any ideas?

 

4th gear requires the 3/4 clutch to be engaged and the band. If you are sure it never acts up or slips in 3rd gear, this is likely the band.
How many miles are on the trans?

These related threads will give you more info:
https://ls1tech.com/forums/automatic...t-trouble.html
https://ls1tech.com/forums/automatic...-gear-all.html
https://ls1tech.com/forums/automatic...ckup-help.html

Checking the servo end play in important in diagnosing this. Note that a few mechanics lengthened their servo pin and got a bit more life out of the trans. Generally by the time the band wears out the trans need a complete rebuild.

 

Thanks for the reply. Transmission had 126000 miles on it. I went ahead and dropped it the night I posted this for a rebuilt one. A good friend of mine owns a transmission shop, he told me it would would need a rebuild. He didn't go into detail, I'm not much of a tranny guy, I know engines haha, but he did say the band was probably "burnt up". Thanks again for you reply though, I appreciate it.

 

You want scanning. See if there is an immature slip code
making the TCC quit trying to lock; see if TCC Mode is
showing it being held off for misfires; see if commanded
line% is unusually low for the load, and whether the
input/output shaft speed ratios are saying slip is in the
frictions or in the converter.

A fact or two, tending to greatly improve the quality
of speculation....