I have a 2011 Express 1500 AWD with 150k miles. It has been through 3 transmissions in 4-5 years. It has been to 3 different shops, and no one has been able to figure out what the problem is. The first time the transmission went out, I had a rebuilt one put in. Shortly after, I began seeing smoke out the back of the van. Trans oil dripping on the exhaust. This smoking never happened with any rhyme or reason, very inconsistent and occasional. Brought it back to the shop, they replaced cooler lines, nothing changed. Then they put a different new tranny in. Symptoms went away just long enough for that tranny to drop out of warranty, then it fully failed this past February. Had another new tranny put in (#3), and it’s still smoking. Seems like maybe the fluid is coming from the vent hose?, but difficult to tell. Please help.

 

I'm guessing the leak is near the tail since you're looking at the vent tube. What part of the exhaust is getting wet with fluid?

These trannys can leak from the slide yolk and sling fluid around the tail by the first U-joint. The slide yolk is hollow with a pinhole right behind the U-joint that can develop a leak over time. It can be easy to miss because it's not a spot where a tranny can leak if not for that pinhole. Even experienced mechanics might not know about the pinhole unless they've dealt with it before.

The seal where the yolk slides in the tail (obvious leak point) will be dry and it will look like fluid is coming from the U-joint which would be impossible. Basically, the tranny is dry and the driveshaft is wet. Without knowing about the pinhole, the obvious thought is that the fluid is dropping on to it from above. The same slide yolk would be used with each new tranny so the problem would carry over.

Mine did it right around the same mileage as yours. Fortunately It happened in the driveway and I saw the puddle (almost a quart) as I was pulling out. Surprisingly, the solution is to plug the pinhole with RTV. Some have had luck cleaning the area behind the U-joint really well and applying RTV but the proper way is to pull the drive shaft and plug the hole from inside. I found this online and confirmed it with a friend of a friend who owns a rebuild shop that works on these gearboxes. He's plugged a bunch of them with no issue.

I pulled the drive shaft and cleaned the inside of the slide yolk with a gun barrel cleaning brush and brake parts cleaner. It doesn't have to be perfect. I found a water bottle cap fits perfectly down the "barrel" so I put a glob of RTV on the cap and rammed it all the way down the shaft until it bottomed out and the RTV came out of the pinhole. It's been bone dry ever since.

There's a bunch of info online about leaking GM slide yolks if you google it. Once you know what you're looking for it's not hard to diagnose and if you're up to pulling the driveshaft it's an easy fix. If this turns out to be the case the shop should handle it for you. Very easy for them.

Come back and let us know what happens.