Just talked to Bob. The transmission now slips in 1st gear. He never really got on it till now to see how it works. This is for my L67 swap. The trans came out of a 3100 century with 86k miles.

Engine mods are 3.4, CAI, headers, high flow cat and 2.5" catback exhaust.

Any ideas? Bob'* guess is that it could be not enough pump pressure in 1st gear so he'* thinking of increasing the pressure and seeing if that helps anything. All other gears work perfectly fine. He even sent me a WOT video from 35 to 70 in 2nd and 3rd gear and no slipping at all.

Anyone run into this during engine swaps? Transmission really only has 86k on it according to where I bought it from and they said they were able to drive it in 1st and 2nd and it didn't slip at all, but for him its slipping even at medium throttle.

Update: he just laid a 30ft stretch of tire marks in 1st gear. He'* thinking it is the pressure. Any ideas?

Ok, so now he'* not sure if the transmission was actually slipping or if it was the tires roasting. The tires don't make much noise at all when they squeal.

What did he use for a pcm, wiring harness and tune? The tune is where he'* upping the pressures. But here'* something to consider, the input seal/piston etc is known to be an issue area in our transmissions.

99 Monte Carlo LTZ PCM, he made the wiring harness himself, and he'* tuning with HPTuners.

Someone else just pointed out that if the trans was slipping, it should throw a code, DTC P1870. He'* going to check the codes later.

The trans did have only 86k miles and was bolted to a 3100 Century, so its not like it was abused. I saw pictures of the car it came out of and looked like a complete old lady car.

I just realized something. Wouldn't a problem with the input seal/piston cause an issue with slipping in all gears and not just first?

Yes it would as the input clutch is on past 1st gear.

An update for you guys. No code showed up, all other gears shift fine, and he'* going to raise the pressure and try it again tomorrow. He said his Impala did the same thing for a while and raising the line pressure fixed it. Here'* to hoping it does the same for me.

Alright, so it seems that when the car is cold, gears engage really harshly. Its more of a jolt than anything. However, I let it sit and went inside to buy something at Target, then came back out, and it was shifting fine. However, it started its stupid light stutter/slip under acceleration from a dead stop. Bob told me his 06 Impala does the same thing. It doesn't do it at any other time except from a dead stop, and even then its not all the time. All the gears hold perfectly fine, and even 1st gear is perfectly fine is you're moving.

I'm not sure what it could be. I'm considering removing the transmission cooler as it seems that the transmission started getting a lot better once it warmed up, and that took a good 3-5 minutes.

Just out of curiousity, what torque converter did you use?

The torque converter off the 3100 that the transmission came with. I used it because it presumably had a higher stall speed...