Finally had enough of the 4l65 so bought a rebuilt 4L80 and had a local shop perform the switch. Tranny worked fine at the start but engine was running terrible. Hooked up laptop and couldnt get engine to stay in closed loop(tuned) which caused it too run rich. Hooked up HP tuners but it wasnt throwing any codes. I had to use truck for some errands and put about 100 miles on tranny and now slippping in 1st gear what a disaster. Only thing I could think of was a bad segment swap from HP tuners. Otherwise did we screw something up by not shutting down something on the computer with the switch. Any thoughts would be appreciated. We are pulling the tranny this week to see the damage. Tranny was all new inside with shift kit.

 

Your sure the issue is (was) not mechanical? I have some home brew HP seg swaps that work just fine, what year is the pcm/truck?

 

Truck is an 03 Denali. We fully built a 408 with a custom HP tuners tune which ran great. Right after the swap and segment change truck runs like crap most of the time. Flashing check engine light but no codes. Hooked up laptop with HP tuners and it just wont run in the closed loop,

 

If the PCM sees a problem with a sensor or other engine management component it will not go into closed loop. It will run in a predetermined open loop condition.

The absolute first thing to do is to determine what is going on with the engine. DO not attempt to drive it any further until you understand the issue thoroughly. You can damage the transmission further, if it is dumping fuel you could hydro-lock the engine for example as well. Get a grip on it before you get much further.

I would seriously consider going back to a stock config on the PCM and then go from there. I have seen segment swaps go terribly wrong in the past. Been there, done that!

As far as the transmission itself... what is the fluid condition? If it is very dark and smells burnt then yeah it probably needs to come back out for inspection and repair. If the fluid isn't compromised then I would work on getting the engine right and then with the rear wheels on jackstands (zero load on drivetrain) run it through all the gears to see how it runs.

If you'd like advice or help along the way just give me a shout.

g

 

Thanks for the help. We pulled the tranny and 3rd gear was gone so probably builder rolled a seal. It is being fixed under warranty. Still trying to figure out the engine issue. Supposedly a stock PCM tune from HP tuners. What was weird is that a small part of the time the engine ran fine. We changed the O2 sensors but no change. Will take a serious look at the tuning again but have reloaded several times. Hopefully HP tuners can resend us a new segment swap or alternatively I have seen that someone claims that the swap is not needed with correct tuning. I will keep you informed.

 

Are you c.w.barber by chance?

 

Nope, nick names Fish.