Misses when drops out of overdrive

My 97 Sierra pickup 350 V8, auto tranny runs fine at all speeds, but when you press the accelerator hard enough to make it shift down, it starts to miss badly. Acts like it's only running on 4 cylinders, chugs and looses power totally. If you ease up and let it shift back up again, smooths out and runs like a top. You can be running along at 70, come to a big hill and press hard enough to make it drop out of O/D and it misses. You can accelerate normally as long as the tranny doesn't shift down.

Does anyone have ideas or suggestions?

Sounds like the Torque Converter Clutch is not unlocking before it downshifts. Could be the TCC solenoid, the converter itself or a sticking valve.

Is the Check Engine Light on? If so, the code may point you to the problem.

//2000CAYukon

The check engine light is not on, and there are no other abnormal indications. I've owned the truck for several months, and the previous owner had just had the tranny rebuilt. This doesn't sound good.

What happens if you manually shift down to 3rd?

Do you know where it was rebuilt at? It might still be under warranty.

//2000CAYukon

I'm going to manually shift and see what happens, will post the results. I do have the paperwork on the rebuild, but it's not still under warranty.

It only seems to do it when you press the pedal hard enough to make it shift down to the next gear. Runs ok if I shift down manually.

It only seems to do it when you press the pedal hard enough to make it shift down to the next gear. Runs ok if I shift down manually.

Well at least you know it is not an engine problem (because it would have done it when you manually shifted). I don't have the factory service manual with me (still at work) but I still think the problem is with the torque converter not unlocking. Manually shifting must be unlocking it but it is not unlocking with throttle position (auto downshift).

Later on, I will look at the factory service manuals to see what the difference is between a manual downshift and an automatic one (as far as valves and the tcc solenoid)

//2000CAYukon

If the SES light is on take it to Advanced Auto or Autozone and have them scan it for you, it's free.

One thing that probably has some bearing here, the previous owner had the trans rebuilt (About a year ago). I'm afraid they didn't get something right, although it shifts smoothly and there is no other indication of a problem.

One thing that probably has some bearing here, the previous owner had the trans rebuilt (About a year ago). I'm afraid they didn't get something right, although it shifts smoothly and there is no other indication of a problem.

It could be something as simple as a sticking tcc solenoid. Of course, it could be something in the valve body, too.

Have you changed the fluid recently? If not, it would not hurt to drop the pan and change the fluid and filter. With the pan off, you could look at the tcc solenoid or even just replace it.

//2000CAYukon

FYI for anyone interested, according to the tranny shop, the front sprag needs replacing. Best case $600, worst case $1100. This despite a trans rebuild less than 15 k miles ago.