97 LSS has no pep after warmup

[sappdw]

My 97 LSS with 200K miles has no pep after warmup. When first started it seems to run normally. Check engine is not coming on. I am suspicious that it is running fine in open loop mode when first started and when it goes closed loop at 160 degrees some senor is passing bad data to the ECM. It runs fairly smooth in this under powered warmed up state but feels like it has the power of a 4 cylinder. I have to really put my foot into the gas to get it up to speed. Shift points are also abnormally high in this warmed up state. All electronic sensors are original. Fuel pressure is normal in the low 40s. Vacuum is a little low at 17 inches. I suspect this is due to warn rings at 200K miles. This problem first started about 5 minutes before the heater core ruptured (burning my accelerator foot). The heater core has been replaced and I can't see any relationship between the heater core blowing and the poor acceleration after warm up. Does anyone have any bright ideas before I just start swapping parts? The only thing I have replaced since this started was the ignition wires thinking perhaps they were antifreeze soak. No improvement.

[Tech II]

Sounds like it's not breathing....suspect a plugged cat....can do a back pressure check or try driving with the O2 sensor removed to see if power returns.......

Assume you checked for codes and there were none?

Good indicator of this, is from a dead start, lightly accelerate and slowly bring the vehicle up to 50-60mph......can the car do this?

Now, from a dead start, lay into it....does it bog down and rev out around 3K and not shift? If yes, sure sounds like a cat......

[sappdw]

Originally Posted by Tech II

Sounds like it's not breathing....suspect a plugged cat....can do a back pressure check or try driving with the O2 sensor removed to see if power returns.......

Assume you checked for codes and there were none?

Good indicator of this, is from a dead start, lightly accelerate and slowly bring the vehicle up to 50-60mph......can the car do this?

Now, from a dead start, lay into it....does it bog down and rev out around 3K and not shift? If yes, sure sounds like a cat......

A plugged cat is a good thought. But here is what I am seeing. If I accelerate slowly it will get up to highway speed. However it will take more pedal depression than normal to get up to speed. If I lay into the gas, it will accelerate at about half normal speed and the rpms will go up to about 6000 before upshifting. This is a later shift point than normal. It is as if the ECM thinks I am going up a steep grade. A vacuum gauge will hold steady if I slowly increase rpm from idle. I have read a plugged cat will cause a vacuum gauge to slowly go down to zero if the rpms are held high. It does not. This is a non-supercharged engine. It does have a small oil leak at the plastic intake manifold. The check engine light has not come on but I have not actually hooked a scanner up to check for codes. I guess that would be the next thing to do.