'97 Lumina strange lack of power---HELP?

[decollect]

Car has 97k religiously maintained miles. Plugs & wires good, fuel and air filters replaced recently (in attempt to fix problem). No oil in water or vice-versa so intake gaskets still good, I think.
Car has no acceleration under moderate load. At idle it's smooth as silk, no misses. Rev engine in neutral, and it runs right up to rev limiter, again smooth and strong. Drive the car easy and as it tries to accelerate you can feel it almost surging and trying to rev up, and the transmission shifts as if it's really losing power, way high in the powerband so the shifts are really hard. It doesn't feel like it's missing, it almost feels fuel starved. However, and here's where the strange part is, if you are at W-O Throttle, it runs like a bat out of hell, no problem, and then when you let off the gas after that it runs ok for a little while.
At cruise and light acceleration it runs just great and still gets decent gas mileage. It's just under moderate acceleration is when it dogs out. And when the A/C is on as now in Summer, it's even worse because of the extra load on engine.
No Service Engine light on, ran thru state inspection while problem exists where our inspection requires OBD II plug in, and NO trouble codes listed on computer.
HELP? Thanks

[richtazz]

Your problems sound like a vacuum leak. When the engine is under load or part throttle, the vacuum leak will cause the most trouble. At WOT, the car is running very low vacuum, so the leak will not cause any problems. The vacuum lines are a combination of rubber and hard plastic, and become brittle with age. I'd start by looking at the line that goes to the transmission vacuum modulator first.

[jeffcoslacker]

Originally Posted by decollect

Drive the car easy and as it tries to accelerate you can feel it almost surging and trying to rev up, and the transmission shifts as if it's really losing power, way high in the powerband so the shifts are really hard. It doesn't feel like it's missing, it almost feels fuel starved. However, and here's where the strange part is, if you are at W-O Throttle, it runs like a bat out of hell, no problem, and then when you let off the gas after that it runs ok for a little while.

Ever notice the idle being too fast or too slow occaisionally?

You might try this, if you don't find a vacuum leak like Rich said. Take the TPS off the side of the throttle body. Just a couple of small Torx screws. See if you feel and roughness when you rotate the switch, or you hear any rattle when you shake it...

It's possible the TPS has a bad spot in it, in that range of the sweep it can't sense throttle angle, and will surge, flatten out, and shift erratically like you describe. The TPS won't always set a code unless it becomes so plotzed that the ECM recognizes the reading as erratic...otherwise it just thinks YOU, the driver, are doing that with the throttle, see?