Re: 2001 Buick Century with 225k miles. The car starts, idles & revs fine. When driving on level road at steady speed it runs fine but when the accelerator is pressed it bogs down and has to downshift to accelerate at all or make it up a moderate incline. It’s as though it had a bad vacuum leak or the spark won’t advance. The vacuum and compression measure within specs. It doesn’t set any codes except an occasional generic EGR DTC when it has revved really high. I tested the EGR valve and even blocked the inlet port without effect. Sometimes if I drive 50-60 miles it will suddenly run right until I shut it off. The last time this happened I shut it off for 3 minutes and when I restarted it it was out of whack again for the next 30 miles. This has gone on unchanged through many tanks of gas.
BTW: it has new plugs & wires and good fuel pressure under way.
If this is the original fuel pump I would not go any further than what is required to get a fuel pump installed. GM fuel pumps are about the only part I will replace simply on a guess and mileage. GM really dropped the ball with pump quality. I don’t guarantee this is the culprit but I would not put one dime into diagnosis until it had a new pump installed, not real scientific I know but these cars will come in on the hook and then you can drive them into the shop. Safe thing is install a new fuel pump.
Have you looked at the fuel tank at all? any dents or caved in places? this can cause issues like you discribe.