UPDATE: I bought a scan tool and downloaded the torque app. I don't get any error codes. I decided to drive it again today. I drove about a quarter mile and barely got to 20MPH. I turned around and pushed the gas pedal to the floor and car never made it above 25 MPH in about a quarter mile. The RPMs fluttered a bit, between 2000 RPM and 3200 RPM with the gas pushed down all the way. I parked the car and there were some strange smells. I am not sure how to describe them, at one point almost the burning rubber.
I own a 2001 Pontiac Grand Prix. Last week it started accelerating very slowly when pushing on the gas. Going about 20 MPH I pushed the gas to the floor and almost nothing happened. The RPMs did not go above 3000. The RPMs also got a shaky and moved back a forth a bit.
On the highway the car slowly got up to 45 MPH and 3500 RPM, then it was stuck there for about 15 seconds - it would not go any faster and RPMs would not go higher. After those 15 seconds it finally went above 3500 and got up to 60. Once at 60 everything seemed normal, normal RPMs and no weird sounds.
The next thing I tried was in neutral. I pushed the gas down all the way and it went to about 3500 RPM. Keeping the gas pushed the down the same amount, the RPMS slowly fell to about 2500 RPMs before I let off.
Could anyone give me advice on basic ways to troubleshoot this? I checked the transmission oil after letting my car warm up and it was low (just under the whole OK marker). I don't know if this would cause these symptoms, but was not sure what else to check.
The only check engine light has been on for a long time and must have a bad sensor. I have a low engine coolant light, but I have checked it a few times and even added to it and it seems to be at the correct level.
EDITS: Automatic transmission. The CEL is for "Low engine coolant".