I have a 2003 nissan sentra gxe 1.8 and for the past 2 weeks it's been accelerating very slow when it first started I was gradually able to accelerate to 70 on the freeway but now it's gone down to where I can only slowly accelerate to 40 mph I mean even to get to 20 mph is ridiculously slow and flooring it will definitely not help it doesnt do much just slowly go up to 20 to 25 you would have to put your foot off the gas and lightly press on it to gradually reach 40 , theres no engine light that comes on plus I just replaced my fuel pump and that didn't help I have 136,500 miles on it , Can any one help with giving me some ideas of what this problem can be ?
I had a very similar situation on my 2004 Dodge Stratus. It struggled to reach any speed over like 20 and pressing the throttle further made the acceleration worse. In my case it turned out that the catalytic converter was completely clogged and not allowing exhaust flow. This could also be a fuel pressure but since you just replaced your pump this doesn't seem likely.
An easy way to figure out if a plugged converter is your problem is to pull the pre cat o2 sensor and rev up the car and see if a lot of exhaust shoots out of the hole. If it is plugged a large amount of pressure will escape from here. if it isn't the exhaust there shouldn't be a massive amount of airflow, some is normal but high pressure air shooting out is an exhaust restriction. Also if you do find the exhaust to be restricted make sure to perform the same test with the post cat sensor as well as the muffler could be clogged instead of the converter. Granted, it's not as likely but still possible.
In my case with the converter being plugged the exhaust pressure was in excess of 11 psi, so if a restriction is present it will be obvious. Best of luck.
This can happen because of few things, clogged or not working fuel pump, clogged fuel filter, leak in the fuel lines, vacuum leak, clogged air lines or air filter, dirty air sensor, clogged exhaust system, clogged or failing catalytic converter.