My dad has a 92 5.0 V8 caprice classic. As of tonight the car had no pep when he turned into a gas station to fill the tank from a half a tank. And now the car still has no pep, but from 0-39 mph the car is fine as long as you do not floor it. Because when you floor the car it like stumbles as if the exhaust is not flowing right(From experience with exhaust), There is also a nosie when the car starts to stumble it sounds like marbles and it is not coming from the engine. and from 40+ mph when giving it gas does the same thing and when you are at 40mph and try to get up to 55mph fast it takes a long time to, it is like the car loses power from 40 and up. Also my dad said when i steped on the gas in the drive way that some black somke came out too. and when the car is idling the car is fine. and also can someone tell me where the fuel filter is because i am having trouble finding it?

Fuel filter - Passenger side rear underneath the car in line with the fuel line. If you're lucky, the car will have the quick disconnects, squeeze and pull the line from either side of the filter... remove the bolt holding it to the frame, and replace. It may also be a good idea to clean the throttle body with some spray cleaner.

Is the car throwing any codes?

I do not know if the car is throwing any codes but the engine light is not on but that does not mean there aint any codes.

Aslso Check To See If If There Is Any Spark Plug Wires Ching The Engine Or The Exhaust

The first thing I would do is change the fuel filter. The same thing happen to my 90 Caprice PD car. The car would run fine around town, but if I needed to pass someone, it was like shutting the car off, untill I backed off on the gas.

Well the marbles sound, that doesn's sound like it's coming from the engine, leads me to think the convertor is plugged.....