1999 Chevy Suburban Drop in RPMs while driving

Ok so does the rpm drop when you are at a constant speed? Has is died anymore? You said you replaced the fuel pump but how old is the fuel filter? What side of the tank do you normaly run on, do you run on the lower end or upper end of the tank? See if you like to run empty it is hard on the fuel pump as the liquid in the tank 1/4 tank and better keeps the pump in the fuel which helps it stay cooler and helps it last longer. Also with gm the tight media of there fuel filters and with the ethenol in today's fuel (which absorbs moisture from the air) they should be changed once a year. Is the engine light on? Do me one test ok, drive at a steady 45 let the rpm drop then hold it steady and light step on the brake with the other foot just enough to get the brake lights to come on but not enough to cause any brake drag did the rpm come back up? Get back to me with what you can and we will take it from there.

OK good the rpm change in that manor tells us the computer see's the brake lights and thus locks the convertor and to give the engine more power. This means the computer is working and the transmission is seeing the signal as it should. What you did by pressing the brake pedal was make the truck think you had a load thus taking it out of overdrive. Now I would change that fuel filter they arn't very expensive and it is more then needed and then track how it runs. I would also like you to check you local parts store to see if they do a free scan if so then scan you truck and write down what codes are there and then have them clear them. Get back to me with what codes are there, if there are any and I will look them up and see if any of the codes will tell us what happened. Thanks.

Yes you can change your fuel filter it isn't that hard to do. Once you have it in hand just look on the inside of the frame under truck about there the drivers seat is. As for the codes if the truck picked up any codes from the engine light being on before and it has never been cleared then they should still be there. Let me know how it goes. Here is a pic of the fuel filter location.

OK let go back a step and clean the Mass Air Flow Meter ok. I will need you to get some mass air flow cleaner (nothing else) and then remove the mass air flow sensor and spray the inside of it and then put it back in and see what it does. If it still acts up then we will check the TPS and the MAP sensors ok. Let me know how it goes.