OK guys I'm stumped I have a 1995 Chevy k1500 4x4 with 5.7 350 v8 a few months ago it started blowing smoke from tail pipe every one said it was oil and my rings we're gone but looking at it closer it was black smoke so I took it to the shop and they said it was the tps(throttle position sensor) so I replaced it that didn't fix it then I replaced (map) sensor still nothing so I rebuilt my throttle body and still nothing and after a few more parts and 500$ later it still smokes it will burn a full tank in two days..... I've tried every thing I could think of some one said it could be out of time because when I get up to 55 MPH the rpm drops and it is very sluggish and when I get on the gas when leaving a stop sign the rpm will tap out but I'm not even over 35 MPH I could really use some help on this one

 

My take, injector leaking. This is a mechanical issue (if I am correct), fuel leaking into the cylinder

 

Is the check engine light on? If it is get it read to help you figure out what is going on.

 

+1 for codes
+1 for injector.
the spider assembly or poppet injectors may be leaking down. install a fuel gauge and check for fuel pressure leakdown.

black smoke is overfueling. the plugs are probably coated also.

 

Check your air intake system. Sounds like it is running rich. Lack of combustion air. Dirty filter, collapsed hose, intake obstruction ie rats nest. Good luck.