I have a 99 tahoe 5.7 vortec. My check engine light came on and threw codes po171 and 174 bank 1 and 2 are too lean so i checked for vacuum leaks found none then i replaced mass air flow sensor and problem consisted ses light stayed on truck idles rough tranny shifts rough and shuts off sitting at a stop light. my fuel pressure will jump to 60 psi then down to 50 with a fuel pressur tester but if i clamp off a return line to the tank then it will sustain 80 psi. Can anybody help do i have a fuel pump problem,fuel regulator,or spider injector?

 

It could be a number of things the most common is that the injectors will get cloged with carbon and stay open for to long. The other is that yes the regulator could be bad, other than that you could have a bad sensor like a temp sensor or o2 sensor telling it to stay open but it seems more like a regulator or injector problem. Along with that the fuel pumps have been known to go out completely or just go weak.

 

On the vortec I have been the fuel pressure regulator is in the intake plenum? is this correct

 

Yes in the upper plenum on one side of the spider. It is a small bowl a little larger than a quarter in size with what looks like a vacum nipple