GMC Repair: Misfire at Idle, fuel pressure regulator, ac delco parts

Question
We own a 1999 Chevrolet Suburban with 193K miles.  Over the last month, it has been idling very rough.  We did a full tune up with AC Delco parts (including plugs, wires, rotor, fuel filter, distributor cap) however the rough idle stayed the same.  We own an Auto Tap diagnostic scanner, and were able to pull trouble codes and monitor the fuel trim and 02 sensors and misfires.  The codes we got were P0300, P0172 and P0075 which are random misfire, and rich on bank 1 and bank 2.  The engine only misfires at idle on cylinders 1 and 3, and maybe on cylinder 5 from time to time, but it only does this at idle, no other time.  The 02 sensors switch appropriately when the throttle changes, so I don't think they are the cause.  Also, at idle the short term fuel trim on bank one is -30% and on bank 2 it is about -20%.  The trim values both return to near zero once we are driving.  I'm thinking it is a faulty fuel pressure regulator, what are your
thoughts?  It also is harder to start, and you can smell fuel near the exhaust pipe at idle. How hard is it to change a FPR on this engine (5.7L Vortec), I've
heard varying stories...  Thanks in advance!!!"

Answer
Hi
Sounds like you done your homework...
From what you are telling me, I would also concur based on the STFT values. The PCM is trying to subrtact fuel to get the O2 sensors to read in specs.
The Regulator MAY be leaking and the fuel is running into the #1 and 3 runner channels.
The FPR does not have a vacuum line going to it it uses Internal Manifold vacuum to keep pressure.
The Upper intake Plenum needs to be removed.
To me it is about a 30 min job. But I've done 100 or more upper plenum removals.
I will give you the EXACT GM removal process and if you need ANY other assistance, please follow post back.

Intake Manifold Replacement (Upper)
Removal Procedure
Disconnect the battery negative cable.
Drain the engine coolant.
Remove the air cleaner assembly.  
Remove the vacuum brake booster hose, if equipped.
Disconnect the accelerator control and cruise control cables (if equipped).
Remove the accelerator control cable bracket and set it aside.
Remove the following electrical connectors:
  -  The SCPI injector harness connector
  -  The EVAP canister purge solenoid valve
  -  The ignition coil
  -  The ignition control module (ICM)
  -  The generator
  -  The idle air control (IAC) motor
  -  The throttle position (TP) sensor
  -  The oxygen (O²) sensor
  -  The exhaust gas recirculation (EGR) valve
  -  The engine coolant temperature (ECT) sensor
  -  The engine oil pressure sensor  
  -  Disconnect the fuel pipes from the fuel pipes.
  -  Remove the AIR check valve (crossover) pipe, if equiped.
  -  Remove the distributor cap and reposition the distributor cap out of the way.
Remove the PCV valve hose assembly.
Remove the engine wiring harness bracket from the manifold.
Remove the EVAP canister purge solenoid valve.
Remove the studs holding the upper intake manifold to the lower intake manifold.
Remove the lower intake manifold.  
Clean all the surfaces.

I will also tell you ta MAKE SURE you get a new Upper Intake gasket (seal) kit and USE THE ONE that goes around the oval shaped electrical connector for the injector passthrough. The old one will NEVER seal correct again.
Pawl