New guy with a problem

[Whitedog]

Greetings and salutations to all here. I have been searching to solve a problem on a van and you guys seem to have a lot of information here.

Van is a 2003 Chevy 210000 miles 6 liter engine. Van was running rough at low idle so I did a tune-up on it and no change. I cleaned the idle air control valve and changed the fuel filter. I change the mass airflow sensor with no change. I read codes and had a code for gross vacuum leak and found the evap canister was broken underneath. I replaced that and no change. Also had codes for both Banks running lean and when its running rough it is running very rich. You can smell it in the exhaust in the side of the van is actually covered in soot. He doesn't run rough all the time just most of the time.

Searching here I found I should check the intake gaskets, EGR valve, the throttle body and fuel pressure. To check the intake gaskets I think I can spray water at the base of the intake while running to see if things change. Does this sound right? I may have clean the throttle body but I do remember it's been awhile. But I will check that anyway next chance I get. I also have a fuel pressure gauge to check fuel pressure. I have also heard about the fuel pressure regulator so I can pull the vacuum line off of that to check for fuel there correct? The EGR valve I don't know how to check other than pulling it apart and looking.

So what else does that leave me? The spark plugs were Platinum plugs with new wires.

Before I changed the evap canister the long term fuel trims were maxed out, but afterwards they were OK until I revved it and then one went to 22. O2 sensors were e okay but not great in my opinion but I'm not good at judging that. The front O2 sensors would cycle up and down but not consistently and smoothly but each cycled just like a mirror of the other. That is something I can check again with my torque app. Something I just thought of is if it's running rich at the exhaust pipe how can the O2 sensors be saying that it's running lean? The three codes were P0455 for the gross leak, P0171 Bank 1 lean and p0174 for bank 2 lean.

What else do we need to know to help get the van running right again? Thank you for your time helping me with this problem.