i am new and really need some help withe check engine light

I finally bought a 99 vx, and could not enjoy it more, clean and well taken cared of.....however
check engine light is on.
first I put a stock muffler on by a muffler shop, he did an excellent job,
then the code read o2 sensor, so I replaced all four. then I got a code for engine running lean, so I replaced the mapp sensor. still got a lean code. I had a shop go over every vacuum line and checked the intake gasket. they found no problems. its still reads lean bank 1 & 2 .
They are dumb founded and they are ready to call it quits and I need in an inspection.
the car runs perfect and smooth.
suggestions given to them was to clean the injectors, but they do not have a machine/setup to do it.
my question is could it be a computer needing to be refreshed...the shop says that happens with other makes and models.
I read the posts here and it seems there are lots of people familiar with these cars, its amazing the amount of knowledge here.
can anyone help, or has anyone had the same problem.
thank you
greg

What code numbers is it throwing out?

first a small correction. the sensor that was replaced was a MAF sensor, not a mapp. the codes that showing are P0171 and P0174
thanks

Have you replaced your fuel filter yet? It's a possible but I don't think very likely cause. You can double check the intake seal by spraying carb cleaner all around the intake gasket while the vehicle is running and see if your idle changes.

Have you tried resetting the comp by unplugging battery for 15 minutes or so to clear the code? And see if it pops up again when you replug?

A code P0171 may mean that one or more of the following has happened:
•The MAF (Mass Air Flow) Sensor is dirty or faulty
Note: The use of "oiled" air filters may cause the MAF to become dirty if the filter is over-oiled. There is also an issue with some vehicles where the MAF sensors leak the silicone potting material used to protect the circuitry.
•There could be a vacuum leak downstream of the MAF sensor
•Possible cracked vacuum or PCV line/connection
•Faulty or stuck open PCV valve
•Failed or faulty oxygen sensor (bank 1, sensor 1)
•Sticking/plugged or failed fuel injector
•Low fuel pressure (possible plugged/dirty fuel filter!)
•Exhaust leak between engine and first oxygen sensor

A lot of times, cleaning the MAF sensor and finding/fixing vacuum leaks fix the problem. If you're on a tight budget, start there, but that may not be the fix for certain. So, possible solutions include:
•Clean the MAF sensor. Consult your service manual for it's location if you need help. I find it's best to take it off and spray it with electronics cleaner or brake cleaner. Make sure you are careful not to damage the MAF sensor, and make sure it's dry before reinstalling
•Inspect all vacuum and PCV hoses, replace/repair as required
•Inspect all hoses and connections in the air intake system
•Inspect and/or test the intake manifold gaskets for leakage
•Check for a dirty fuel filter and proper fuel pressure
•Ideally you'll want to monitor short and long term fuel trims using an advanced scan tool
•If you have access, you may want to run a smoke test

This is a handy document to keep available. It contains hyperlinks to what I posted above.

update: fuel pressure is fine, although fuel volume is hard to check and considering the size motor and needs.. volume was not my first thought. we sprayed a few cans of cleaner around the intake and under hood hoses and no change in anything. there are no exhaust leaks anywhere, and all is tight.
No, I have not change the fuel filter, but will do that in the morning.
I do not know what a 'smoke test is", but if I am told I will try it.
the computer has been cleared and re-examined several times.

thanks to all. I will not give up if you don't

update:
talked to the shop just now, they explained to me the smoke test, kinda like a dye test for a sewage system and is low pressure. although they do not have one, the head mechanic can borrow one from another shop if need be. he is changing the fuel filter in the morning.
thanks again.

The next easy/cheap thing to do is replace the PCV. Ours are notorious for clogging. It doesn't seem like it would be the cause but it is listed as one of the possibilities.

Another thing to try is to add a bottle of fuel treatment to the tank. Another cheap/easy thing to try but I wouldn't hold out much hope of it making a significant difference.

The last easy thing to try (if you can find it in your area) is to fill up with a full tank of ethanol free fuel. When my Proton's MAF was acting wonky, it only threw a code whenever I was below 1/2 tank.

Pvc cannot hurt to try. the gas treatment was also recommended by the shop, but finding no ethanol gas around me may be difficult. but I can google it. I have tried to keep the tank close to full with fresh gas, as it sat for about 9 months in the guys garage I bought it from. he said it had a code for 02 sensors when I bought it......but not the other code.
thanks