GMC Repair: 2004 Impala - electrical issue, fuel pumps, pigtail connector

Question
I have a 2004 Chevy Impala with one issue that is hemorrhaging money. At first, it gave a code for MAF sensor. The car sputtered like it ran out of gas, so we had the sensor replaced. Mechanic thought it would be a good idea to replace fuel filter and O2 sensor at the same time. Then the check engine light came back on, gave us a code for low fuel intake. Car would not properly start. it would sputter and quit right away. No battery issues, etc. It seemed like the car would just turn itself off. We now found out that if you rev the engine before it sputters out about 6-8 times, it'll work, but works like a dog. In the off chance it starts fine, it works fine. But when it works like a dog, it gets 13 mpg, we're used to 30. (there are about 104,000 miles to this car, it's no where near done, and I've taken good care of the thing, I just don't want to lose it to a stupid electrical issue).

They checked the fuel injector switch and all fuel pumps were performing well, so they diagnosed it as a bum MAF sensor. we had it replaced again. still no luck. then they thought it was the computer, so we had that replaced. no luck there. still thought it was a short in the MAF sensor, so we replaced it a third time. and wouldn't you know, the check engine light came on this morning and keeps giving the code for low fuel intake.

Any suggestions? If they tell me MAF sensor one more time, I'm going to go insane. I know you are a GMC expert and not Chevy, but I thought I'd try anyway. Thank you for your time.

Answer
Hi
Not knowing which engine you have (3.8 or 3.4) I will give you possible reasons for either engine. Also by saying "low fuel intake" I am assuming the code is a P0174 which is actually fuel system lean.
For the 3.8L If it also set's the MAF sensor code, have them check the pigtail connector that actually plugs into the sensor. This is a common fault on the 3.8L. They (or someone who kinda knows about cars)while the engine is running wiggle. that harness/plug.If the fault (sputters) while wiggle testing the plug occurs, there is your fault. Also on the 3.8 would be the plastic upper intake manifold (plenum). Again, if mechanic could spray carb cleaner around the plenum while it acts up and it smooths out, the plenum could be warped or cracked.
For either engine,  Low fuel intake may also mean high air intake. Any unmetered air entering the engine ( vacuum leak, EVAP system fault) could cause your code and symptoms.
If the 3.4 the more common fault would probabally be the lower intake gaskets failed, again creating an internal vacuum leak.
Have them check the basics, air leak, vacuum leak. Low fuel intake, as stated can also mean high unmeterd air.if these are your only codes that is where I would start.
You MAY also have the Catalytic converter checked, I doubt it , but the symptoms you state also sond alittle like the Converter may be plugged or partially plugged.
Hope this helps
Pawl