Got these engine codes but I'm trying to find ways to pin point the exact problem of what it is.

PCV valve is new, idle has no problem constantly at 600 rpm, but i have terrible mpg (8)
Spark plugs? wires? what could be the cause and how can i pinpoint the issue?

2001 Blazer LT 4wd

 

Those are trim codes, P0171 is fuel trim system bank 1 too lean P0174 is bank 2 fuel trim system too lean. What about the MAF? When those go bad or get dirty, they don't always set off the SES light or even set a pending code. The PCM is getting the wrong signal from the MAF, and it's either dumping way too much fuel or not enough. If that is indeed the problem.
Try cleaning the MAF and see if that helps

 

Last time I had a MAF problem I had idle issues too..
thing is before the light came on I also had a leaky pcv hose that I fixed but I am not sure if the computer picked it up in 5 miles of driving

 

I had MAF problems with my Alero. Different vehicle,I know. But when the MAF was cleaned, the acceleration was much sharper and fuel economy went up by as much as 20% The Alero would also run rough at WOT when the MAF went hard altogether. When I unplugged it, it ran fine.
Same component, just different engine.

 

I see.
Well Ill try cleaning it again and run some fuel injector cleaners see what happens

 

By the definition of the codes, the MAF makes sense. If the MAF is dirty or bad, the engine isn't using the correct amount of fuel, so the O2 sensors are picking up only a certain amount of fuel being burned, inconsistent with what the PCM knows it's supposed to be according to base values and sensor input.
Another possible culprit might be the fuel filter. When was that last changed? What of the intake air temp sensor? The engine uses fuel at a different rate, depending on temperature of the fuel itself and the temperature of the air.


Sorry if it seems like I'm grasping at straws here, I'm just thinking of the cheapest fixes first.

 

Fuel Filter was changed like 6k miles ago.. and the intake air temp sensor.... I have never touched it once..

And what would be considered an expensive fix? besides changing the injectors

 

Low fuel pressure and or volume will cause those two DTC's to set simultaneously, and cause poor fuel mileage. https://blazerforum.com/forum/2nd-ge...eakdown-88305/ Post all 4 numbers and we can analyze them.