P0171 p0174 lean code

Hi on the 2002 chevy 1500 with lean codes p0171 and p0174 I did replace the intake gasket it was bad I checked intake for cracks found none cleared codes took for test drive drove it down the hyway for 5 -6 miles at 65 mph check engine light never came on long term fuel trims were still at 25 when holding steady rpm and mph lean codes came back no check engine light I have a snap on apollo scanner I went to check freeze frame data under generic functions and there is none or active codes under generic functions and still no check engine light the customer replace the mass air flow and all 4 oxygen sensors with correct parts I checked the part numbers my question is what I'm I missing why does it show nothing under generic functions and what should a good long term fuel trim be

in generic mode, the car has to see the fault 2 times to set the check engine light.

25 in long term is not good. that is basically saying the computer is adding as much fuel as it can and it is still running lean..

remember,, short term fuel trim, drives long term... example, short trim will add or remove fuel, up to a certain amount, like 20 %, when it reaches its limit, it then increases long term, and the short trim then resets and the process repeats.

I would smoke test the complete intake system and make sure there is no exhaust leaks.

What are is your total fuel trim at idle on each bank? (total fuel trim = Short term + long term)

To answer your question, domestic vehicle manufacturers typically allow for total Fuel Trim of +/-10%. But I personally don't like to see it deviate much beyond +/- 5%, especially on late model vehicles.

As a general rule of thumb for the top 3 things that would cause lean on both banks...

Vacuum leak/unmetered air
FT@idle: High +%
FT under load: Gets much better

Poor fuel supply/low fuel pressure
FT@idle: normal (near zero) or a little high
FT under load: Very high +%

MAF
FT@idle: normal or slightly high +% or slightly high -%
FT under load: Gets worse (+%)


There are a whole bunch of other things that can cause these codes, but I always start with what I just mentioned above if I am lean on both banks, as these are the most common causes.

have you done wiring integrity testing... check power, grounds, heater circuits, 02 voltage oscillation. manually tested the 02's with propane and induced vacuum leak... did you clear codes and erase the block learn after repairs.

smoke test engine if you have a smoker, if not use water. even if you do not find an external leak, could have an internal... +25 is high, suggest a big vacuum leak.

good LTFT's = 0 best... no more than +- 10 max...

watch the STFT's on your scantool when doing your testing, they will be first to show any change

I cleared the codes checked wires working on a smoke machine tried water didnt find anything yet I plugged off the brake booster to see if that would do anything and so far nothing