my brothers truck is driving me insane, it keeps setting p0171 system lean and p0174 system lean codes, i have replaced the mass air, checked all over for vacuum leaks, replaced the fuel filter, i did a fuel pressure check and found i had between 30-34psi, so i went and got two new fuel pumps, put them in and ive got the normal 42-45psi of fuel, i cleared the codes and about 40 miles later its popped right back on, anybody got anymore ideas before i pull the bulb out the dash for the check engine light???? this truck runs fine but the light wont go out

 

What kind of milage does the truck get?
Are there any noticable exhaust leaks?
How many miles on the O2 sensors? If fuel milage is typical(mid teens on the highway) and the exhaust seems solid then it's probably time to replace the O2s.

 

Has he put headers on the truck and now the O2 sensors are move back to a longer distance from the engine?
If so this will cause it to give those codes and the engine will also be running rich at the same time..

 

the exhaust is new from the converters back, i havent replaced the o2 sensors yet, with the obd II plugged in the o2s appear to be working they constantly change voltage and what not, the truck still has the factory exhaust manifolds there and i put stock walker exhaust on it when i did that, the truck is bone stock no aftermarket parts except for a drop in k&n filter

 

oh and shes getting about 15mpg she'll go 425ish miles on both tanks

 

I was having the same codes on my '96 a couple of months ago, just prior it needing a new inspection sticker, so I put a new O2 sensor on the bank one manifold and it fixed the problem. I found a Bosch sensor on e-bay for $19 with free shipping and had enough time to wait the five or so days it took to get here. I now wish I would have bought all three! It also improved my mpg.

 

You can try to watch the o2 voltage while under a partial load and see how they react. Typically though a scan tool being digital is probably not quick enough to see the actual fluctuations real time so you may not see anything significant. Usually the only times I have seen a bad o2 with a scan tool it was so bad it never changed.

If we are shooting from the hip and replacing parts my vote is o2 replacement or you have an air injection check valve stuck open and its dumping air into the exhaust all the time.

Dunno

 

I'm with Paul, Bill, and others... Time for o2 sensors.

 

o2 sensors it is, im going to get all 3 today so i got my fingers crossed, thanks for all the help, i will post back to see if it fixes her up

 

lets see.....o2, o2 and o2....check them and try replacing them...don't know much about the engine yet but I know enough to want to say its sounding like the O2s are possibly going on there way out to the death bed