lean fuel trim codes

[csdragracer]

I have a '96 S-15 4.3 vortec with a 5spd trans. It is all factory stock. It has had p0171 and p0174 codes for 5 years. I have replaced everything on the truck, fuel, exhaust, intake, and all the sensors. Nothing has stopped the codes from coming back. The truck runs great and the fuel mileage is ok. Is there anyone else who has had this problem? I have changed the injectors to MFI, still has the codes. Any ideas would be appreciated, thanks.

[Sparky2263]

Under what conditions do the codes set?

[csdragracer]

The codes set when you are cruising at a steady speed. It has never set while accelerating. Usually about 30-80 miles after clearing the codes, is when the come back. Around 30 miles with in town driving and around 80 miles on the highway.

[csdragracer]

I have just fixed this problem on my truck. Oil from the K & N air filter, was getting on my MAF sensor and messing up the readings. I changed the filter and cleaned the MAF and the code went away. If you run a K & N with a MAF beware of problems. Thanks.