system too lean codes ?

The system too lean codes come from feedback from the oxygen sensors, but the O2 sensors are not the cause. The computer takes data from the MAF and compares it to the factory set ideal fuel mix. I think there's like a +/-20% window. If the computer thinks that for a given air flow, it has to lean out the mixture 20% below normal to keep the oxygen sensors from reading too rich, then the computer sets the too lean code.

What you probably want to do is update the table in the computer that converts MAF readings to air flow values. With your new replacement MAF, it doesn't read the same as stock. The aftermarket unit will read plus or minus at different flow rates. Maybe the vendor can supply you with the values. I haven't programmed or re-flashed any ECUs though.

The other thing to do is check the fuel delivery. If your boost-a-pump is adding to the fuel pressure, it will put more fuel in for a given pulse width. The same thing could happen with high flow injectors. This affects the calculation the same way. Too much fuel means the computer trims the fuel down, and when it gets to -20% of normal (compared to mass air flow), then it sets the too lean code.

The problem is that it won't adjust the fuel mix beyond the 20% window. This could result in running a little rich at times. If the MAF is dirty, or if you have an oiled air filter like K&N, then you may need to clean the sensor in the MAF.

If you're getting codes indicating too lean fuel mixture at banks 1 and 2 and their upstream codes that tells you that both sides of the engine have a rich fuel mixture problem.It could be the MAF sensor mod you made but since it ran fine for a couple weeks after you had changed 02sensors I'd check the plugs and wires,plus the air filter etc.

Hope that helps and best of luck.By the way sometimes the plug wires connector at the coil etc get rusty or corroded so check them and clean them at both ends.If that engine has a IAT sensor check it too.

the concern could be interior the exhaust gadget, including a undesirable flex coupling interior the header pipe, or only be brought about via a gasoline filter out that needs replaced. Now the gasoline filter out replace is extra handy (and extra less expensive) then an exhaust leak to restoration. yet once you have observed a upward push in engine noise suspect which you have a leak some place between the engine and the front O2 sensor.

I had the same symptoms on a Windstar with the same codes, it was the upper intake seals, and isolator bolts. It is most likely related to your intake.

u have a vacume leak bout 90percent of the time on a 2 lean code vacume leak sure