I have a 1999 Explorer. I'm getting adaptive fuel too lean on bank 1 and bank 2. I changed the oxygen sensor on bank two. The code is erased but comes back in three days. I looked for a vacum leak but can't find one. Any known cause?

 

I've been reading about the problems with mass airflow sensor causing these code problems. It gets dirty so you may want to clean the sensor. BE CAREFUL, it's a very delicate piece of equipment, try using electrical cleaner.

 

Did you replace all the oxygen sensors if its a 4.0L it will have 3 sensors just did that on mine. Also those codes will come up if you have a leak on the intake manifold especially if its plastic its quite common with those.

 

What was stated already is very good. Another possible air leak are the manifold gaskets. Upper and lower. If you hear an air leak, but cannot locate it, consider that. When you do the lower manifold, you can look at the injectors in place. Just a thought. Good luck.

 

since both banks are lean, you need to look at items that would effect both banks. bad maf is valid but rule out other items first.
-2 individual 02 bad at the same time is unlikely. the rear 02 are not used in fuel metering.
-look at the freeze frame data and see what rpms the dtc set at. Look at live data and read the fuel trim when in closed loop operation. Is is lean at idle, at 2500rpm or both?


fuel trim within +/- 10% is ok, 0% is ideal. over10% is a concern. the adaptive limit is 25%. +reading indicate a perceived lean condition and fuel is being added. - reading indicate a perceived rich condition and fuel is being cut.


if lean at idle but normal at 2500rpm it is not a vacuum leak.
if lean at all times, check fuel pressure at the fuel rail. If low, change the filter, if still low change the pump.

 

OTE=Shadow5;17937]I have a 1999 Explorer. I'm getting adaptive fuel too lean on bank 1 and bank 2. I changed the oxygen sensor on bank two. The code is erased but comes back in three days. I looked for a vacum leak but can't find one. Any known cause?[/QUOTE] I had same problem and found out that one of the 2 vacuum lines on the pressure regulator had a hole in it from wearing on the lower valve cover