Yet another rough idle question. 2004 f150 fx4 w/ 5.4 .I have a code that read " system adaptive fuel to lean bank 1 " and the same for bank two. Soooooo... I got on here and did some research. Cleaned the throttle body, maf sensor, air filter, looked for vacuum leaks , changed the fuel filter and it still is running very poor at idle and it seems to be getting progressively worse any other suggestions!!!!???

 

Sounds like it's getting too much fuel. Usually when you get a code that says bank 1 and/or bank 2 too lean the PCM is trying to dump more fuel in. Could be O2 sensors. I would find a good Emissions Contol and Powertrain shop manual and follow the diagnostic tree for the code you are getting. Factory shop manuals will tell you what test equipment is required. Most cases a digital multimeter is needed.

 

Check the brake booster hose connection to the back of the intake. It is on the left side all the way at the bottom of the intake at the back. You will need a flashlight, mirror, 2 elbows, and a double jointed wrist to get to it System too lean bank 1 and 2 means un-metered air is getting in from somewhere and the PCM is having to add fuel beyond acceptable range to get the O2 sensors to switch. If it was getting too much fuel you would have system rich codes meaning the PCM is trying to take away fuel but extra is coming from somewhere, usually a bad injector or 2.