My daughter returned from college with her 2000 f150 4.2 idling and running rough on acceleration. It is throwing a P0171 code only. I replaced the isolator bolts/plenum gaskets four years ago because of a P0171 & P0174 codes.

So far I have replaced the MAF, new PCV valve and elbow, checked all obvious lines for vacuum leaks (though I suspect a leak somewhere still).
Also, a quick check of the plugs show some lean color/deposits on the drivers side only. Any thoughts or troubleshooting I should try next?

Thanks,

'bob

 

A misfiring plug will cause that code.

 

Thanks for the reply. I did replace the plugs today when I pulled them but it did not change the rough idle/acceleration issue. One had a huge white deposit built up on the electrode. Could this be a coilpack or plugwire issue?

Thanks, 'bob

 

Either/or.

 

Well I checked the coil pack and plugs wires and everything seams to check out ok. Any other ideas?

Thanks,

'bob

 

You do know that bank 1 is on the pass side, right?

 

your probably looking for a small vacuum hose that is hard, cracked, brittle. Pull them off one at a time and check EVERYTHING real good. 15 year old hoses laying in engine compartment are suspect.