Hi Alloro...I was told that if there was a bad vacuum, PCV valve, etc that the PCM would lean out the engine, causing the misfires.
Vacuum leak misfires are old school predating engine computers as we know them today. In today's world with a vacuum leak the computer can compensate to a point by enriching the mixture. OTOH misfires allow excess fuel into the exhaust, in turn the O2 sensors see less oxygen and figure the system is running rich, so the computer leans out the mixture, hence the P0174 code.
OTOH misfires allow excess fuel into the exhaust, in turn the O2 sensors see less oxygen
Actually, the misfire causes the oxygen to not be consumed, and therefore the O2 sensor sees too much oxygen and tries to compensate by adding more fuel.
You want to check the freeze frame data to see if the engine was under load when the code set. If it was , check the fuel filter and the fuel pump pressure. Inadequate fuel supply can cause a lean misfire and/or the lean code. Additionally the freeze frame will show the fuel trims so you can see if the PCM was trying to compensate, and if it was compensating on one or both banks.
If the engine was unloaded look for a vacuum leak.
Not too familiar with this thread but have worked on an 03 Expy and now have an 04. I have codes P0171 and P0174 currently on my 04. It had jumped to some other ones combined with the P0300 but got those to go away.
So, after reading numerous things online I am still trying to resolve the codes. I have replaced all 8 spark plugs with the OE motorcraft plugs, and I replaced the Heated PCV valve and line(infamous rear intake elbow) and still have the 171 and 174 codes.
Had it looked at by a mechanic who had checked for vacuum leaks...
Checked the MAF sensor and had cleaned it before the codes came on...
Any ideas what it could be? I am deployed so can't try any fixes right now but trying to think of things it could be so can tell my wife to tell the mechanic if it comes to that. Or, I will try when I return...Thanks for any input.