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I have a '99 4.0L Ranger, and the check engine light is on. I had the codes read at Advance Auto and it came back P0171 & P0174. I have searched the forum, and I have found a few things to check other than the obvious vacuum leaks. The only time the truck runs rough is on cold mornings (below 40F) and this is when the light comes on. I first time I got the light was on a cool morning right before the summer. I had the code read and reset, and did not have trouble again until a few weeks ago. I have not had too much time to work on it, but I am trying to start researching the problem so when I get a chance I can check a few things.
I'm having the same issue up here on my '98. Mine similarly started just after a cold snap. Right now, I'm still trying to find vacuum leaks. Other things could be low fuel pressure, or an exhaust leak.
Lean codes for bank 1 & 2, kinda sounds like a temperature sensitive vacuum leak & seeing as how it's the 4.0L, the lower intake manafold gasket leak should probably be high on the suspect list.
If the fasteners are just loose & the gasket hasn't sucked in, maybe retorquing them will suffice.
After that, any vacuum hose down stream form the MAF sensor, including the PCV system would be good suspects.