Hello all, I have a 1999 B4000 V6 4.0L with 78K miles on it. I recently had P0171 and P0174 error codes come up. I replaced the MAF sensor and have not had the codes return but now the truck will basically stalls out when sitting at a stop light. It does not do it all the time just randomly. It has never done it when engine is cool but only once up to temp. The idle is smooth at cold engine but begins to get rougher once warmed up (nothing too bad just noticeable). I cleaned the IAC with MAF cleaner but no joy. I reseated all plug cables at the coil but no change. RPM around 600-700 then fluctuates up a bit then truck will stall quickly( I usually cannot put other foot on gas in time to get it to increase RPM before it dies)Thoughts?

 

Welcome to FTE.
I see you don't have any replies, so i'll see if I can get things started.
Those codes are for a lean air/fuel ratio both cyl banks. Why did you replace the MAF sensor???? Did it test bad???? While replacing the MAF sensor, did you disconnect the battery B- cable to wipe the corrupt fuel trim tables & if so, did you perform the cold & warm idle relearn strategy that's listed here Ford Truck Enthusiasts Forums - View Single Post - high idle problem that also gets wiped with a battery disconnect????
The IAC is responsible for maintaining idle speed, so does it do its thing when you turn on a engine load, like the AC, or turn the steering wheel so that the power steering pump loads the system, or what happens when you shift it into gear from P, or N????
If its an auto tranny, is it stalling when coming to a stop??? If so, try quickly shifting into "N" & see if that'll stop the low rpm/stall, if it does, suspect the torque converter isn't unlocking when coming to a stop. SO, using your ELM scantool running FORScan or the like, scan the tranny computer for trouble code clues & post All Numbers..

 

So I replaced the MAF because of the p0171/174 codes. Those no longer come back. The idle will fluctuate with load of ac or no ac, only occurs once it is warmed up. Shifting to neutral will not prevent the stalls as it has stalled in drive or neutral in the past. I cleaned the IAC because of other posts with similar issues. I was thinking about changing plugs, wires, and coil pack next because it feels like a misfire. Thoughts?

 

Don't throw parts at it on a hunch, take the time to perform a proper trouble shoot to find the problem part, Then replace it.
If your behind on due scheduled maintenance replacement parts, like air & fuel filters, pcv valve & the plugs & wires are due, then replace them with the specified type that are designed to take the double work load our waste spark ignition system puts on them & Motorcraft are recommended as they're of good quality & designed for the job.
Perform a "wet down" test with a spray bottle of water on the, plugs & wires, one at a time & coilpack, at night with the engine at warm idle, to see if you can spot an arcs & sparks light show from a faulty part, before throwing a wad of your gold at expensive ignition parts on a hunch.
I'd also consider scanning the computer again for trouble code clues & post All Numbers found.
If you have, or can come by a ELM scantool, run FORScan or the like software to your viewing device, as it can scan All of the vehicle controllers/computers & thus perform a more in depth look for trouble shooting code clues than just a code scanner that just looks at the engine controller/computer. It can also monitor live sensor PID feeds to controllers & their response to actuators, so that we can get a more in depth look at what's going on with the vehicles various systems. A good quality ELM can be had for $25 or less, so is a good investment for our electronic tool box.
More thoughts for consideration, let us know how it goes.

 

I forgot to ask if you've performed a fuel pressure test yet????

 

Since you were getting lean codes, you most likely have a vacuum leak. Have you tried looking for a vacuum leak?