Poor idle (sometimes) poor fuel economy, need help!

[RogerT]

Ok, here's the issue;
1996 Ford Explorer, 4.0L
Truck will run fine, then suddenly exhibit rough idle and sometimes stall.

OBD code tossed, P0171, and P0171, Lean both back 1 and 2
During a test run, when under throttle, Short term Fuel Trim bounces around -3 to +3%, when off throttle, like decelerating off a highway ramp or coming to a stop, Short Term Fuel trim jumps to around 43% and smooth idle. After a short time, 2 or 3 minutes, short term drops to near 0% and idle becomes rough. When Idle is ok, Short Term Fuel Trim is around 43%.

Found no vacuum leaks, used both carb cleaner and propane to test, nothing found.
Tested IAC, found to be working fine,
Tested DPFE, appears to work fine, control voltage changes under pressure and vacuum.
Checked MAF sensor, that too appears to be fine, cleaned it also.
Checked EGR Solenoid, also appears to be ok.
Fuel pressure is around 38psi when engine not running, but drops to around 30 when idle, and down as low as 28 psi when under throttle.

O2 Sensors appears to be ok, since when in closed loops mode the engine runs smoothly, and the ECU is compensating by increasing the STFT to run, yet under open loop mode, the idle is rough and STFT drops to 0. The high STFT would indicate a lean condition the ECU is adjusting for. (If my thinking is correct!)

Is the fuel pressure too low? causing the lean condition? (Not sure what the pressure should be when running, I know around 35 is considered ok) Any suggestions would be greatly appreciated.

 

[Stefanlg55]

try changing the fuel filter.

But thoes codes will cause ur problems. I would recommend pulling off your vacuum hoses and inspecting them for punctures. Especially get the pcv vacuum system.

 

[Hoof Hearted]

I take it, as you hae done several checks that you done the intake gaskets?

 

[RogerT]

changed fuel filter early on, checked vacuum lines with carb cleaner, might pull them off and check one by one up close. The gaskets are the next thing going to get replaced, I sprayed all around with carb cleaner and the engine made no change even tried propane, but again no change in engine, but the gasket are a likely cause so I will be changing them just because... There doesn't seem to be any hesitation when accelerating. I am still leaning towards a vacuum/gasket leak someplace. Thanks for all your suggestions.