2001 F150 7700 GVW 5.4L approx 320,000 kms (192,000 miles)
Using a Bluetooth OBD2 adapter and Torque Pro App.

Thanks to this forum and others and many You Tube videos, I have learned so much about my engine, diagnosing and repaired many small items myself. My STFT's are +/- 5% at idle, accelerating or steady cruising. The LTFT's (both banks) however, range from 12 to 22% at idle and 7 to 10% during acceleration and cruising. O2 data is always oscillating the usual Sine wave between .1 and .9 volts. Occasionally a lean code will show on the scanner (both banks) and rarely set the CEL. Everything I've learned so far indicates a vacuum leak based on the LTFT variance.

What I have done:
-Cleaned IAC (this fixed idle hunting problem I had).
-Replaced rubber elbow for PCV line at the back of the intake manifold (the original was actually still good).
-New PCV valve.
-Ran 2 cans of Seafoam in a full tank of gas.
-Used Seaform spray in throttle body and manually cleaned TB with rag and cleaner.
-New fuel filter.
-Cleaned MAF sensor and tried another used one (no change in air flow readings).
-Checked operation of EGR, DPFE and associated regulator - all ok.
-Changed EGR gasket and cleaned EGR even though it was not badly fouled up.
-Inspected intake tube from air filter to TB - no holes or cracks. Grommets are good at MAF and IAT sensors.
-Had the fuel pressure checked at the rail. Not certain but think it was 25 to 30 psi, maybe higher. Can't remember the KOEO and idle pressures.
-Used a volatile spray along the edges of the intake manifold (engine at idle) while watching fuel trims and O2 data. No variation in data or sound that I could tell. Not confident I sprayed every spot because it's a birds nest in there as you all know.
-Inspected all vacuum lines and the vac reservoir behind the battery. I applied slight air pressure to all vac lines a section at a time and sprayed with soapy water. Found some minor leaks at connections and repaired all. The EVAP purge valve mounted on the firewall had soap bubbles originating from the seam in the round valve part. A hissing sound was noticeable. Acquired two other used EVAP purge valves and both have same leak at the same seam.

So finally a couple questions:
1) Is it normal for the EVAP purge valve to leak slightly from its casing? Maybe some vacuum valves need a little atmospheric air to allow the internal pintle to move??
2) As an engine ages and wears, is it normal for the LTFT's to creep higher and create a new baseline? The STFT's continue to do their +/- 5% in order to maintain the new LTFT baseline? Maybe I am chasing a suspected vacuum leak that doesn't exist?
3) What else can I check that would cause higher than normal LTFT's?

Thanks in advance.

 

You should know by now that the Ox sensors are sensing excess oxygen left over from combustion.
That will shift the LTFT per bank rich as a loop function attempt to keep the engine running within limits until the shift limit has been reached, then codes are set.
If both are shifted, it's a common fault to both banks in the intake system.
if one bank only, look at the exhaust system header to pipe for a leak.
No mater what you checked and fixed, you missed the one causing the table shifts.
The Ox sensors in the exhaust is at the end of the trail and detects the issue no matter where is comes from..
The software can even set a code for a out of balance table condition before either one reaches the 25% limit.
Doesn't often happen but I have seen it.
Good luck.

 

Thanks Bluegrass for your explanation. It's also my opinion that I have still missed the vac leak and the intake tract (manifold and/or gaskets) is the culprit. I should have mentioned that both STFT and LTFT are in the teens and occasionally low 20's (both banks) at start up even after closed loop. As the engine warms, the STFT's make their way down to +/- 5% and the LTFT's also drop but stabilize above 10%. So it seems the vac leak is engine temperature related. In my mind, I see a cracked intake manifold gasket closing up as the engine warms and parts expand and the result is lowering of the fuel trims. I plan to make a poor man's smoke generator and smoke the vac lines/intake tract when the engine is stone cold to check for leaks.

Can you or anyone also comment on the EVAP purge valve leak I had a question about in my OP. Thanks again.