P1506 Code and Slow return to Idle

So I've been Lurking around trying different tips on solving this P1506 code and I cant seem to solve it or my Idle problem.

The Engine idles at 900 when stationary, but the main issue is when moving and pushing in the clutch or shifting to neutral, the idle stays or even revs up to 3000 revs. Often it stays there until I slow below around 15mph, such as at a light. And also there is the P1506 AIC over speed code.
I have:
- replaced the AIC (the old one looked fine, didn't seem to be sticking or dirty)
- replaced the PCV and vacuum hose
- When around with carb cleaner checking for vacuum leaks, but no leaks as far as I can tell. All hoses look great.
- Tested the TPS (around .9v at idle, 4.8v ish at WOT) which is fine from what I'v read
- Tested EGR valve by applying vacuum to it, and seeing if it stalled the engine (which it did) so I know at least it can open.

I checked mode 6 with OBD II scanner and Torque app.
all normal except :
TID:$10 CID:$21
-
Max: 64
Test result value: 65,535
FAIL
----
TID:$41 CID:$11
-
Min: 64,640
Test result value: 48
FAIL
----
TID:$41 CID:$12
-
Max: 896
Test result value: 48
PASS

I believe there is no Bank 2 o2 sensor, and 65,535 is the highest return possible, so I am ignoring that.
$41 relates to DPFE sensor. CID $12 and $11 relate to the upstream and downstream pressure reading (I forget which is which) but it seems like they shouldn't have the same reading.
- DPFE sensor replaced
Idle issue seems better, but haven't driven enough to be certain. P1506 reappears after clearing computer.
Mode 6 gives new reading:

TID:$41 CID:$11
-
Min: 64,785
Test result value: 65,523
PASS
----
TID:$41 CID:$12
-
Max: 751
Test result value: 65,523
FAIL

65,523 is a max readout so maybe the sensors need to recalibrate?

On another note, unplugging the AIC eliminates the Idle issue, so its not stuck open (as expected, since stationary idle is normal)
From what I understand, the computer is programmed to keep the idle higher by opening the AIC after shifting out of gear to allow for easier shifting and emissions. My hypothesis is that the computer is sending inappropriate open commands to the AIC because some sensor is is giving a bad readout. This also explains why the idle is normal when stationary, as the computer is programmed to keep idle a little higher when moving, in case I decide to put it back in gear.
So:
AIC closed when stopped, normal IDLE
AIC commanded to open too much when shifting out of gear.

Sorry for the long post! What causes this? I've seen several posts with similar symptoms but no real solutions.
I'm planning on testing the Mass Airflow Sensor and EGR solenoid with a multimeter, but That doesn't seem very promising.

Is there some other piece of the puzzle I'm missing? Anyone seen this before? Any help would be great!

With IAC(idle air control) valve unplugged, the engine idle should be 500 rpms or engine may even stall, either is fine.
Only air coming in should be through the gap at throttle plate

If idle is above 600 rpms with IAC unplugged then you have a vacuum leak.
Point of the IAC is to control idle, so idle RPM without IAC connected must be below 600-650, the target idle for manual trans engine, or IAC can't set minimum warm engine idle.

CKP(crank position) sensor tells computer the engine RPM, but if it's not working you get a no start.

ECT(engine coolant temp) sensor(2 wire), tells computer the coolant temp so it can set a higher idle when engine is cold, these rarely fail and you would usually get a code for Open Loop or Closed Loop error if this was a problem.

Yes, above 5MPH computer will keep idle around 1,000-1,200.

Warm up engine.
Press gas pedal down and release it, see if RPMs drop like they should
Shut off engine
Now remove throttle and cruise cable from throttle plate, this will tell you if one of these is binding, causing slow return to closed.
Start engine and then manually open and release throttle, see if RPM drop changes.
Give a close look at the throttle return spring.
See if you can manually close throttle faster than the spring does, it should be equal, spring should be strong enough to overpower the air stream that will hold throttle open.

I've got the same exact problem as you SMBA2003. RonD was helping me in my other thread, but I'm interested to see if you figure this out before I do. I have to find my bluetooth obdII to hook up to Torque to run the mode tests to see if I get the same results as you. I also see a bunch of other threads with people having the same exact issue, but there was no solution. My old '99 ranger did the same exact thing and it drove me crazy so much that I just ended up selling it. This is a very annoying and quite embarrassing as it makes me look like I don't know how to drive stick, if you know what I mean.

Here's the code I'm throwing


I'm going to have a mechanic buddy with an advanced diagnostic computer check this out, hopefully tomorrow, so I'll post up my findings as it may help you out!

Found this today. http://www.ranger-forums.com/2-3l-2-5l-i4-tech-31/blown-maf-hotwire-slow-return-idle-137535/

Might be relevant to our issue.

Well I don't think I'm going to be of any more help. I replaced my IAC today and it seems to have cured all of my idle issues including the hanging idle between shifts. I'm not convinced it's fixed, so I hope the truck proves me wrong.

Sorry Its been a while to reply, I havent had much time to try solutions because of classes.

tsayyote, I read the link and it seems he had a firmware issue after clearing the PCM memory. It relates to a TSB for 2007 rangers. I've considered that perhaps I have a firmware issue, as the PCM seems to be commanding higher Idle on a functioning IAC. Iv read around for a similar TSB for my problem in a 2001 2.5 ranger but I can't find any.

Anyways, with the IAC unplugged I do indeed get an idle around 550-600, and as stated, no hanging idle between shifts. So normal normal. In fact I get about this idle once the engine is warmed up. Pushing the gas at stop has a normal return to idle. I havent tried removing the throttle cable, but after manually actuating the throttle the spring is pretty snappy to close so I think it is normal, I cant imagine it is sticking. Revolutions drop just as if I opened and let off with the pedal.

The code has cleared on its own on several occations, and on these days, the hanging idle is unoticable. On other days the code returns and the idle still hangs at 3000 when in neutral and coasting. There seems to be a trend that cooler days have more issues, especially once the engine has warmed up.

Still stumped

I took the truck to a shop to get the steering aligned, and the mechanic noticed the idle. He said he thought he smelled valves burning and recommended I go to a full service shop and get a valve job. Would a burnt valve cause this issue? Or maybe The high idle is burning valves (or something unrelated). Any ideas?