P1506 IAC Overspeed -- Whats your advice

[cavity.]

First off, been reading this forum religiously since I got this truck so I appreciate all the posts. I wish I had more to contribute myself Now to my problem...


Symptoms:
P1506 Check engine code (Idle Air Control Overspeed error)
Slow to rev back down when I step on the clutch or put it in neutral. Usually have to wait till I fully come to a stop for it to slow down
I'm not sure what my actual idle rpm is, but it does sound a little high. I ordered a tach which is on the way.

Things I've done:
1. Checked for intake leaks, sprayed all over with carb cleaner -- found nothing
2. Replaced MAF sensor with clean one from junkyard (cleaned it too) -- no effect
3. Replaced IAC with what looks like a brand new one out of an explorer from junkyard-- no effect
4. Replaced PCV Valve (old one seemed ok, but figured it was worth a shot)
5. Disconnected IAC -- no effect on idle !!
6. Measured TPS Voltage: Measures 1.04V to 4.76V


So my questions:
What does it mean if disconnecting the IAC has no effect on idle? (I do get a P1504 IAC circuit malfunction code when i disconnect it)
I have a theory that the idle screw was messed with by previous owner because it was misfiring when i got it (he had two plug wires swapped). He might have increased the idle to prevent the engine from dying while misfiring. Is 1.04V high enough to assume this might be the case? Can I just release some tension in the throttle cable and call it a day?

 

[tomw]

By golly, I think you have pointed to the solution yourself. I agree that the idle hard stop screw has likely been 'adjusted' to cover for the crossed plug wires.
There may be an adjustment procedure, but I would just unscrew the stop, cycle the throttle open to increase revs, and let it close. Observe idle speed, and if too high, unscrew some more.
If you have a scan tool, you can get the idle rpm using the tool. They are available on web sites such as ebay and amazon. Software to use them with smart cell phones and laptop computers is available at no or low cost. An OBD-II USB or Blue Ray, or even WiFi scan tool can be had for less than $25. Get one from a vendor that has a US presence, as many of the HK vendors have a return policy that is useless. There's a US vendor in AZ that sells and backs up their product.
Once you have a 'decent' idle speed, try disconnecting the IAC plug, and the engine idle should drop so low you think it is going to quit.
tom

 

[cavity.]

By golly, I think you have pointed to the solution yourself. I agree that the idle hard stop screw has likely been 'adjusted' to cover for the crossed plug wires.
There may be an adjustment procedure, but I would just unscrew the stop, cycle the throttle open to increase revs, and let it close. Observe idle speed, and if too high, unscrew some more.
If you have a scan tool, you can get the idle rpm using the tool. They are available on web sites such as ebay and amazon. Software to use them with smart cell phones and laptop computers is available at no or low cost. An OBD-II USB or Blue Ray, or even WiFi scan tool can be had for less than $25. Get one from a vendor that has a US presence, as many of the HK vendors have a return policy that is useless. There's a US vendor in AZ that sells and backs up their product.
Once you have a 'decent' idle speed, try disconnecting the IAC plug, and the engine idle should drop so low you think it is going to quit.
tom

Thanks for the advice! I have a free version of one of the scanner apps so i didnt realize some of them display engine speed. I'll try it out tonight!

 

[Mark_88]

When you disconnect the IAC the idle should drop...if it does not drop that indicates a possible vacuum leak...I know you searched for vacuum leaks and didn't find any...

Changing the IAC should require a computer reboot as it is a learned control process and can take time to reset...rebooting (clearing) the computer is needed so the computer will relearn the idle behavior.

Once your reset the computer if there is no vac leak and the idle should drop if the IAC is functioning...

Pulling an IAC from another vehicle may result in the same issue...look inside the IAC and if there is visible rust give it a shot of PB Blaster or similar...hold the IAC upside down so that the spray does not run into the stem where it may or may not cause problems (I read this somewhere and did it when I cleaned mine). Once the PBB has been allplied you can use a small brush or whatever to scrub the insides...I think I used Q-tips but can't remember now...

Reinstall and make sure you reseal the IAC...I was able to use the original gasket but my IAC actually worked fine...afterwards...because the idle dropped when I unplugged the IAC.

 

[tomw]

You should be able to visually check that the hard throttle stop is holding the throttle plate open or not. If there is a gap, the hard stop is not the cause of the high idle.
If there is no gap, adjust as you wish, blipping the throttle a bit between adjustments so the plate settles to its 'normal' position.
If there is a gap, the IAC is possibly keeping the idle high. Fiddle as above.
tom

 

[chewy012]

I myself have made the mistake of playing with the ol' idle adjust screw.... which as I've read, i'm sure from RonD or ADSM, isn't really an idle adjustment screw. My IAC also didn't do anything when I unplugged it, then I backed the screw out a bit and then all of a sudden haha it worked again!

1.04V from the TPS? if so, yeah that's high. I think .7-.9 is normal... mine reads 1.0 flat.

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