Its been a few months, but I'm working on my 300 again. After fiddling with the truck for months prior because it ran so terribly bad, with no codes, I pulled the ECM. Well, the damage from leaking caps ruined the board. Finally found another ECM, and truck finally runs again, but with a problem. I have a nasty idle problem. All vacuum lines are good. Anything above 1000 or so rpms, and it does fairly well. Below 1000, and its missing.

Here's a rundown of troubleshooting.
Fuel psi is 50'ish
No IAC control ( Unplugged doesn't affect idle and RPMs dont go down. Checked for voltage and its good at connector.)
With TPS unplugged, CEL comes on, but there's no change in RPMs, still idles crappy.
With vacuum line removed and capped from MAP sensor, idle increases to around 1300 and runs fairly well with CEL.
Removed IAC valve and plugged TB opening for it. No actuation of IAC.

Codes pulled are 411 and 311. 311 was my fault but for testing but the 411 I understand from how the truck ran, but not sure why the IAC isn't working. I know that when cold, it does the high idle for startup and then kicks down, but why it doesn't change the idle speed when unplugging and plugging it in has me lost.

I'd appreciate any help. Thanks again.

 

Bumping after 118 views and 12+ hours.

 

Did you try the key on, engine running self test or just the engine off? One thing it tests is the iac, and when it functions correctly, you can hear the computer varying the rpm during the test.

The iac may be stuck open and letting too much air thru... That's my best guess at the moment.

Best of luck.

 

Tried KOER.
Tried 3, known good, Motorcraft IAC valves. No solenoid actuation when unplugged or plugged in. No change in RPMs, same goes for when the TPS is unplugged.

 

If the idle remains high after unplugging the IAC then you may have a vacuum leak (unmetered air) and/or someone in a previous life has jacked with the throttlebody stop screw.

Start with the KOEO tests and CM display before running the KOER tests.

 

EGR leaking/stuck open perhaps?

Do you still have one of those extra IACs handy? If you do you might try unplugging the connector from the one on the engine and plugging it into one of the spares and watching it.

 

The idle is under 800 and lower rpms. It idles so crappy, that it can kill itself if I pull a intake leak and then rapidly cap the intake leak. Looking at the TB, the stop screw has never been tampered with. Still has the paint locking it in place.

 

With the IAC unplugged the truck should run at a very low RPM (400-500ish) or die. If it remains the same then you have unmetered air in the intake (based on your throttle stop screw statement) or the throttle cable is not fully releasing.

 

Spray water, oil, carb cleaner, etc or used an unlit propane torch with a rubber hose attached to locate the vacuum leak. Check around the intake gaskets, both upper and lower as well as the vacuum hoses.