93 3.0 Ranger idle issues (code 412 and noticeably weak idle)

[CrocsandRangers178]

So, second time posting in a few weeks.

My recently acquired ranger throws the code 412 (check engine light is not on), and the engine dies down a bit here and there while idling. I checked the IAC and it looks like its been replaced at some point and did not have much of a build up inside. I sprayed some brake clean in there just for the hell of it. No difference in idle. Any ideas?

A few other things to note is that the oil/plugs smell a bit like gas, so maybe fuel injectors? The fuel filter looks old and the cat sounds/smells clogged up. I also know from the guy who sold it to me that the throttle body is on its way out b/c it sticks in cold weather? Idk, this is what his mechanic was telling him/my memory not serving well.

I recently changed the plugs, distributor, and ignition coil. Still have the old wires on there b/c i couldnt find any by motorcraft locally.

Thank you for any help!

 

[RonD]

412 (R) Idle speed system not controlling idle properly (generally idle too low)

The IAC Valve is a 12volt solenoid with a spring that holds it closed
It gets full battery and then alternator voltage with key on, on its Red wire
Other wire color is its Ground, controlled by the computer
Computer Pulses the ground to set a lower voltage which closes the IAC Valve so computer can set Target Idles it has in memory
PWM(pulse width modulation) is what this type of circuit is called, it is used on several other systems, like EVAP and EGR solenoids, very reliable setup.

3rd party IAC Valves are not all that reliable, so grain of salt on "new" IAC Valve, lol
Check the wires for corrosion
Test voltage between the 2 wires at IAC Valve, key on engine off, should be Battery Voltage, 12.3-12.8volts, computer grounds IAC Valve fully to open it all the way for start up

Test battery voltage after engine is running, should be above 14volts, and then after running for a few minutes it should be under 14volts, 13.5 to 13.8volts
If its under 13.4volts then you may have charging issue, i.e. dimming headlights at idle, and that would also effect IAC Valve because computer would have to change the Pulse rate for lower "battery" voltage
Computer "learns" the pulses per second it needs to set, say a 700rpm idle, or a 1,000rpm idle, so if "battery" voltage changes, idle could drop lower than it should and computer has to correct for that, and thats what sets the 412 code

This change in voltage can be charging system or corroded wires, or IAC Valve itself

 

[CrocsandRangers178]

RonD to the rescue! Thank you for getting back to me.

So i went out there this morning to check the voltages with the multimeter set to 20v. With key in 'on' position, engine off, the IAC wires read 11.9v. The battery measured normal with engine off (above 12v), but with the engine running, the voltage would not go below 14.77v (it was at 14.9-15v when it first started up). I let the engine run long enough to get up to normal operating temps (on a cold start), so almost 10 minutes. All accessories were turned off with the wheels straight. The inch of wires i can see coming out of the IAC plug look to be in good shape, but thats about all I can see with the rest of it being wrapped up and leading into tight spaces. I will keep the multimeter in the truck and when I drive it later today, I will check to see if the 'battery' voltage goes down after running longer. Not sure how much this matters, but the multi-meter i have is extremely cheap.

Any ideas on what this higher voltage could be?