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1993 Ranger 4.0L V6 Idle Air Control solenoid not working


iheart2code

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Hey everyone! I am having issues with the Idle Air Control (IAC) solenoid on my Ranger. It's a 1993 Ford Ranger STX 4.0L V6 with manual transmission.

I am getting the following codes on a KOER test:

- 326: HEGO (H2OS) sensor fault, always lean (driver side)
- 411: cannot control RPM during KOER (low RPM check)

I think the oxygen sensor fault might be caused by the IAC solenoid not operating as expected. I have replaced the oxygen sensor as well as the wiring harness for it (it was melted). I have also performed a smoke test of the air intake and exhaust systems. Only a few small leaks where the air filter box seats together.

I know the IAC solenoid is not working because I purchased a new one and connected it to the harness off the engine. The new solenoid stays in the open position and does not close when connected to the harness with the engine running.

I have also taken a multimeter to the two connectors. The red wire (coming from the fuel injectors) will read full voltage (14.5 VDC) while the engine is running. The white/blue wire (coming from the PCM) will read about 0.15 VDC when the engine is running. I have performed these checks with the engine cold and then again when warm.

I'd appreciate some help identifying where the problem is in the loop between the IAC solenoid and whatever is causing the problem. Am I looking at a blown fuse, damaged wire, faulty PCM?

The engine runs just fine, and fairly smooth aside from a slightly rough idle (especially when cold). I'm mostly trying to address this so I can pass a smog check, as these codes do trigger the check engine light.
 


iheart2code

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I have performed a test of the IAC solenoids I have (the original part and the new one) with a 9v battery. They don't seem to be faulty.
 

SenorNoob

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Sounds like a classic vacuum leak to me, but; if the O2 sensor wiring was melted, it may have blown something in the PCM
 

iheart2code

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That's where I'm at now. I performed a smoke test and only noticed leaks in the air intake tube. A quick duct tape patch did not resolve the codes I'm seeing. So I have ordered a refurbished PCM. I guess that one symptom of a bad PCM in this car is improper signals to the IAC valve.
 

Mechrick

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Did you check the voltages with the IAC plugged in?
 

iheart2code

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Yes, see the original post at the top.
 

Mechrick

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The IAC is a normally closed device. The PCM will ground it to open with a variable duty cycle square wave signal. Sounds like the ground side from the PCM is shorted directly to ground, either in the harness or the PCM pin itself. You could try clipping the wire right at the PCM and see if the valve closes. That would rule out the PCM.
 

iheart2code

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I pulled and opened my PCM. It looks like there are several damaged components on the board.

PXL_20240418_032336051.jpg
PXL_20240418_032344031.jpg
 

sgtsandman

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I believe @Ron D says those blue things are replaceable. I think they are capacitors.
 

Mechrick

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Yep, caps are bad.

It's possible to source tantalum replacements that would last longer. They are polarized, if you give it a go make sure you solder them in the right way...
 

iheart2code

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I've decided to replace the PCM with a remanufactured version from RockAuto. I'm not super confident in my soldering skills and I'd prefer to get something that's been thru some thorough testing. Hopefully replacing the PCM and fixing the air intake tube (it has a few holes) will clear the two codes I'm seeing.
 

iheart2code

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Replacing those things did not solve the issue. Also cleaned fuel injectors & replaced fuel pressure regulator, still the same 411 and 136 codes. I guess I am hunting down a hard-to-find vacuum leak at this point. Perhaps I did not perform the smoke test well enough last weekend.
 

iheart2code

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I have checked the vacuum with a gauge and am noticing the needle slowly bouncing up to 1 inch in time with changes in RPM. Does anyone know what this could indicate?
 

Mechrick

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Do you have the capability to look at data? If your truck has a MAF with 3 digit codes there should be data output. If so, look at long and short fuel trims at idle.
 

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