High idle disappears when I unplug the IAC valve, but it is brand new

[RonD]

But valve IS moving, so is capable of moving and not stuck wide open, just not capable of "following instructions", lol
So I would say defective IAC Valve, return if possible

 

[jcook100]

Okay, sounds good. I will try ordering a Motorcraft one. Hopefully that does it. I will follow up here once I try it. Thanks!

 

[jcook100]

Okay, so I tried a brand new Motorcraft IAC valve and the issue remains the same: high idle of about 1700 - 2000, which drops to about 900 when I unplug the IAC valve.

I should note that while driving the car and when it is in neutral (e.g., I am coasting/not in gear), often times the RPM's "bounce" between a lower and higher value - say between 1500 and 2000. That is, the RPM's jump back and forth between those two (or similar) values.

What else could the issue be? @RonD mentioned the ECT sensor, and I have yet to mess with the anti-diesel screw.

 

[RonD]

Computer is setting the high idle then
So computer issue

ECT can set high idle on cold engine but 1,200rpm never above 1,500 even on the coldest days, and it would set a code if engine(ECT) wasn't warming up

Open up computer and have a look at circuit board, especially the 3 blue capacitors, seen here: http://www.auto-diagnostics.info/ford_eec_iv

Won't be the same exact layout, they change year to year and engine to engine
under $5 to replace the 3 capacitors if one looks like its leaking or top is not flat

 

[jcook100]

@RonD I opened up the ECU.

A couple things to note:

1. The blue capacitors (there are 2) do appear to have a slightly misshapen/non-flat head (see pics), though they do not appear to be leaking.
2. There is some white/cream-colored goo that I circled in one pic, near where there is a black removable plug in the outer casing. Could this be an issue? Or maybe this is just some sort of connectivity grease?
3. I did notice a tiny bit of corrosion on one "chip", circled in one of the pics. I used a dental tool to very gently scrape away a bit of the corrosion. When I put everything back together and started the truck, the high-idle issue remained (no change).

Does anything jump out to you? Is there any way to test the computer? As in, plug it into a machine that tests it or something?

 

[franklin2]

You can check the computer for error codes. All you need is a testlight. Here's how to do it.

How To Retrieve Trouble Codes On Your EEC-IV System Without Using A Code Scanner

As of late I am seeing a lot of people asking about pulling the Fault Codes from the EEC-IV systems on their Ford Trucks. This is what I have always...

www.fordtruckzone.com

 

[RonD]

Replace capacitors for sure they are just about to leak and are no longer working

Clean up corrosion on the chip with Q-tips, isopropyl alcohol(90%+), small screw driver to scrape hard stuff off

White "stuff" looks OK, most likely heat sink silicone

 

[jcook100]

How do I replace them? And where do I buy new ones?

Should I take it somewhere or is it simple enough to do myself? I have only done minor soldering before.

 

[RonD]

If you solder you can do it yourself

Google: salt lake city electronic component parts

Looks like there are a few stores that sell board level components, i.e. capacitors

Or Google: salt lake city electronic repair

These shops can replace the capacitors and clean up the circuit board, they will have the parts

 

[jcook100]

Okay, thanks.

Is there anyway to test the computer while it is in the car, just to confirm that that is where the issue is? For instance, would it throw a code, and if so, how would that manifest on the dash - the check engine light?

 

[jcook100]

Update - I will try the EEC- IV system that @franklin2 mentions up above.

 

[jcook100]

I ended up ordering a rebuilt ECM for about $150, with a $45 core. I figured it was worth $100 or so to just swap it out. It is on the way.

In the meantime, I was able to do the EEC-IV test.

For Key On Engine Off (KOEO), I received two codes at first: 24 and 67. After a couple of repetitions, however, the 24 fell off, so I was only left with 67. My manual shows that KOEO code 67 is "Neutral Drive Switch (NDS) circuit open/A/C on". What does that mean? I made sure the AC and all interior electronics were turned off.

I did not receive any Continuous Memory codes.

Strangely, or perhaps due to the ECM damage, I was not able to get the test to work with the engine running. I tried several times.

The idle is still around 2,000 rpm. Hopefully the new ECM solves the problem.

 

[jcook100]

Welp. I replaced the ECM and it did not resolve the issue. Idle is still around 2000 rpm and drops to 900 when the IAC valve is unplugged.

To review: no vacuum leaks, new PCV valve, new IAC valve (Motorcraft brand), and new ECM (computer).

I even tried unplugging the battery for a few hours with the new computer but nothing changed when I reconnected it.

@RonD or others - what would you try next?

 

[rubydist]

Its been a while since I read through this entire thread, so maybe you already looked at this, but if you are sure there are no vacuum leaks, then I would look at tweaking the throttle plate. There is a setscrew that acts as a throttle plate stop to keep the plate from going totally shut - maybe that screw got bumped in all your work and is holding the throttle plate a bit too far open. If you haven't looked at that already, I would adjust that a very little bit.

 

[jcook100]

Thanks @rubydist, I will take a look.

@RonD what would you try next?