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

[RonD]

"Idle is still around 2000 rpm and drops to 900 when the IAC valve is unplugged."

IAC Valve or computer
IAC valve is obviously open since unplugging it/cutting power lowers RPMs

So either it is misinterpreting the voltage the computer is using or computer is sending the wrong voltage for warm idle

 

[jcook100]

I just don't get it because both are brand new, and this is the second brand new IAC valve I have tried (and it is Motorcraft).

 

[RonD]

I know you have, but its pretty obvious the IAC Valve is passing too much air when its connected

 

[jcook100]

Does the voltage of ~5v passing through to the IAC valve seem about right?

 

[RonD]

Seems low, 7v to 10v is what I have seen, warm engine idling, page here says 8v to 11v, pin 21: http://www.auto-diagnostics.info/ford_eec_iv
More likely to be alternator output differences at idle

 

[jcook100]

Okay, I may have found something.

When I check the voltage coming out of the computer for the 21 pin, I am seeing 9-10v. When I check that same wire at the IAC valve connector, touching the other lead to a ground, I still see 9-10v. However, when I touch one lead to the "in" wire (is that the positive I think?) at the IAC valve connector and one lead to the "out" (negative?) wire at the connector, I only see around 4v.

So is that second wire the issue? Is it a ground? So a bad ground?

 

[RonD]

The red wire on the IAC valve should show Battery voltage
So key on engine off
Unplug IAC connector
Test battery voltage first, 12.2v to 12.8v, remember it

Test red wire voltage
Red wire to volt meter
other volt meter wire on battery negative
Should see exactly the same volts

If not matching volts then red wire is damaged, it comes from EEC/PCM relay

The Other wire on IAC Valve connector is "the ground" for IAC Valve but its a Pulsed Ground, thru the computer, not just grounded to battery negative
With key on engine off
Put volt meter on both wires, red positive other wire negative, should see 11v-12v, computer Open IAC Valve full for Start Up so full voltage

Ranger IAC Valve is a true Solenoid, meaning it has a wire wound around a metal tube, one end is red wire other end is the "other wire"
This is an electro-magnet, the higher the voltage passing thru the wound up wire the greater the magnetic force
So same setup as a Relay but relays are on/off not variable voltage, unless battery is dead, lol, i.e. "click, click, click,........"

Anyway, the valve inside is attached to a metal rod with a spring that holds it closed
As voltage is passed thru the coil of wire the metal tube becomes a magnet and pulls the rod into the tube, the higher the voltage the more it pulls the rod in against the spring pressure trying to hold it closed
Very simple setup really
And quite accurate, +/-4rpm is spec

The computer has feedback from crank sensor for RPM, so it adjusts IAC Valve voltage accordingly
Computer has "Target idle RPMs" in memory, 1,100rpm cold, 750rpm warm, and it varies by year and Emissions standards

 

[jcook100]

Thanks, @RonD .

For the first test, I am seeing 11.98v at the battery and 11.91v at the red wire of the connector (key on engine off). Is that close enough or different enough to be a problem?

For the second test, with red lead on red wire of connector and black lead on the other wire (key on engine off), I am seeing 11.6v.

With the truck running and the IAC valve connected, pins through the back of the connector (red on red and black on the other) only show ~4v.

 

[RonD]

Both key on tests look good

Did you test key on engine off with connector plugged in?

And what was the engine idling at on last test?
2,000?

 

[jcook100]

KOEO with the connector plugged in, red lead on red and black on other wire (using pins from behind) I am showing 0.02v.

Engine idling at 1800 rpm.

 

[RonD]

Unplugged connector showed 11v and plugged in 0v?
Key on engine off

Key off
Switch meter to OHMs or continuity
Unplugged connector
hold either meter's probe to metal case of IAC valve, touch the other probe to either connector pin in IAC valve, should be NO change in meter, no continuity
test other pin the same way

 

[jcook100]

Unplugged connector showed 11v and plugged in 0v?
Key on engine off

Yes. At first I tested with pins into the back of the connector, but then to make sure I stuck a pin in each of the two wires (red and white). This is with the connector plugged in. When I touch one lead to the wire and the other to a ground, I see voltage. This happened for BOTH wires. But when I touch one lead to the pin in one wire and one lead to the pin in the other wire, it shows 0.02v. That is what you are asking, right?

I am so confused lol. Could it be a bad connector?

No continuity on the IAC valve.

 

[jcook100]

Also, @RonD can you confirm that the 8-11v I should be seeing at the IAC valve with the engine running is with the red lead touching the red wire and the black lead touching the other wire (with it plugged in)? When I do this I see ~5v.

 

[RonD]

11-12volt key on engine OFF................computer is holding open the IAC Valve all the way for START UP

Engine running 7-8volts at 700-800rpm warm engine, a little higher for cold start and higher idle
There is no set/comparable voltage, i.e. 7v = 700 RPM doesn't work that way
The computer pulses the ground to to change voltage at the IAC solenoid, to set a "target idle" which it Learns on the fly
Computer knows the RPM from crank sensor signal, and in its memory it has "target" idles based on engine/coolant temp, it pulses the ground to open and close IAC Valve solenoid to match "target" RPM, and accurate +/- 4rpm

5v engine running would mean the computer has reduced the voltage to closed the IAC Valve all the way to STOP a higher idle
If engine RPMs are still high with 5v then either IAC Valve is not closing or issue is not related to IAC Valve

 

[jcook100]

Well, I have an update, but not much good news.

After working on this issue for months, I finally decided to take it to a mechanic in the area who has a lot of experience working on older cards. Long story short - he checked everything he could think of and spent many hours investigating, but was only able to drop the high idle by adding a new IAC valve (note that I tried two different brand new ones before taking it to him). This did seem to drop the idle moderately (1500-2000 down to 1200, so still high), but the "surging" or "spiking" idle issue while the car is in neutral still exists. The mechanic confirmed that all sensors seem to be functioning properly.

Please see this Youtube video to see the idle spiking:

The issue occurs when the car is in neutral, like when I am coasting to a stop light. It doesn't matter whether or not I have the clutch depressed. The engine basically revs from about 800 rpm up to 1200 rpm then back down to 800, and repeats about once per second until I come to a stop.

To make things more complicated: it is important to note that the tachometer does have issues and isn't always accurate, which was confirmed by the mechanic. I do not think this negates the surging issue, since I can literally hear the engine and transmission revving when it happens (it is hard to hear in the video). But this does make things more difficult, since we cannot be super confident in the tachometer readings.

A couple of thoughts since taking it in:

One of the mechanics thought that the truck may have had the transmission swapped from automatic to manual in the past, although the stock sticker inside the driver's door does seem to indicate that it was originally manual (see pic).

I do know that the steering column was replaced before I bought it. This is especially evident because I have to use a custom ignition push button on the left side of the steering wheel. This may be why the mechanic thought it used to be automatic.

Any idea why someone would have swapped out the steering column in the past? Could this be the cause of the issue?

Also, is it possible that the issue is related to the transmission? The "surging" sound that I hear seems to be coming in large part from the transmission tunnel. I am wondering if the issue could be some feedback problem between the engine and transmission.