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

[jcook100]

I have been trying to troubleshoot a high idle. It idles around 1600 rpm. I replaced the IAC valve (so it is brand new) and the PCV valve, but the issue persists unless I unplug the IAC valve. When I unplug it, the rpm's drop to about 800-900.

Any idea what is going on?

 

[rubydist]

The new iac has a different performance curve than the old one. It can take up to a couple of weeks for that to resolve itself. It is likely to go faster if you disconnect the battery so you reset all of the pcm functions to factory, because then it seems to go through the learning curves faster.

 

[Broosedamoose]

Just because it's a "NEW" part doesn't mean it's a "GOOD" part. I've taken back several "NEW" parts that were no good!

 

[RonD]

Assuming 1991 4.0l Ranger
Manual or automatic?
They have different idle parameters

IAC Valve needs to be a true solenoid valve, so Motorcraft or Hitachi brand, other are hybrid solenoid/stepper valves

In any case, unplugging the IAC Valve and having the RPM drop is what should happen
But RPM should drop to 500 or so, barely running

There is an Anti-diesel screw on the throttle linkage, looks like an "idle screw" but fuel injected engines can't use an idle screw so its an Anti-diesel screw, lol
With engine warm up and idling, unplug IAC Valve
RPMs should drop to 500 or so
If not then slowly turn Anti-diesel screw counter clockwise(unscrew) to see if RPMs drop more, if so keep turning screw until engine is barely running
If turning screw DOES NOT lower RPMs then turn it back to where it was, you have an air leak somewhere, vacuum leak

Leave IAC Valve unplugged
Remove 1 vacuum hose at a time and plug its intake port with your finger
If idle drops you found the leak
If idle doesn't drop then plug hose back in and try next hose
Start with power brake and PCV hose

Could be a bad gasket but eliminate the obvious first


Computer is setting the high idle if it happens only with IAC Valve plugged in
This could be faulty or wrong brand IAC Valve or could be faulty sensor
ECT sensor tells computer Coolant Temp(nothing to do with dash temp gauge)
If Computer sees Cold coolant it sets a high idle

 

[jcook100]

Hi @RonD , thanks for the help!

Yes, it's a 1991 4.0l Ranger. It has a manual transmission.

I just looked up the order and the new IAC valve that I installed was produced by WVE. Could that really do it? Should I go ahead and order a Motorcraft one?

Yesterday I unplugged the IAC valve and the rpm's dropped from 1600 to about 900. I read that unplugging the battery could reset the computer, so I left the battery unplugged overnight. Today when I tried it, the rpm's were still high upon start- about 1600, but dropped to about 600 this time when I unplugged the IAC valve. It was just barely running as you suggest.

I should also note - I replaced the intake manifold gasket and others within the past year or so. That doesn't necessarily mean that it isn't the issue, but I am pretty confident that it is not.

I already tried plugging the vacuum hoses to check for leaks, but I did it with the IAC valve plugged in. I will try with it unplugged.

 

[RonD]

Good that the warm idle dropped to 600 with IAC Valve unplugged that would be correct

IAC Valve is a solenoid, it gets 12volts on its red wire with key on
The other wire is the Ground and it goes to the computer
Computer Pulses the Ground wire to lower the voltage at the solenoid, called PWM(pulse width modulation), used by most car makers when they need to vary voltage for anything, and they use the Ground side not the 12v side because its safer
Its a vary accurate system, +/- 4rpm

Key on and computer opens IAC Valve all the way about 11volts
After start up Computer will start to lower the voltage to set "target idle", i.e. if engine is cold target idle will be higher, there is a table in computer memory that has target idles based on temp of coolant(ECT sensor)

At warm idle you would see about 7 volts, you can test voltage with IAC Valve plugged in
If Volts are at 6 to 8 and idle is still over 1,000rpm then IAC Valve is most like the culprit
If volts are still 11 or so then its a computer issue

Computer should set a code if idle is too high, or too low, after it has lowered voltage an IAC Valve doesn't respond

In 1991 its most likely a 2 digit code
12 Idle Speed Control motor or Air Bypass not controlling idle properly (generally idle too low)
13 Idle Speed Control motor or Air Bypass not controlling idle properly (generally idle too high)

A Stepper motor is a digital motor, many car makers use these as IAC valve or EGR valve
You send these a specific voltage and they turn to a specific position, moving "the valve" in or out

A Solenoid valve uses magnet and spring
Spring holds the valve closed
As voltage is applied to a coil of wire wrapped around metal core in the solenoid it becomes an electro-magnet
The higher the voltage the higher the magnetic force
This pulls the valve open against the spring pressure holding it closed
(relays are the same but they just get full 12v to "close the relay", no volts and spring pushes relay open)

Problem with Hybrid solenoid/stepper IAC valves is that they can misinterpret the voltage coming from the Ford computer as it pulse the ground wire
These can work just fine with a Ford computer but NOT 100% of the time and not with 100% of the Ford computers, so............

 

[jcook100]

How do I test the voltage of the IAC Valve ground with it plugged in?

 

[RonD]

If your volt meter probes don't fit in backside of IAC connector then get 2 sewing needles/pins and pierce each wire, far enough apart so the needles can't touch, lol
Then use the probes on the needles

You can buy needle tip probes, and they work great........until tip breaks, lol
I have a box of needles and use pliers to push them in

But if you are testing a bunch of wires in a harness then the needle tip probes work well

 

[jcook100]

But how specifically do I test them?

Do I touch the red lead to the red wire and the black to the black then test for voltage?

Or do I touch the black lead to the negative battery terminal then the red lead to each of the two wires (one at a time) to test for voltage?

Thanks so much - I realize I am a total novice here lol

 

[RonD]

Yes, red lead to red wire on IAC and black lead to other wire on IAC
DC Voltage

 

[jcook100]

Okay, I tested the voltage and saw 5.5v right when I started it. It then dropped slowly - 5.1, then 4.7, then 4.3 - over the next 10 minutes or so as the engine warmed up. The whole time RPM's stayed high and constant at ~1500. Note that it was cold out -- about 28 degrees F.

Do those voltages seem correct? If so, this would point to the IAC valve being faulty, correct?

 

[RonD]

Yes, new IAC should be closing with those voltages, so RPM dropping
Try it with old IAC Valve, assuming old one was Motorcraft

 

[jcook100]

Thanks. I'm afraid I chucked the old one when I put on the new one (smacks forehead).... I'll order a new Motorcraft one and see what happens.

 

[RonD]

Just to confirm, when you unplug the new IAC Valve engine RPMs still drop to 600, so its valve can move when power is cut, just not moving when it has any power

 

[jcook100]

Well, shoot. Now it only drops to about 900 when I unplug it. So yes, it does drop, but not all the way down to 600.