Bad IAC Out of the Box??

rangerenthiusiast · May 7, 2018

Sooooo.... I bought a ’92 Ranger (4.0/A4LD) a few months back and I’ve had a racing idle ever since. With the original Motorcraft IAC, the idle would vary between 1100 and !700 RPM, but occasionally dropped down to 900 RPM. I cleaned the IAC, with no change. I unplugged it, which dropped the idle down to a nice 800-900 RPM (which was great!), but after a few days, the computer caught up and started giving me a check engine light and an idle so low that it occasionally felt as though it might stall when starting up from a dead stop.

Bought a brand new Hitachi IAC and installed it. IAC is usually around 1100 now (way too high). I’ve had it on there for several weeks now, so today I disconnected it while the truck was idling, after driving it for about 20 minutes. This time, there was ZERO change to the idle with the IAC disconnected. Nothing. At all.

Does this mean that the new IAC is bad? :icon_confused:

RonD · May 7, 2018

You have a vacuum leak.

With engine fully warmed up and idling, unplug IAC valve, idle WILL drop to 500rpm or engine may even stall, if not then something is wrong.

4.0l w/auto should idle at 750 in Park warm, 800 in gear warm, computer will open IAC Valve a bit more "in gear"
Manual idles at 625 warm

If idle is higher than say 600 with IAC unplugged the first thing to check is the anti-diesel screw, on the throttle linkage, many mistake it for an "idle screw" but we all know you can use an idle screw with fuel injection, thats why IAC Valve needs to be used.

Anyway, with IAC Valve unplugged and idle above 600 turn anti-diesel screw counter clockwise(unscrew) to see if RPMs start to drop, if they do someone has messed with it, keep turning it until RPMs get down to 600, or as low as possible.
If RPMs don't change put screw back were it was

If higher than 700 after adjusting anti-diesel screw then you also have a vacuum leak
Leave IAC Valve unplugged and pull off larger vacuum hoses, one at a time, from upper intake vacuum manifold, plug port in manifold with your finger to see if RPMs drop, if not replace hose and move to next hose.
If idle drops then you found the hose/device with the leak.

A stuck open PCV Valve is a vacuum leak, when was yours changed last?
Should change it every 2 or 3 oil changes
At idle, high intake vacuum sucks PCV Valve closed, if its stuck open..............vacuum leak

rangerenthiusiast · May 8, 2018

Hey, RonD! Thanks so much once again for the great step-by-step diagnostic instructions! The first day that I have off from work, I’ll be sure to check those things out.

One question, though. When the old Motorcraft IAC was on there, unplugging it would cause the idle to drop from 11-1700 RPMs, down to about 700-800 RPMs. After a few days of driving, it even sounded like it might stall with the IAC unplugged. However, unplugging this brand new Hitachi IAC with the truck running does absolutely nothing at all. It’s only been on for a couple of weeks, so I doubt that anything else has changed. Does this mean that the new IAC is bad?

Much appreciated,

RE

RonD · May 8, 2018

Could be bad IAC Valve

Take it off the intake and plug its wires in
Turn on the key
Computer will open IAC Valve all the way for starting, key on engine RPMs at 0
So IAC Valve should be open
Unplug the wires, IAC Valve should close all the way
Plug wires back in and it should open all the way
Repeat as you see fit

Volt meter can tell you if the 2 wires have Ground and 12volt key on
The computer Pulses the GROUND wire to vary the voltage at the IAC Valve(open and close) when engine is running, 12volt wire is always 12v with key on

rangerenthiusiast · May 8, 2018

RonD said:
Could be bad IAC Valve

Take it off the intake and plug its wires in
Turn on the key
Computer will open IAC Valve all the way for starting, key on engine RPMs at 0
So IAC Valve should be open
Unplug the wires, IAC Valve should close all the way
Plug wires back in and it should open all the way
Repeat as you see fit

Volt meter can tell you if the 2 wires have Ground and 12volt key on
The computer Pulses the GROUND wire to vary the voltage at the IAC Valve(open and close) when engine is running, 12volt wire is always 12v with key on

RonD, thank you SO much for this, bud! I will definitely try that on Friday when I have the day off and report back! :icon_thumby:

Craig0320 · May 8, 2018

Also when you unplug the iac the engine should idle down to barely running. If it just stays the same or barely drops then you most likely have a vacuum leak. Normally a iac is open 15% or so at its closed position on a handheld. This is for the pcm to maintain desired idle speed. My pcv valve went bad and was letting the intake suck un-metered air from the crankcase causing my high idle. Replaced my pcv with a motorcraft one and it has ran perfect ever since.
Just my 2 cents.

rangerenthiusiast · May 30, 2018

RonD said:
Could be bad IAC Valve

Take it off the intake and plug its wires in
Turn on the key
Computer will open IAC Valve all the way for starting, key on engine RPMs at 0
So IAC Valve should be open
Unplug the wires, IAC Valve should close all the way
Plug wires back in and it should open all the way
Repeat as you see fit

Volt meter can tell you if the 2 wires have Ground and 12volt key on
The computer Pulses the GROUND wire to vary the voltage at the IAC Valve(open and close) when engine is running, 12volt wire is always 12v with key on

Hey, Ron. Finally got the chance to do as you recommended today. My freakishly long arms finally came in handy, as I was able to cycle the key and watch the IAC at the same time. So there was a little cone-shaped “diaghram” right about in the middle of the shaft inside the IAC. When the key was turned on, the cone would rise up a bit (opening?), then go back down after a second (thus closing?). I put the old Motorcraft one back on and got exactly the same result. So I think both the new and the old are actually working correctly.

I couldn’t really comprehend the part about using the voltmeter though. I have one, but am still kind of weak on using it. Any chance you could walk me through it?

rangerenthiusiast · May 30, 2018

Craig0320 said:
Also when you unplug the iac the engine should idle down to barely running. If it just stays the same or barely drops then you most likely have a vacuum leak. Normally a iac is open 15% or so at its closed position on a handheld. This is for the pcm to maintain desired idle speed. My pcv valve went bad and was letting the intake suck un-metered air from the crankcase causing my high idle. Replaced my pcv with a motorcraft one and it has ran perfect ever since.
Just my 2 cents.

I definitely plan to replace the PCV soon, since it’s a cheap thing to try. The old one is a Motorcraft and looks, well, old. The only reason I didn’t change it before is because it does rattle when shaken, which is supposed to mean that it works. Then again, the guts of it look kind of loose inside, so who knows? I’ll give it a shot.

In regard to vacuum leaks, I pulled a bunch of lines off of what I refer to as the vacuum block (which sticks out from the rear drivers’ side of the air intake plenum and has several vacuum hoses connected to it). Each caused the idle to drop when I disconnected them, except the one running to the fuel pressure regulator, which caused the idle to gradually RISE from 1100 RPM to about 1500 RPM before topping out. Capping the port caused it to drop back down. Does this mean that my fuel pressure regulator is bad? Would that cause a racing idle?

Thanks, folks! :icon_thumby:

rangerenthiusiast · May 30, 2018

RonD said:
You have a vacuum leak.

With engine fully warmed up and idling, unplug IAC valve, idle WILL drop to 500rpm or engine may even stall, if not then something is wrong.

4.0l w/auto should idle at 750 in Park warm, 800 in gear warm, computer will open IAC Valve a bit more "in gear"
Manual idles at 625 warm

If idle is higher than say 600 with IAC unplugged the first thing to check is the anti-diesel screw, on the throttle linkage, many mistake it for an "idle screw" but we all know you can use an idle screw with fuel injection, thats why IAC Valve needs to be used.

Anyway, with IAC Valve unplugged and idle above 600 turn anti-diesel screw counter clockwise(unscrew) to see if RPMs start to drop, if they do someone has messed with it, keep turning it until RPMs get down to 600, or as low as possible.
If RPMs don't change put screw back were it was

If higher than 700 after adjusting anti-diesel screw then you also have a vacuum leak
Leave IAC Valve unplugged and pull off larger vacuum hoses, one at a time, from upper intake vacuum manifold, plug port in manifold with your finger to see if RPMs drop, if not replace hose and move to next hose.
If idle drops then you found the hose/device with the leak.

A stuck open PCV Valve is a vacuum leak, when was yours changed last?
Should change it every 2 or 3 oil changes
At idle, high intake vacuum sucks PCV Valve closed, if its stuck open..............vacuum leak

Thanks, Ron. Yeah, idle is usually around 1100 RPMs. ZERO change when the new IAC is unplugged. Nada.

I think I found what you are referring to as the anti-diesel screw, but it appears to be turned all the way out. If I turn it in, it will just cause the idle to race higher, the same as if you were to grab the throttle linkage and give it a twist. So I really don’t think that’s it. Per Craig’s post above, going to try the PCV. Any idea if my fuel pressure regulator might be bad, based on the fact that the idle races higher when I unhook it?

Thanks. :icon_thumby:

Craig0320 · May 31, 2018

Just because the pcv rattles does not mean its good. My bad one rattled just fine but when you turn it over and look inside all the guts were just scattered around in there. Usually when a fpr is bad it sill be leaking gas out of the port the vacuum hose plugs into in the top.

rangerenthiusiast · May 31, 2018

Craig0320 said:
Just because the pcv rattles does not mean its good. My bad one rattled just fine but when you turn it over and look inside all the guts were just scattered around in there. Usually when a fpr is bad it sill be leaking gas out of the port the vacuum hose plugs into in the top.

Okay, I’ll get a new PCV. Might just as well.

So it’s normal that disconnecting the vacuum line that runs to the FPR causes the idle to race, even though disconnecting other vacuum lines causes the idle to drop? :icon_confused:

Thanks.

EDIT - I just remembered why I never replaced the PCV. When I try to look it up on the websites for Advance, etc, the PCVs offered look nothing like the Motorcraft PCV I have on my truck. I found a Motorcraft one on Rock Auto, which looks like this: https://drive.google.com/open?id=1GJxVU_7bQeFOPmbM7LMFiVvX0loyCHzI

The one I have on my truck looks identical to the white part, but does not have the black attachment with the two vacuum ports coming off of it. Instead, a rubber hose connects the top of the white PCV valve directly to the port on the intake plenum. I have noticed that one of the vacuum ports on the vacuum block (which comes off the back of the plenum) has no line going to it and was capped by the PO. So I’m thinking that the PO did away with the black plastic piece and vacuum hose for some stupid reason, which could be causing some of my idle problems. Sound feasible?

Craig0320 · May 31, 2018

Hook it back up and give a try. Couldn't hurt.

Bad IAC Out of the Box??

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