My brain hurts

[fixing_franklin]

Someone explain to me why I forget to plug in the IAC and my truck sounds healthier, brand new IAC. I am lost for words and I want someone to tell me it's better to leave it unplugged, because when it's plugged in my idle stick like crazy

 

[Dirtman]

You probably have a vacuum leak.

 

[adsm08]

Did you use an aftermarket one, or a Motorcraft one?

A lot of OE IACs have diodes in them to prevent EM feedback from the IAC's motor getting onto the line and messing with the computer, a lot of aftermarket ones don't.

 

[fixing_franklin]

It's a Standard #AC117T, honestly it sounded better with this thing unplugged, I will eventually reconnect it and get my idle properly set, but I can't drive it without it sticking, it's supposed to allow more air into the engine to help with the extra load, turning/ AC but honestly it doesn't sound like it's struggling and I'm questioning why I need it in the first place lol

 

[fixing_franklin]

The idle is sticking, not the IAC

 

[Dirtman]

The engine should barely run or even stall out without the IAC plugged in. If it's running good with the iac unplugged you've got one of two problems. A vacuum leak, or the diesel screw is not adjusted properly. Idle should be 500rpm with no iac.

 

[fixing_franklin]

Right, but I cant adjust the screw correctly without finding the vacuum leak first, so I mean, it can't hurt can it? I'll find the leak but I mean seriously

 

[Dirtman]

Well a vacuum leak acts like the IAC valve. (Letting extra air in to bring the idle speed up) the problem is its unmetered air. Meaning the maf sensor is not registering it and therefore throws off the injector pulse width, which in turn wrecks your air/fuel ratio. Not great for the cats, o2 sensors, fuel efficiency, and performance.

Do a vac leak test and fix it or if none is found set the diesel screw to the correct rpm, tune it to the tps, then plug the iac back in and it should run normally. Provided the issue isnt just a fubar iac in the first place but again, with no iac the truck should idle like garbage so something else must be going on since its idling good without it.

 

[adsm08]

Right, but I cant adjust the screw correctly.

That sentence can stop right there.

You cannot adjust the idle speed on a Ford fuel injected engine without hopelessly messing it up.

There is a way to do, there is a written procedure, and 99.5% of the people on this forum (myself included) do not have access to the tools needed to do it correctly.

 

[fixing_franklin]

That sentence can stop right there.

You cannot adjust the idle speed on a Ford fuel injected engine without hopelessly messing it up.

There is a way to do, there is a written procedure, and 99.5% of the people on this forum (myself included) do not have access to the tools needed to do it correctly.

There is a way you can get it close enough to work properly, you have to get the engine up to temp, disconnect the IAC, adjust the idle down to 500 RPM, turn the engine off, disconnect battery, let it sit for 10 minutes, then plug everything back in, and should reset the computer allowing it to process the adjust a lot faster. The problem I'm having is that no matter what adjustment I make for the anti-diesel screw it doesn't matter because I've got a heavy vacuum leak somewhere and it's causing my idle to stick in neutral.

 

[Dirtman]

That sentence can stop right there.

You cannot adjust the idle speed on a Ford fuel injected engine without hopelessly messing it up.

There is a way to do, there is a written procedure, and 99.5% of the people on this forum (myself included) do not have access to the tools needed to do it correctly.

I was under the impression all you needed was a scanner to read the pid for the tps voltage and idle speed? Adjust to close enough rpm then tune it till the tps voltage reads to the spec for closed throttle. Plug the IAC back in and do an idle relearn.

I did that procedure once on a 5.4 and it ran good as far as I know.

What does the real procedure involve? I assume special ford only computers but do you know what they are doing specifically that makes it impossible for a diyer to do? I'm just curious at this point. Not that I in any way doubt your tech knowledge, just seems dumb the poor guy now has to bring a 20 year old truck to a dealer to adjust a screw.

 

[adsm08]

There is a way you can get it close enough to work properly, you have to get the engine up to temp, disconnect the IAC, adjust the idle down to 500 RPM, turn the engine off, disconnect battery, let it sit for 10 minutes, then plug everything back in, and should reset the computer allowing it to process the adjust a lot faster. The problem I'm having is that no matter what adjustment I make for the anti-diesel screw it doesn't matter because I've got a heavy vacuum leak somewhere and it's causing my idle to stick in neutral.

I was under the impression all you needed was a scanner to read the pid for the tps voltage and idle speed? Adjust to close enough rpm then tune it till the tps voltage reads to the spec for closed throttle. Plug the IAC back in and do an idle relearn.

I did that procedure once on a 5.4 and it ran good as far as I know.

What does the real procedure involve? I assume special ford only computers but do you know what they are doing specifically that makes it impossible for a diyer to do? I'm just curious at this point. Not that I in any way doubt your tech knowledge, just seems dumb the poor guy now has to bring a 20 year old truck to a dealer to adjust a screw.

The actual procedure involves a "calibrated orifice" which for the 2.9 is about .020" that is put in place of the PCV valve during the adjustment. Other than that it is very much as @fixing_franklin said. The idea is that you have to very carefully and precisely control the amount of air coming in for this to work.

The few times I did it on my 2.9 I held my thumb over the PCV hose while making the adjustment, and it worked well enough, but the idle was always a little off after that.

FWIW, the dealers don't have the calibrated orifice tools either, and the shop manual says not to touch the screw, and that if you do throttle replacement is required.

In a pinch you probably could use a scanner to adjust the TPV PID into range.

 

[Dirtman]

A junkyard throttle body that doesn't seemed messed with seems like a better option then. Around here that's about 20 bucks... still need to fix the vac leak for the IAC to function.

 

[fixing_franklin]

The actual procedure involves a "calibrated orifice" which for the 2.9 is about .020" that is put in place of the PCV valve during the adjustment. Other than that it is very much as @fixing_franklin said. The idea is that you have to very carefully and precisely control the amount of air coming in for this to work.

The few times I did it on my 2.9 I held my thumb over the PCV hose while making the adjustment, and it worked well enough, but the idle was always a little off after that.

FWIW, the dealers don't have the calibrated orifice tools either, and the shop manual says not to touch the screw, and that if you do throttle replacement is required.

In a pinch you probably could use a scanner to adjust the TPV PID into range.

Dude I'm gonna be honest, I adjust the **** out of this thing. I wasn't very precise at all, I tried but I got it pretty close last night. Now my idle doesn't stick as much