I am pretty sure the hard stop screw, the screw that can be adjusted to hold the throttle open just a smidge, was set at the factory to ~500 rpm.(vintage dependent?) The IAC is supposed to keep the idle above that, mine ~750 hot in neutral(manual trans). When the IAC is turned off, or actually the key to OFF, the IAC closes, and the engine should stop. Back when, if the idle was set by the 'hard stop', it could allow fuel to get past the throttle plates after the key was turned to OFF, and allow the engine to 'diesel'. More or less, glowing carbon particles would fire off an incoming fuel:air mix, and keep the engine turning. To prevent this, and the emission of the other non-firing cylinders of unburnt fuel:air mix, the IAC was used. It also allowed for better idle control for variable loads such as A/C, power steering, and the load of the automatic transmission in D or R, drive gears.
So, the hard stop was, if equipped, essentially set to 'not gonna run' speed. It keeps the butterfly from mashing into the walls of the throttle body and getting jammed, and gives a tiny bit of cushion when you take your foot completely off the gas pedal and allow the throttle to slam completely closed.
IOW, it is not generally used to set any sort of idle speed. FoMoCo did have a TSB/errata where they were allowed to bump the hard stop to force idle speed of ~900 in CA for the EFI models of at least 1985. I have the sticker to prove it, as it would not pass the first annual SMOG test (emissions) at 3k miles, and there was some sort of exemption.
Enough?
tom

Thanks, Tom!
If it's generally set to not drop below 500 RPM, then I guess mine is fine and I don't need to make any adjustments. I won't worry about it then.

Since replacing my throtte position sensor, I have no more low hanging idles. Any drop to 600 RPM is only momentary before being bumped up to 850 to 900...which pleases me immensely.

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What was your TPS voltage at idle before and after replacement? I know these trucks set idle voltage every time the ignition is turned on but I’m wondering if there is a voltage that still messes it up.

Mine is 1.3 and that seems high to me.


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Automotive Electronics Engineer
2000 XLT 2.5L A/T
Mods:
Explorer OHC
Explorer Autodim Mirrors w/ Autolamps
Explorer Sport White Face Gauges

Sorry, I don't have a working multimeter, so I don't know the voltage.

I just did a bunch of research about the causes of low idling, and narrowed it down to TPS as most likely culprit. I took a chance & got lucky.

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The older(oldest?) TPS were adjustable to set the 'fully closed' position. They had 'arcs' at the mounting bolts so you could turn the whole thing when it was loosely installed on the throttle body. I do not remember the volts at either end of the sweep.
When I take my foot of the gas, coasting to a stop light, I can feel the engine re-apply fuel(?) as it gets down to low rpms(manual). The engine feel will change, from totally coasting to a little bit of fuel being supplied(I think) so the engine will run at idle & stopped.
It's a little disconcerting. But, the point is, the setting of the TPS can/will affect that when you have an adjustable TPS.
I figure over time, the designs changed, and also the fully open, fully closed voltages may have changed, thus making the closed throttle voltage a vintage dependent value.
tom

It's not necessarily because it has been tampered with, its because its a part that is assembled on the line to be within a range that the AISC can control. Remember they made thousands of these, and each one was plastic/metal. The AISC was designed to compensate for this change so they weren't overly concerned with perfection. My Throttle Body has been used long enough it actually "sticks" to the throttle body, and I had to "kick" the pedal so to speak to get it to release. This procedure needs to be done on a fully warmed up engine with the heater controls set to "off" or at least the A/C not running. The AISC motor and all related passages needs to be clean and free of debris to allow a full range of motion.

Essentially what you do is pull a vac line off the engine, preferably a large one like the brake booster, and let it idle up. You'll need to leave it like this for 30 seconds to a minute.

The point of this is to fool the ECU into thinking the idle valve is "open" causing high idle compared to throttle position. I'm not sure the total functionality of the ford unit, but almost all of them do not have a feedback function that states relative position other than full open or full close. Rather it uses RPM/load and throttle position to determine if you are on the gas or if it is in "idle" so its only fix is to close the idle speed passage. And it will continue to do so until it reaches its "end of travel"

So, like stated, pull the vac line, let the truck idle up for a min, then unplug the idle motor. You do this so that the controller doesn't attempt to adjust idle while you do the next bit.

Then, with the booster line still disconnected, turn the idle screw UP at least a few turns to get the throttle body opened enough to keep the truck running. Then pop the line back on, and adjust the screw back down until you hit the idle you prefer. I like mine around 800 or so, but if you really felt like it you could turn it down. I think factory is around 8-900 anyway. I would set it just a hair under what your idle is now with the AISC connected, so that the ECU isn't constantly trying to shut it off.

After you have it set to where you want it to be, then shut the truck off, and plug it back in. You may see a code pop up because it was disconnected while running (essentially a failure state) but I did not experience this at all. The truck should idle up like normal, then drop down to where you have it set. Physically setting the idle via the TB ensures it cannot physically drop any lower than that setting unless you have some other mechanical error.

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This must be a Ford only procedure, as it is a standard procedure for Dodges and other makes. As far as "fuel past the throttle plates", you must be talking about a TBI or carb setup as the only time I've ever had dieseling in an EFI car was when the distributor was too far off.


Idle voltage should be much lower than that, as its a full sweep from .01 V to 4.9 V. They have a range of 0 to 5V and any time it sees either min or max range voltage it assumes a failure state. 1.3 V would be approx 25% so it (ECU) would be under the presumption that you are at 25% throttle.

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2001.5 2.3 213xxx and counting
M501-HD L154 Manual
Don't bother looking for 1st gear synchros, I bought the last one

You must not have read closely, as the comment about 'fuel past the throttle plates' was noted as referring to:
"Back when, if the idle was set by the 'hard stop', it could allow fuel to get past the throttle plates after the key was turned to OFF, and allow the engine to 'diesel'."

Note the first two words...

What is an AISC? That's a new one to me.

tom

Carburetor adjustment is a completely different animal, and is dealt with differently between each manufacture. How you set an Edelbrock up is worlds apart from a Holley, etc.



Air Idle Speed Control

There are abut 8 different names I have heard it called, including IAC (Idle Air Control) and AIS (Air Idle Speed)

As far as the throttle body is concerned, the throttle stop screw is sometimes referred to as a BISS, for Base Idle Set-Screw. But that is usually an actual air pintle screw, much like a carb pintle.

It's really up to the end user how they want their vehicle to perform, I personally would never want my engine to be able to swing low enough in RPM to stall itself out. It's just a recipe for a dangerous situation.

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2001.5 2.3 213xxx and counting
M501-HD L154 Manual
Don't bother looking for 1st gear synchros, I bought the last one

You’re seem to be soley focused on the IAC. Here’s a little bit of a curve ball idea. Check your fuel pressure to see if it’s erratic. See if your rpm dips correspond with dipping fuel pressure. If your fuel pressure is dipping then it could be a burnt up corroded fuel pump connector and/or a dirty ground connection.

Other than that...

- MAF sensor
- TPS sensor
- vacuum leak

Also, I would remove the throttle body, get an aluminum turkey pan from the dollar store and a tooth brush, and a full can of throttle body cleaner and thoroughly clean the throttle body back to factory state. Just because.

Thanks, Turismolover22. But I think I'll not mess with it. I would only want to do it if it has been altered from factory specs in the past, but it doesn't seem to hurt anything with the RPMs drop momentarily to 600 when coming to a stop sometimes, so I'm not going to worry about it.

CP30, thank you for the reply but you should have read through the thread. As stated, my throttle body is clean enough to eat off of...And also as stated, I ended up putting a new TPS in and that solved the hanging low idle problem once and for all.

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Glad to hear it. I should have read the entire thread before I replied.

I watch my tach all the time like you. A rock solid idle is a well running engine. I’m not happy myself when my is idle if off.