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How To: IAC Rev Mod (Fixing the hanging RPMs)

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Old 10-13-2011, 02:58 PM   #81
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so i did this mod and it friggin awesome! totaly works and idle was not effected only thing is you WILL have more backfire i had some to begin with so it was a tad more i might end up putting a bigger hole in the copper piece (put 1/4 inch hole) depending how much it bugs me or doesn't overtime.
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Old 10-05-2013, 02:14 PM   #82
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Images not showing up for this or is it just me? Hate to post on something this old but Im trying to do this mod and this is the only write up ive been able to find.
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Old 10-05-2013, 02:50 PM   #83
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Images not showing up for this or is it just me? Hate to post on something this old but Im trying to do this mod and this is the only write up ive been able to find.
Use a 1/2" copper pipe cap as described in this post; it does the same thing and is a lot less hassle than playing around with gaskets and sheet metal plates...
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Old 10-05-2013, 04:55 PM   #84
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My car idle hangs a bit, but I'm hesitant to do this because my car idles kind of low when im at a stop and in gear, at about 550. I imagine that doing this would cause my car to shut off since there would be less air going into the engine?
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Old 10-06-2013, 05:33 AM   #85
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you must adjust the idle screw higher now
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Old 10-06-2013, 08:16 AM   #86
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you must adjust the idle screw higher now
There is no "idle screw"--here is how to adjust the preset for the throttle stop screw...
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Delta Force tuned, Steeda UDPs, Ralco flywheel,
RAM HDX clutch, 3.73s, 262 rwHP/305 lb-ft.

New ride (7/1/2013) 1998 Mercedes SL500-5.0L 32V VVT 326/347 HP/tq


Engineering is, at its base, making what you want from what you have.
At its extreme it is making what you want from what you can get.
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Old 10-06-2013, 08:54 PM   #87
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Jeez I love this forum, I can't wait to try this trick, I'll hit the hardware store for the copper cap on the way to work tomorrow.

Cliff, you're the man, now dump that Mercedes and come back home to Mustang Land where you truly belong.
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Old 10-07-2013, 06:14 AM   #88
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Quote:
There is no "idle screw"--here is how to adjust the preset for the throttle stop screw...
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idle screw...throttle stop screw....same screw dude lol
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Old 10-07-2013, 07:54 AM   #89
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Whatever it is, how does one move that screw since its just threads. I wish it was an idle adjuster because my car idles kinda low while in gear and stopped.
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Old 10-07-2013, 08:19 AM   #90
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Originally Posted by Grey-03-GT View Post
idle screw...throttle stop screw....same screw dude lol
It is not the same thing, as the screw in question's function is not to control or set the idle; that is the PCM's job, using the IAC and ignition timing.

The throttle stop screw sets the position of the throttle plate when "closed" so that a certain fixed amount of air "beeds" passed the throttle plate. Properly set this is about 0.44 lb/m of air¹--not enough air to support idle. The PCM then opens the Idle Air Control valve, ideally to about 35 to 40% of fully open, to maintain the idle speed set in the tune.

In fact a better way to set the stop screw is to monitor the duty cycle of the IAC control signal and adjust the screw until the d/c is in the 35 to 40% range. It is NOT an idle adjustment screw and it should not be used as one.

Idle speed is set in the tune and the PCM will do all it can (again, using the IAC and ignition timing) to maintain that idle speed regardless of the stop screw's setting. Because of this if the stop screw is not properly adjusted all sorts of idle problems can occur.

If too far open too much air will bleed passed the plate and the PCM will not be able to reduce idle speed as needed (engine speed will hang between shifts, backfiring on throttle cut, etc.), even if it closes the IAC; the AFR at idle can go quite lean if far too much air is passing through the TB at closed throttle.

Too little bleed air and the PCM will not be able to add enough air, via the IAC, to increase idle speed when additional engine load is added (such as the AC, generator, p/s pump).

-------------------------------------------------------------
¹ - This value is set in the tune, usually labeled Air Flow Through Throttle Body. The PCM needs to know this value, and the stop screw need to be adjusted so that the actual flow is near this value, so that it can better control AFR at idle where the MAF output is not very precise.
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SOLD! - 2003 GT, UPR X, FRPP 24lb/h, Magnaflow, PP 70mm TB & plenum,
Delta Force tuned, Steeda UDPs, Ralco flywheel,
RAM HDX clutch, 3.73s, 262 rwHP/305 lb-ft.

New ride (7/1/2013) 1998 Mercedes SL500-5.0L 32V VVT 326/347 HP/tq


Engineering is, at its base, making what you want from what you have.
At its extreme it is making what you want from what you can get.
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Old 10-07-2013, 07:09 PM   #91
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Well I just gave this a shot, got the 1/2" cap, put a 9/32" hole in it. It fits in the center section of the IAC inlet hose perfectly.

I disconnected the battery for 10 minutes then started the car. It started totally normally, let it warm up for I think at least 15 minutes and during that time the idle dropped down as expected.

But then problems. When you goose the throttle it still hangs up, probably even more. But then it never settles down to a normal idle, it hunts between 1000 and 1200 or so.

While this is going on you hear a weird buzzing noise. I thought it must be the IAC valve so I put my hand on it but didn't feel anything so I can't say for sure that's where the noise is coming from.

I took the cap out, connected my data logger and restarted and then the car was back to normal.

At idle the IAC parameter sits constant at 33%. If you goose it hard it rises up to 50%, stays there for a couple seconds holding the rpm's up then it drops down to 33% and the car comes back to idle. The car is running great so it doesn't have any vacuum leaks or other problems.

Any idea why the trick isn't working on my car?
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Old 10-07-2013, 07:19 PM   #92
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You need to drive the car normally/somewhat agressively for 25 to 50 miles; "goosing it" from idle accomplishes nothing. The PCM needs to relearn the idle air and virtual "dashpot" trims (and other adaptively learned values) after resetting the KAM...
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SOLD! - 2003 GT, UPR X, FRPP 24lb/h, Magnaflow, PP 70mm TB & plenum,
Delta Force tuned, Steeda UDPs, Ralco flywheel,
RAM HDX clutch, 3.73s, 262 rwHP/305 lb-ft.

New ride (7/1/2013) 1998 Mercedes SL500-5.0L 32V VVT 326/347 HP/tq


Engineering is, at its base, making what you want from what you have.
At its extreme it is making what you want from what you can get.

Last edited by cliffyk; 10-07-2013 at 07:21 PM.
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Old 10-08-2013, 05:50 AM   #93
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In plain english...ok then after the mod I adjusted the" throttle body stop screw" to 0.010" with a Allen wrench in those threads JJ Glo... and it runs and idles just right.
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Old 10-09-2013, 09:51 AM   #94
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Just wanted to say this is a great forum, we new guys need this. I have the same problem with 03gt. Thought maybe the iac was going bad. Think I give this a try seems simple.
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Old 10-09-2013, 10:38 AM   #95
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Just wanted to say this is a great forum, we new guys need this. I have the same problem with 03gt. Thought maybe the iac was going bad. Think I give this a try seems simple.
A couple of points:
  • It is normal for the engine speed to hold high an annoying bit between shifts--this is what the IAC mod is intended to "fix";
  • It is also normal for the idle to hold high while rolling to a stop. This is done to ensure sufficient pressure in the hydro-boost braking system for safe braking--making the IAC mod too restrictive can cause the engine to stall at closed throttle, causing sudden loss of power-assisted braking; this is bad;

The IAC can be fully tested as described here...
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SOLD! - 2003 GT, UPR X, FRPP 24lb/h, Magnaflow, PP 70mm TB & plenum,
Delta Force tuned, Steeda UDPs, Ralco flywheel,
RAM HDX clutch, 3.73s, 262 rwHP/305 lb-ft.

New ride (7/1/2013) 1998 Mercedes SL500-5.0L 32V VVT 326/347 HP/tq


Engineering is, at its base, making what you want from what you have.
At its extreme it is making what you want from what you can get.
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Old 10-09-2013, 11:12 AM   #96
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Ok well this seems normal with my car it seems to run great. I've notice some humming coming from my IAC every since I cleaned it out. I will test it as described tho. I remember when I first bought the car new from ford, it seemed when I engage the clutch my rpm's would be around 1000 now its more like 1300-1500 then come back down to regular around 8-850 or so. Your right losing my brakes would be bad...
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Old 10-09-2013, 11:35 AM   #97
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Ok well this seems normal with my car it seems to run great. I've notice some humming coming from my IAC every since I cleaned it out. I will test it as described tho. I remember when I first bought the car new from ford, it seemed when I engage the clutch my rpm's would be around 1000 now its more like 1300-1500 then come back down to regular around 8-850 or so. Your right losing my brakes would be bad...
You will not completely lose the brakes, just the power assist.

The IAC hums because centering vanes on the valve plunger are worn allowing it to vibrate in the housing--more about that here. In your case it is likely that cleaning it removed some carbon build up that was dampening its motion.

There is no repair for this, the IAC needs to be replaced--in fact Ford makes it clear in the shop manual that the IAC "cannot be cleaned"...

Click the image to open in full size.
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SOLD! - 2003 GT, UPR X, FRPP 24lb/h, Magnaflow, PP 70mm TB & plenum,
Delta Force tuned, Steeda UDPs, Ralco flywheel,
RAM HDX clutch, 3.73s, 262 rwHP/305 lb-ft.

New ride (7/1/2013) 1998 Mercedes SL500-5.0L 32V VVT 326/347 HP/tq


Engineering is, at its base, making what you want from what you have.
At its extreme it is making what you want from what you can get.
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Old 10-09-2013, 11:47 AM   #98
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Man great right up I've always wondered what in the world was going on. Makes total since, good job cliffyk
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Old 10-28-2013, 05:28 PM   #99
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Thumbs up for the 1/2" cap in the IAC tube, worked like a charm.
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Old 10-28-2013, 05:28 PM
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