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Discussion Starter #1 (Edited)
A few months of observations with an UltraGauge and some (apparent) ECU adaptive learning that was adverse for MPG led me to experiment with deleting the IAC (idle air control) valve today. The result is a stunning increase in MPG.

The delete was made by cutting a block-off gasket to go under IAC valve unit, and reinstalling the unit. This prevents the unit from affecting airflow, and allows it to still be connected to the wiring harness (to avoid the CEL message that would be generated if it were simply disconnected and the passages covered). Also, the base-adjustment idle screw on the throttle body needs to be un-epoxied, and preferably replaced with a more accessible machine screw, with lock nut. With the IAC deleted, the idle speed is controlled primarily by the base-adjustment screw; with the IAC the ECU programming gets in the way.

My test result after several before and after tests is an increase from 41 to 49 MPG (plus/minus a little on both), on six mile runs on a rural road with modest traffic and opportunities for pulse and glide. So, nearly a 20% increase. Speeds were generally 40-55 MPH, with short stretches of 30-35.

The factors involved here are the idle speed (and consequent idle GPH), how fast the idle drops to its lowest speed, and the coasting speed.

With IAC my idle was in the range of 1700 RPM at 0.48 GPH to 2000 RPM at 0.54 RPM. Coasting in neutral at 45 MPH, this means a coasting mileage of 83 to 94 MPG.

With the IAC deleted, my coasting idle is 700-850 RPM (varying) at 0.23 GPH, which at the example 45 MPH speed is 196 MPG. The example speed is a mid-point of a typical glide from 50 MPH down to 40 MPH (speed limit is 45), over which the range is 217 declining to 174 MPG. The key to taking advantage of this is not only the low idle RPM, but getting down to that RPM immediately. With the IAC valve deleted, the RPMs drop like a rock when you come off the gas pedal, just like in older carbureted cars.

So, while a 20% gain in MPG from this simple modification may at first seem to you to be a fantasy, I think you can see from these stats why this is a real number, based on this style of driving in light to moderate traffic. (I gain on a car at 55, then fall back; and I take advantage of terrain, slow at a crest and gaining speed on a down grade.)

I have noted many threads on this and other websites about "hanging idle" (i.e. a long delay, 10 seconds or so, before the revs decline at a slow rate) and a too-high idle on the Focus. This seems to be a good solution.

I set the idle for 730 RPM (low end of the recommended range that I saw somewhere). When parked, the idle is fairly stable; 715 to 780. When coasting however, the range is more like 650-850 RPM, with some rapid hunting going on. So, the ECU is trying to do something when the car is rolling, but the GPH is stable nonetheless.

One thing that helps with my low idle setting is the simple "$0 Thunder Throttle" modification (http://www.focusfanatics.com/forum/showthread.php?t=186991), because it makes the throttle response quicker and smoother.

Thoughts and advice on this IAC delete would be appreciated. When the car is rolling, what do you think the ECU may be trying to do, when it doesn't get the response it expects from the IAC?
 

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Manual or automatic? You didn't happen to specify.

My '04 duratec manual doesn't have an appreciable idle hang, it's primarily noticeable when slowing down in gear and it takes a sec. or two before it starts slowing down faster. How long did yours hang at that high idle speed? (I'll have to check & time it for a drop to neutral, don't usually do that & don't remember anything odd when tried before)

Cold start high rpm might be interesting to handle without it being automatic.
 

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Discussion Starter #3
It is a manual, 2005 ZX3.

> How long did yours hang at that high idle speed?

It seemed that it would hang there forever as long as the car was rolling. When I coast to a stop and put then put the foot brake on, it would hang for 5-6 seconds, then slowly drop to 1100-1200, and finally drop sharply to 850 (and 0.30 GPH).

Ambient temp is mid-30s. In summer (ambient = 60s) the idle GPH would drop to 0.19 to 0.21, but with some delay to get there (but I don't recall the RPMs).

ADDED EDIT: Also, the car has no A/C.
 

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my 05 2.3 does the same it seems, hangs a bit when I come to a stoplight and seems to take longer than necessary to drop back down. Between shifts when going through the gears as well.
 

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All that hanging can be adjusted through a tune , with out the IACV and you use the heat , A/C , pwr steering your IACV adds idle needed to hit the idle target

Your better off with the IACV

Tom
 

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Discussion Starter #6
Tom, how fast can you make the idle drop, when foot is taken off the pedal? Also, can you get me down to a smooth, low idle at 0.19 to 0.21 GPH at an ambient temperature in the low 30s (F)?

EDIT: (Added the GPH, above.)
 

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I don't have any hang idle at all,,,,,,
 

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My idle doesn't necessarily "hang" but my D20 sure is slow on the RPM drop and will kind of hang around 1300rpm when I shift to neutral to coast to a stop. I just assumed its because of no DFCO and a heavy flywheel holding momentum. If these engines do have DFCO my scangauge isn't picking it up. I've also noticed that my engine doesn't compression brake very well, another bit of evidence (I think) for no DFCO and reason why RPM tend to "hang".
 

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Is this motor stock?
 

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Tom, how fast can you make the idle drop, when foot is taken off the pedal? Also, can you get me down to a smooth, low idle at 0.19 to 0.21 GPH at an ambient temperature in the low 30s (F)?

EDIT: (Added the GPH, above.)
I can make the idle drop so fast it will still , not what you want to happen but it shows how fast the tune can make it happen

Idle can be set to what ever you want , 750-800 is about as low as I would recommend going but some want less

Tom
 

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My idle doesn't necessarily "hang" but my D20 sure is slow on the RPM drop and will kind of hang around 1300rpm when I shift to neutral to coast to a stop. I just assumed its because of no DFCO and a heavy flywheel holding momentum. If these engines do have DFCO my scangauge isn't picking it up. I've also noticed that my engine doesn't compression brake very well, another bit of evidence (I think) for no DFCO and reason why RPM tend to "hang".
All adjustable with a tune and a very common problem , if you have a TB added and dont have the idle screw properly adjusted then that adds to the problem

Tom
 

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All adjustable with a tune and a very common problem , if you have a TB added and dont have the idle screw properly adjusted then that adds to the problem

Tom
Little confused by your statement, Tom. Of course I have a throttlebody. My car is relatively unmolested but not sure if the PO did anything under the hood since I don't have a stock engine to compare to.
 

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I ment after market TB , if you add a aftermarket TB then you need to properly adjust the idle screw and most people dont do it right or at all

The idle screw on the TB isnt what sets the idle

Tom
 

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Discussion Starter #15
I can make the idle drop so fast it will still , not what you want to happen but it shows how fast the tune can make it happen

Idle can be set to what ever you want , 750-800 is about as low as I would recommend going but some want less

Tom
Not sure what you mean by the first sentence. Do you mean "stall"?

Regarding desired idle speed. If you were to set it for 750-800, how long would it take the car to settle into those low revs from a cold start. How much would warm or cold weather affect how long that takes?

(And thanks for your help on this topic!)
 

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Discussion Starter #16
A further question. For this kind of tune, should the ECU's adaptive learning feature be turned off? I'm thinking that if it is left "on," maybe over time that would muck up the idle-drop and idle settings you would make with your tune.
 

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Discussion Starter #17
... The idle screw on the TB isnt what sets the idle

Tom
To be clear, this is true when the IAC valve is operative. With my modification (IAC valve deleted), the idle screw is the only control for the idle.

FURTHER PERFORMANCE REPORT: On a cold start this morning the idle was too low (~650 instead of my warm ~730 setting the other day -- a bit ragged and so quiet I couldn't tell whether it was running or stalled). So I upped the idle a bit. After some driving that gave a thorough warm-up, I ran the same test in the same place and appx. the same traffic and ambient conditions. This setting dropped my MPG from the previous ~49 to 47.3. That's still an impressive 15% gain in MPG. The UltraGauge is still showing 0.23 GPH at idle, but the prior and current readings must be very nearly 0.01 different, given the slight drop in MPG (and there could be some driving variability, since I ran only one test today).

By the way, my car has new snow tires (no studs) and the gas is winter-grade.
 

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Not sure what you mean by the first sentence. Do you mean "stall"?

Regarding desired idle speed. If you were to set it for 750-800, how long would it take the car to settle into those low revs from a cold start. How much would warm or cold weather affect how long that takes?

(And thanks for your help on this topic!)
I mean is in the tune I get to aggressive with the dashpot in setting how fast it idles down the engine will fall in RPM so fast that it will stall ( engine shut off )

It should drop to the set idle instantly , clutch in the tach should go straight to set idle just like the IACV unhooked

Cold starts , it all depends on what you want , this is where custom tuning comes in vs a canned tune , a custom tune I can adjust your Focus to how you drive and want your Focus to act , I can set the cold start to idle up to what ever idle you want and to hold there for how long you want or as short as you want it to , its all adjustable

Tom
 

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Checked the drop to idle tonight when driving for a reference.

The only "hang" is that I need to take my foot off the gas before clutching, or the RPM's will bump up before dropping to normal idle.
 

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When people ignore problems and come up with something they call a 'fix', then post about the astounding results ... I just laugh.


So you're telling me that under same/similar conditions not having an IACM results in less fuel consumption?
The only way this is possible is if there was a vacuum leak the IACM was compensating for or the IACM was damaged in some way. In other-words, you've covered up and ignored the initial problem while ignoring the evidence that it was ever there.

And now as you've found out, a manual adjusted TB can not compensate for all climactic conditions very well.


But best of luck with your jerry-rigging and misunderstanding.
 
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