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Old 03-27-2014, 11:55 AM   #11
Tdubbs1989
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You could always try to make it relearn the idle and see if that helps. Disconnect the battery, wait 5 min, reconnect battery, start car with everything turned off and let it run till cooling fans kick on, then flip on a/c and let it run for at least a minute, then go out and drive it. Kind of a shot in the dark but it may help.
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Old 03-27-2014, 12:27 PM   #12
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Quote:
Originally Posted by Tdubbs1989 View Post
You could always try to make it relearn the idle and see if that helps. Disconnect the battery, wait 5 min, reconnect battery, start car with everything turned off and let it run till cooling fans kick on, then flip on a/c and let it run for at least a minute, then go out and drive it. Kind of a shot in the dark but it may help.
I've always done that when doing work on my car. But ill try it your way cause i would just let the car warm up and wouldn't use a/c till later.
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Old 03-27-2014, 12:39 PM   #13
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Here's a quick link to the thread I found that info in. Ignore the part about the auto trans, but the idle strategy relearn should be the same. Again its a shot in the dark but you said when the a/c is on so worth trying, worst case it doesn't help but we can rule that out as a possible issue.

http://www.focusfanatics.com/forum/showthread.php?t=181786
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Old 04-01-2014, 07:48 PM   #14
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I would look at the ignition coil. I've been having this issue and after replacing wires and plugs and it still doing it (along with misfiring now). Upon close inspection, I have found small cracks in my ignition coil. Waiting on part in the mail to see if this clears up my problem.
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Old 04-01-2014, 10:00 PM   #15
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So Megawar, any update on if your car runs better?
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Old 04-02-2014, 04:04 PM   #16
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What flywheel are you running?

My SVTF does the same thing, has been doing it occasionally for years and 145k miles. My stock clutch failed at 45k miles so I replaced it with a 12 lb Al flywheel. I'm now at 190k miles...

The issue is infrequent and minor so it never bothered me. On the extremely rare occasion it stalls it always fires right back up for me.

Here is my thought process based on my 12 lb flywheel, and why I never let the idle issue bother me over the years. Though to be honest take it with a grain of salt as I look at my ECU as a "black box", meaning I know "what" it does, but not exactly "how" it does it's thing. Where as if Tom can chime in, he actually understands what the ECU is doing and perhaps better explain the issue.

So the stock ECU is programmed to maintain idle RPMs with a certain presumed rotational inertia of the entire rotating assembly, crank, rods, pistons, etc; which includes the stock 24.5 lb flywheel. When a heavy accessory load (AC, alternator, etc) causes a drop in RPMs the ECU adds a small amount of fuel to compensate and maintain the idle RPMs.

By cutting my flywheel weight in half, I have drastically changed the rotational inertia in my engine. So under accessory loading, when RPMs drop, the ECU has no idea my flywheel is only 12 lbs, adds enough fuel to maintain the idle RPMs with a stock rotating assembly and 24.5 lb flywheel. With less rotational inertia of my engine, that now results in a higher RPM then the ECU was expecting for that given small amount of added fuel. Same fuel for less mass = more RPMs than expected.

Now the ECU in my car sees RPMs above the idle set point, so naturally it cuts the fueling a little bit. But this little bit of fuel cut is again based on a stock system with a 24.5 lb flywheel and equivalent rotational inertia. So in reality the small fuel cut is too much fuel cut for my engine. Since my rotational inertia is lower, my RPM's drop faster than a stock engine would. So for the same smaller amount of fuel = More RPM drop than expected. The ECU now sees RPMs drop below the idle set point.

The ECU now says "oh crap, RPMs are down, add fuel" and the process repeats the cycle. It's my thought that by cutting the rotational inertia of my rotating assembly I have inadvertently pushed the ECU control algorithm into a region it wasn't designed to control, and it's "underdamped". Letting this pulsing RPM cycle continue.

Without getting into too much engineering, a properly "damped" electronic control system is in fact very similar to a good suspension on your car. With your spring rates and dampers set properly your car stops bouncing after a bump in less than 1 cycle of up-down. A blown damper makes the suspension system "underdamped" and your car bounces up and down many times after a bump.

ECU control systems can behave in much the same way, by taking away mass that the ECU thought was there I've push the EU past where it was designed to work, kind or like blowing a damper on a suspension.

The very rare stall presumably results from the RPMs dropping faster and farther than the ECU ever expected, the ECU doesn't know what to do and the car stalls. Since the pulsing of the RPMs when this issue happens seems relatively stable, and stalling is very rare, I suspect the 12 lb flywheel is just barely outside of where the ECU was designed to handle. I wonder if perhaps an 18 lb flywheel might let the ECU behave normally under heavy accessory loading (more of a compromise between 24.5 lb stock and 12 lb aftermarket AL).

Of course I LOVE the rev happy nature of my engine with the 12 lb flywheel, so I'll gladly take this occasional idle RPM bouncing as penance for making my engine more fun to drive (no free lunch so to speak).
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Old 04-02-2014, 05:16 PM   #17
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I've been having a similar issue. I went out to the car this morning and pulled the coil. There's oil leaking out of the bottom of the coil=bad coil.

I am at the dealership now with a new coil in hand.

I will post the result tonight.
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Old 04-02-2014, 06:27 PM   #18
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Quote:
Originally Posted by Tdubbs1989 View Post
So Megawar, any update on if your car runs better?
No update yet... I've yet to try the reset. Been super busy lately and haven't found the time. I'm going to tomorrow for sure. I'll let ya know what happens.

Quote:
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What flywheel are you running?
I'm running stock flywheel. Thanks for all that info though! Helped give me a better understanding how the ECU works.

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Originally Posted by freemind View Post
I've been having a similar issue. I went out to the car this morning and pulled the coil. There's oil leaking out of the bottom of the coil=bad coil.

I am at the dealership now with a new coil in hand.

I will post the result tonight.
O man... That blows. Please let me know if anything changes. I'm gonna take a look at mine tomorrow also. Was it easy to pull the coil out?
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Old 04-02-2014, 07:10 PM   #19
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It's super easy to pull off.
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Old 04-03-2014, 09:54 AM   #20
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I went to check the part number on my coil and it had: 988F-12029-AC
I havent checked if it was bad, but was gonna look up prices if i do need to get another one.
I went to Tousley's website to search the part... results came up as "Part Number Not Found"!
Did the part number change? What part number do i need for 2003 SVTF?
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