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Old 08-10-2018, 09:17 PM   #1
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How to fix misadjusted idle screw?

I have a 2002 Focus SPI. When I first got it I was unable to keep it running so I messed with the idle screw to keep it from stalling out. I have since fixed all of the problems keeping it from running and tried to put the idle screw back to its correct position but some of the mark I put on the screw came off so I'm not sure if I have it correct. It idles ok but it seems to lunge forward a bit when put into gear and seems to coast a little too quickly .

I have FORScan set up. Is there a way to get it exact?


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Old 08-10-2018, 10:37 PM   #2
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https://youtu.be/ayjhL9jpFH8

Basically you get your TPS voltage down around or below 1v with the IAC unplugged, engine warmed up, and set your base idle with the screw to ~475 rpms.

Last edited by FordNLNR; 08-10-2018 at 10:40 PM.
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Old 08-11-2018, 01:25 AM   #3
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Too low, the IAC will be toward one end of its' range and then takes too long to respond leading to problems. Also, being at the end means you run out of bumpup adjustment say with a/c switched on. It'll work but not optimum.

Don't even need software. Warm engine up and disconnect the IAC harness and then adjust the idle speed to say 650-700 rpm then plug the IAC back in. That will have the TB butterfly set a bit low, the IAC then comes on to adjust slightly upwards (750ish) and still plenty of bump room for it at say a/c coming on. Older higher mileage engines will love it too.

You want the IAC not working speed slightly lower than with it on to have full IAC control but still be close to the proper centered-in-range position.
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Old 08-11-2018, 04:00 AM   #4
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AMC, you underestimate the range that the IAC has. If you set the idle to 700 with it unplugged you will likely get an error code. The TPS voltage will also be too high. The PCM often desires / commands an idle speed below 700. He is looking to set the absolute minimum base idle. The IAC is always working, and it should be working at 35-45% duty cylce on a warm idle without much electrical draw. If he sets the base idle to 475-500, that's about where it will sit.
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Old 08-11-2018, 05:51 PM   #5
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I turned the screw past where I knew it was before and the idle is still high. TPS voltage was about 0.97 before adjusting and 0.94v after. I was able to get it down to about 980rpm but I didn't want to go down further without checking in here first. I have attached a screenshot after plugging the IACV back in. What else could be causing the high idle?
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How to fix misadjusted idle screw?-idle.jpg  

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Old 08-11-2018, 07:17 PM   #6
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Vacuum leak.

To other...........

'The PCM often desires / commands an idle speed below 700.'

I disagree with that. The absolute hot idle speed is set in software whatever it is (MTX may be slightly lower) and the PCM seeks to hit that number except at cold start which bumps up in a ramp based on it as well. There is NO reason to go lower, and it increases emissions to do so. Several other bad things happen at lower rpm as well.

'AMC, you underestimate the range that the IAC has.'

Uh, no. I am fully aware of the wide range it has, in this case you misunderstand the importance of the IAC ranging being kept closely to the software idle speed. It's not nearly so important on newer vehicles but on older ones it helps idle performance immensely as there is no 'older engine' software update provided by Ford. The wide range is actually PART OF the problem.

You will get NO CODE at that close to the idle speed, the only IAC codes there are are for at end limit either way and the IAC limit on the upside has to cover cold idle as well. You have to look at the IAC pintle itself, these have a seek-to-off position of still open quite a bit meaning there is a lot more shutoff to that direction as well, again meaning no limit code that way either. You gotta look at and understand your parts.

The TPS is not settable at all so why you bring that up I do not know. Mine is modded to be settable but most do not have that, the PCM has an algorithm that takes variance into account anyway. A normal correct TPS will be under one volt regardless of where the idle is set if it is in the right range.
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Old 08-11-2018, 11:10 PM   #7
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I agree, vacuum leak most likely. Are your long term fuel trims positive, say past 10%, at idle? Try holding revs while monitoring fuel trims, see if they creep back towards 0. That would indicate a vacuum leak. Try dripping / spraying water around the intake connections and listening for vacuum to pull the water in. You'll want to sort the leak out before dialing in your minimum / base idle speed.
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Old 08-11-2018, 11:24 PM   #8
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AMC, I mentioned TPS signal voltage because it is related to throttle angle. He's changing the closed position throttle angle by turning the stop screw, and thus the signal voltage.

I know how the IAC works. It's a ground-side switched, pulse-width modulated solenoid controlled by the PCM. It's commanded nearly full open when you cycle the key on for second, and then again once cranking is detected. Its duty cycle corresponds with throttle while driving, and at idle it works to hit the commanded RPM by the computer. DSDRPM is the PID if you want to monitor it with live Scan tool data. If you did that, you might see a PCM commanded number below 700, even below 600 at certain times / conditions. On Fords, a duty cycle around 20% is effectively closed. So the 27% we see in the above screenshot makes sense. I think he's got a vacuum leak somewhere.

Last edited by FordNLNR; 08-11-2018 at 11:28 PM.
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Old 08-12-2018, 10:02 AM   #9
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Quote:
Originally Posted by FordNLNR View Post
I agree, vacuum leak most likely. Are your long term fuel trims positive, say past 10%, at idle? Try holding revs while monitoring fuel trims, see if they creep back towards 0. That would indicate a vacuum leak. Try dripping / spraying water around the intake connections and listening for vacuum to pull the water in. You'll want to sort the leak out before dialing in your minimum / base idle speed.
I'll have to look into the fuel trims later today. I'm assuming I can do that with Forscan (haven't tried before). I thought with vacuum leaks idle speed usually went down? I did a makeshift smoke test before through the valve cover vent but never saw any smoke come out anywhere . I can try the water test. Are there any common leak locations on these SPI engines to check first?

I don't know the cars history but I'm pretty sure at least the head was swapped in the past. I'm wondering if they didn't reassemble something correctly.
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Old 08-12-2018, 02:22 PM   #10
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I'm not familiar enough with the specifics on that engine but yes generally, it's a MAP system that will idle higher with a leak, not a MAF system. Yes, you can monitor STFT1 and LTFT1 with ForScan.

So, IAC unplugged your idle is 980, and then plugging it in jumped it to 1238? Have you taken the IAC off to inspect clean it? Perhaps it isn't fully closing.
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