Idle Undershoot. I need some clarity.

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Idle Undershoot. I need some clarity.

Post by keeper1016 » Wed Mar 28, 2012 9:56 am

Hey all,

My Neon has the dreaded idle undershoot issue when braking to a stoplight in drive. :angry6:It causes it to shake pretty bad. Putting it in neutral wakes the motor back up and lessens the vibes.

I read a lot on here and elsewhere about fixing this with PCM reflash, drilling the throttle plate, throttle set screw adjust, inducing a filtered vacuum leak, etc, but all of these seem like bandaid fixes or repair of symptoms, not the problem.

I recently read on a couple other forums that the idle undershoot actually is a fuel saving design implemented into the PCM by Chrysler. This would make sense because my car is throwing not one DTC. This would also mean it has always done this, and it's only being noticed now because a possibly worn motor mount which makes the car shake more (or maybe a combo of things).

I've read the owner's manual, and it does not mention idle undershoot. I can't seem to find an instance of it in the Neon FSM.

So, my question is, which is it? A fuel saving design or a problem with the PCM? :scratch: I just want to fix the problem (if it is a problem), or fix the component that makes it noticeable now. I haven't done this TSB Reflash, yet. I've been wanting to learn more about this problem before I take any corrective steps. Sorry for the long post, but I appreciate yall reading it and thank you in advance! Tell me what you think!

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Post by titansxt » Wed Mar 28, 2012 10:07 am

I've driven other Neons that do not have this, they idled much better. If I remember right this problem is mostly with 2003 2.0s. Recently mine has become a lot worse at lights. Pretty much to the point where I think it is about to die and I do something to correct it. Oh, and all my lights go real dim all the time at idle.

I just took out my SRT and realized that even with the solid mounts, it does not vibrate as hard at lights. Now if I have the A/C on, that is a different story. Haha.
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Post by occasional demons » Wed Mar 28, 2012 10:12 am

Fixing the actual problem may be difficult. The geniuses at chrysler haven't been able to fix it for the past 15 years or more....

My V6 Shadow with the ATX would die just about every time you came to a stop with it in gear. That was in 1993.... and two more cylinders.
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Post by keeper1016 » Wed Mar 28, 2012 11:16 am

Thanks for the quick replies! So yall are on the "problem" side. I agree. I think it is a problem, designed in or not. Check out this allpar thread. They talk about it being an intended design around the end of it.

http://www.allpar.com/forums/topic/1126 ... 2002-neon/

My thinking is, I'm not against replacing the car's computer if it will fix it if it's a computer problem. But would a new computer even do anything for it if it's built to do that to the idle? I guess I'll just have to start with getting the PCM flashed and go from there. It would be interesting to change the motor mounts first though and then see if it's even noticeable.

Wow, I didnt know it has been that prevalent for that long. Yeah, I just wish they would say straight out its this or that. Then you at least know what to do to fix it.

Thanks again for yall's responses.

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Post by titansxt » Wed Mar 28, 2012 11:28 am

Don't go get the TSB reflash. I tried that, my car did not need it.
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Post by keeper1016 » Wed Mar 28, 2012 3:29 pm

Yeah, I've read that for some people it works, and others it doesn't. I don't really get that at all. Mine's also exhibiting the RPM loss when the A/C compressor engages while your'e driving at speed. You can hear and feel it. I know that TSB is supposed to correct that too, so I figure it couldnt hurt. I still need to read up on it more though.

My lights dont dim or anything. Your AIC might need a cleaning or something. Thats pretty low!

The flash isnt on the top of my list right now, I have a control arm to replace. Im just trying to get a handle on the :quotes: undershoot :quotes: . I mean, is it meant to do it or not meant to do it?

Thanks!

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Post by Danteneon » Wed Mar 28, 2012 4:08 pm

I was finally able to get rid of mine. 60mm MPX TB with the stop screw adjusted slightly more open. Just enough to open it a little more but not enough to have the TPS read any movement.
If I could just figure out how to meld the Outback and the Neon into one car...

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Post by occasional demons » Wed Mar 28, 2012 4:44 pm

Danteneon wrote: Just enough to open it a little more but not enough to have the TPS read any movement.
Exactly. If you raise the TPS idle voltage, the "in motion" idle goes to about 1400 to 2400 rpm, depending on how much you adjust it.


I doubt a new PCM will help; they have to flash it to your car when installed, so it is going to have w/e (I would think) the latest program is, and like said, it might help, might not.

Now if it was non SKIM, you could try a used PCM of like kind, but no promises there either. But that is only for the 95 to '02 folks...
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Post by keeper1016 » Thu Mar 29, 2012 11:13 am

Okay,
It seems that a guy just has to come to terms with the fact that it's going to do it, and then develop a workaround mentality.

So, the key is to adjust the throttle plate stop screw to open it more WITHOUT the TPS seeing it. Is there an accepted way of doing this? What I mean is, do you count the flats as you turn it? If so, is there a "known" number of flats or turns that get the desired effect? Also, does having the larger throttle body make a huge difference? I would be attempting this on a stock one.

Yeah, unfortunately, I am a SKIMer. I lost one key to the car a while back and only have one now. I suffer wallet paralysis when someone wants that much money for a key.

Thanks for the insights. I know yall feel like your'e beating a dead horse on this subject probably. I thank you all the same!

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Post by occasional demons » Thu Mar 29, 2012 12:34 pm

Wallet paralysis comes with neon ownership. :lol:
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Post by Danteneon » Thu Mar 29, 2012 12:56 pm

I haven't tried this on a stock TB, but by slight, I mean like a 1/16 - 1/8 of a turn.
If I could just figure out how to meld the Outback and the Neon into one car...

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Post by titansxt » Thu Mar 29, 2012 1:19 pm

I thought we came to a consensus that undoing the screw was a bad idea. No?
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Post by Danteneon » Thu Mar 29, 2012 1:44 pm

If it is turned enough for the TPS to read movement, then the PCM will just re-calibrate to bring the idle down. If moved slightly, it won't be enough for the PCM to know any better. It is akin to drilling a small hole in the throttle blade.
If I could just figure out how to meld the Outback and the Neon into one car...

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Post by Donkeypuncher » Thu Mar 29, 2012 11:08 pm

Would it be possible to clock the tps slightly to allow more room for adjustment? That was one of the things the old n/t eclipse guys used to do, but it wasn't to fix idle issues.

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Post by keeper1016 » Fri Mar 30, 2012 7:54 am

I thought we came to a consensus that undoing the screw was a bad idea. No?
Yeah, I agree there. Every manual I've read even my 91 F150's FSM say dont touch those screws. I figure as long as I can scribe a mark someway to make sure I can bring it back to exact factory positioning, I dont see a problem then.
by slight, I mean like a 1/16 - 1/8 of a turn
OK, cool. Thank you. I imagine that would be about a flat, maybe a little less. I like this idea better than drilling the throttle plate.
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Ha, yeah. Among other things. :tongue: Wait, what?
Would it be possible to clock the tps slightly to allow more room for adjustment?
That's an interesting idea. Ive never heard of it. Hopefully I wouldnt find myself needing to do that. I guess it would depend how much play you already have to work with.
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Post by occasional demons » Fri Mar 30, 2012 8:54 am

Reclocking the TPS has the same effect as moving the throttle rest. Too much and the in motion idle goes too high. IDK I tried driving it around for a while, but the PCM never really adjusted the moving idle back to a reasonable rpm.

Plus it would take a lot of effort to make it adjustable as it sits in stock form. Mine is adjustable, because I had to make an adapter to get it to work on the 5.0 TB.

My initial objective to raise the TPS voltage was to hopefully match the 60mm opening a little better. With the moving idle at 1500 -2000 rpm+ it was really hard to tell if it really made a difference. :lol:

I really don't think the rpm has much to do with the vibrations tho. Mine can be idling at 800 and be ratting the dash, and sometimes it doesn't. It even occasionally idles at 500 RPM, and is smooth as glass, even moving the car.

I have come to the conclusion, it is more of a how lean the PCM is making the A/F ratio at idle. Because it will sit there and rattle, then it is like the A/C kicks off and smooths out. RPM never changes on the tach. A/C nor defrost are on, fan can be in the off position.
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Post by Donkeypuncher » Fri Mar 30, 2012 5:15 pm

It's pretty easy to make it adjustable, just drill the out the 2 screw holes a little larger and use some tiny washers on the screws. I don't know what the voltage threshold is at idle and wot though. It may cause more trouble than it's worth, who knows what problems may arise.

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Post by occasional demons » Fri Mar 30, 2012 5:31 pm

The problem you may have with drilling them, is the holes are metal sleeves molded into plastic. If the plastic doesn't hold to the sleeves as they heat up, or the bit catches, it is game over. I would "practice" on a known bad TPS before attempting to drill out the holes on a good one.
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Post by Donkeypuncher » Fri Mar 30, 2012 6:41 pm

Technically you only have to slot the hole in the direction that you want to clock it, I did it on my old avenger. I used a smaller drill bit and used to side of the bit to chew through the metal sleeve inside, if that makes any sense. But yeah, if that metal sleeve breaks loose it's gonna be hard to get back within spec.

It would be a lot of trial and error adjusting the throttle stop and tps to find a sweet spot. I think I'm gonna drill a 2nd hole in the throttle plate since the 1st one seemed to help quite a bit.

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Post by keeper1016 » Sat Apr 07, 2012 12:33 am

Hey all, I just wanted to say thanks again for all the input. I'm gonna be replacing the motor mounts on this beast, and I'll probably wake this thread back up to report back. Just finished replacing the lower control arms. A tight fit to say the least. Anyway, I'm off to ask another electrical question! Thanks!

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Post by gilly02le » Sun Apr 08, 2012 7:45 am

1/8" hole in the throttle plate, simple solution. The pcm won't see any adjustment at the throttle body, so it won't recompensate, and bring back the 550rpm idle in drive. the plate is plastic so drill it slowly so it doesnt crack. Once you are done, no-one will ever be able to tell you've done it, aside from the better idle!


Check rockauto.com for prices on control arms, cheapest i have seen.
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Post by keeper1016 » Mon Apr 09, 2012 1:34 pm

Check rockauto.com for prices on control arms, cheapest i have seen.
Yeah, that's where I bought mine from. I bought the Moog problem solvers. Rock Auto is great.
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Post by Donkeypuncher » Mon Apr 09, 2012 5:30 pm

The 1/8" hole will bump the idle up about 100 rpm, at least that's what mine did. From what I can tell 850-900 is the sweet spot for mine, but it usually sits at 800 so I need just a hair more. I think a 1/16" hole should do the trick, don't want to overdo it.

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Post by Danteneon » Tue Apr 10, 2012 9:11 am

Donkeypuncher wrote:The 1/8" hole will bump the idle up about 100 rpm, at least that's what mine did. From what I can tell 850-900 is the sweet spot for mine, but it usually sits at 800 so I need just a hair more. I think a 1/16" hole should do the trick, don't want to overdo it.
Psst...1/16" is smaller than 1/8". Methinks you meant 3/16"?
If I could just figure out how to meld the Outback and the Neon into one car...

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Post by occasional demons » Tue Apr 10, 2012 9:41 am

I would go to 9/64 or 5/32 before all the way to 3/16.

It helps to get one of those cheap bit sets that go to 1/4 so you have a selection to gradually increase the size. Otherwise you will be plugging the hole with JB weld and starting over.
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Post by gilly02le » Tue Apr 10, 2012 12:43 pm

^i agree. start very small, and test as you go, its easy to make the hole bigger, smaller is another story.
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Post by occasional demons » Tue Apr 10, 2012 4:45 pm

If you don't want to keep removing the TB, You could make a Tee with one end filled, and tee it into the vacuum line on the TB. Then just remove the tee to get the optimum hole size needed. Of course you would want the engine warmed up, but you could do this much faster than removing the TB.

It would take a tee, two pieces of rubber hose and some epoxy or JB weld to fill the end of the tee. Just enough to make a plug. A rubber cap would be too hard to get an accurate hole size.

I think the tube is 6mm/1/4 ish (The one that goes under the brake booster, at least on '00 - '02 cars)
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Post by Donkeypuncher » Tue Apr 10, 2012 4:57 pm

Danteneon wrote:
Psst...1/16" is smaller than 1/8". Methinks you meant 3/16"?
I meant drilling a 1/16 hole in addition to the 1/8" hole that is already drilled. I didn't want to go all out and drill two 1/8" holes and overshoot the idle.

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Post by Danteneon » Tue Apr 10, 2012 5:16 pm

Ah, got ya. Why not just enlarge the existing hole an additional /32" or so?
If I could just figure out how to meld the Outback and the Neon into one car...

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Post by occasional demons » Tue Apr 10, 2012 5:25 pm

Because his neon is craving cheese? :lol:

Sorry, had to give an Ask Dante answer to a Dante asked question. :beatstick:
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