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Out of control Idle Speed Controller; Trying to troubleshoot
Topic Started: Nov 27 2012, 11:03 AM (1,544 Views)
Goodman98155
New Member
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I have been trying to troubleshoot an out of control idle speed control (isc) motor on my beloved 1998 Chev Metro 1.0 base.

This thing has gone crazy! When starting from cold, the idle is way too low (the ISC is withdrawn fully.) Once the engine warms up (a few minutes) the idle increases over the course of another few minutes until the engine is racing so fast I have to shut it down. I don't have tach on this car, but it seems to be floored. The 'piston' or actuator or whatever on the ISC seems almost fully extended and will not retract on its own.

To work around this, I have had to manually energize the ISC with 12 volts from the battery to withdraw the actuator, then unplug the dang thing and just deal with a low warmup idle speed. This works ok for now, but it is getting colder and I can't warm up the car without my freezing foot on the accelerator.

It seems as though the ISC is being commanded to do this.

Otherwise the car runs perfectly.

What I have done so far:

- Swapped the relay for another one
- Removed, cleaned and lubed the ISC, including electrical contacts.
- Cleaned the connection to the Throttle Position Sensor.
- Disconnected the battery for several minutes (to reset the computer)
- I actually reversed the polarity of the motor connections to the ISC thinking that maybe it was trying to retract but actually extending for some reason. Didn't work. Crazy I know but I am out of ideas!

I am curious about a schematic I saw that describes a idle position switch incorporated into the ISC itself, but I can't see how that would work mechanically. If that switch were to fail, would the ISC try to chase the idle speed if it thought it weren't idling? I don't understand its purpose enough to simulate it in my mind.

If anybody has any suggestion, I would sure appreciate it. I am trying not to just throw new parts at it.

Thanks,
Chris
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Geo Glenn
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What's in your engine?

Is the service engine soon light on?
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Waldo
Jack of all trades, Master of none
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I may be wrong, but the only time I notice the idle speed controller actuate is when the A/C compressor kicks on. Other than that, mine stays retracted. Of course it doesn't really get "cold" here in South Louisiana either, lol
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Goodman98155
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Good question, Glen. No Engine light, but I scanned it anyway and got code P0123, which is Pedal Position Sensor High Input according to the device. I am assuming that is becuase the ISC was extended at the time of the test and the RPM was 2560.
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Goodman98155
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This Metro has no AC, but the ISC does help during cold start in the winter here by very slightly adjusting the idle RPM up for a minute or two, then it does (normally) seem to retract. The crazy thing to me is that the ISC seems to have enough authority mechanically to redline the engine!
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AmmoHouse
Fresh Fish
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I'm having the same problem with my 97 Metro. I haven't gotten it figured out yet.
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starscream5000
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Got 70 MPG?

Remove the ISC, superglue a dime to the throttle stop, and run the car like that until you locate a new ISC to put in it. Remove the dime when you get a NEW ISC of course.

The dime is just the right thickness to let the motor idle correctly when warm. You may need to feather the throttle on first startup, but once you get going, you'll be fine :thumb
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Goodman98155
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starscream5000
Nov 29 2012, 01:32 PM
Remove the ISC, superglue a dime to the throttle stop, and run the car like that until you locate a new ISC to put in it. Remove the dime when you get a NEW ISC of course.

The dime is just the right thickness to let the motor idle correctly when warm. You may need to feather the throttle on first startup, but once you get going, you'll be fine :thumb
Now that's a fix I can afford! I think it has come to that. So you think the ISC itself is the problem?
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cwatkin


I seem to be having an issue with my ISC and an oscillating idle. It is sometimes low, sometimes changing, etc. I also have the idle drop off after I come to a stop after being under a load. I am going to change my O2 sensor today as it is well past due and might be contributing to this and other minor quirks. I am wondering if it is slow/lazy from being old and having been on an oil burning engine before this one and isn't adjusting the the conditions fast enough. I will let everyone know what happens.

Conor
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starscream5000
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Got 70 MPG?

Goodman98155
Dec 8 2012, 11:24 AM
starscream5000
Nov 29 2012, 01:32 PM
Remove the ISC, superglue a dime to the throttle stop, and run the car like that until you locate a new ISC to put in it. Remove the dime when you get a NEW ISC of course.

The dime is just the right thickness to let the motor idle correctly when warm. You may need to feather the throttle on first startup, but once you get going, you'll be fine :thumb
Now that's a fix I can afford! I think it has come to that. So you think the ISC itself is the problem?
I suspect it is. You'll know in 5 minutes time as that's about all the time you'll need to do what I said above.
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Jittney
Anchorage 92 XFi

When it's 15 below....
My neighbor's 2000 3/5 sometimes goes to redline at start up.
My 92 XFi did the same thing this week.
I have ISCs for both of us, but have not installed them yet.
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Goodman98155
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Jittney
Dec 9 2012, 02:35 AM
When it's 15 below....
My neighbor's 2000 3/5 sometimes goes to redline at start up.
My 92 XFi did the same thing this week.
I have ISCs for both of us, but have not installed them yet.
Wow- doesn't get that cold here thankfully but i guess that is why the ISC has the authority.

I think I will break down so to speak and source an ISC.
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Goodman98155
New Member
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Well, I took apart my suspect ISC (or IAC depending on who you ask) today and inspected the motor, which operates fine.

However, I dug deeper into the unit and uncovered a switch with mysterious magnetic mechanics that, according to a schematic I found is supposed to be open when there is pressure on the actuator (implying that one's foot is off of the accelerator and the engine is at idle)

That switch was not closing at all while I worked it internally or once the unit was back together. Basically I guess the vehicle thinks it is always at idle. This certainly is a problem, but not sure if it is THE problem. The computer does seem to 'chase' the pedal with the ISC.

I wasn't able to make the switch work, but it did give me the impetus I needed to order a new (used) ISC.

I will install it next week and reply back with results!
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Jittney
Anchorage 92 XFi

I put my good used ISC on the neighbor's car.....fixed the idle issue :)
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GeoPete
AAS Auto Technology past ASE Master Tech

I tried the ball pen spring fix I found on the forum and it did not work on my ISC. Bought a new one from CarQuest, as they seem to be the only one that stocks it. Works fine now.
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