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Idle surge problem; Idle surges with lights on
Topic Started: Oct 10 2017, 10:31 PM (876 Views)
backyardmechanic93
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I could use some help diagnosing an idle problem with a 95 geo metro 1L 3 cylinder, 5 speed manual trans. The car idles up a couple hundred rpms when the lights are turned on, then the idle slowly drops down to what it was with them turned off and then jumps back up about 300-400 rpm and keeps repeating over and over. The problem only happens once the engine is at operating temp and the headlights (or a few other electrical devices) are turned on. No issues while driving or with lights/electronics turned off. I've replaced most vacuum lines, throttle body gaskets, diode module. No luck. I have a running parts car (smashed up but no engine issues) so swapping parts over to test shouldn't be an issue. Didn't throw a code (no engine light) and the car has been in the family since new with no modifications to anything other than a CD deck installed in 1990 something. Where would I start with diagnosing? :hmm Forgot to mention: Compression is good (200 000 km on engine), O2 sensor, plugs and wires changed and everything requiring maintenance has been done at regular intervals.
Edited by backyardmechanic93, Oct 10 2017, 10:40 PM.
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suzukitom
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Tom

Do you have a tachometer? What is the car idling at when warmed up with lights and accessories turned off? 850 rpm?

If your base idle is higher than that rpm level, (for various reasons) the initial surge when you switch the lights is just the normal PCM response to try to move the ISC motor plunger in order to 'maintain' a constant 850 rpm target rpm.

A 300-400 rpm rise is an excessive response to the idle up event; normally the 'idle up' or 'VSS (speed >0) bump up only increases by 250 rpm or less, and then it should gradually return to the target rpm.

So first we need to know what your warm idle rpm is.
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suzukitom
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Tom

Picture of a ISC motor plunger (retracted) at its idle switch on position.

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Edited by suzukitom, Oct 10 2017, 11:34 PM.
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freegeo
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Quote:
 
I have a running parts car (smashed up but no engine issues) so swapping parts over to test shouldn't be an issue.


What year is the parts car? Since you have a 95 you have to be careful what parts you swap. Could make it worse not better.

Edited by freegeo, Oct 10 2017, 11:52 PM.
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backyardmechanic93
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freegeo
Oct 10 2017, 11:52 PM
Quote:
 
I have a running parts car (smashed up but no engine issues) so swapping parts over to test shouldn't be an issue.


What year is the parts car? Since you have a 95 you have to be careful what parts you swap. Could make it worse not better.

They're both '95. I haven't got a tach so I'm not certain of the exact rpm jump however, I am 100% sure the idle screw settings haven't been messed with from factory and aren't an issue. It goes to high idle on cold startup like it should, then comes down to slow idle as it has for the last 20 years. still runs like new until there's an electrical load. I tried the spare idle speed motor as well for the heck of it, made no difference.
Edited by backyardmechanic93, Oct 11 2017, 01:11 AM.
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suzukitom
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Tom

Quote:
 
I am 100% sure the idle screw settings haven't been messed with from factory and aren't an issue.


Well that is an encouraging sign, that none of the factory settings has been tampered with so far.

However, one of the common problems of the 95-00 Metro 1.0 is that the throttle plate bushing wears over time, and as a result air gets past the throttle plate at idle. This physical wear messes up the base idle speed, as well as the relationships between the ISC, TPS. The engine computer gets confused trying to manage idle speed with inaccurate sensor data.

Your idle symptoms usually mean that you will need to 'tamper' with the throttle body, in order to restore the throttle valve to its proper closed position, so that the relative position of the TPS and ISC will also be restored.

Your other parts car engine probably has a throttle plate that is not as worn as the one on your current car. You can also try swapping the entire throttle body assembly, if you don't want to adjust your throttle plate, but it isn't that hard to do.

Your 1995 Metro is likely to pre-date OBD2 standards, so you probably wont be able to use an OBD2 scan tool for RPM or to view TPS data. You might still consider getting a tachometer though, in order to check rpm more accurately.


You may want to read this lengthy post. Most of the resolution is in post #156 of this thread:

http://geometroforum.com/topic/8091153/1
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geogonfa
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Just a few simple Questions: :type
Does it happen when;
1.) lights are turned on? of course, right?
2.) if it has rear defroster, when its turned on?
3.) when you turn on the blower motor (any speed)?

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freegeo
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Quote:
 
diode module


In your 1st post you said you replaced the diode module. Where was that located? Was it for the idle up signal?

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Edited by freegeo, Oct 11 2017, 08:53 AM.
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backyardmechanic93
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geogonfa
Oct 11 2017, 07:06 AM
Just a few simple Questions: :type
Does it happen when;
1.) lights are turned on? of course, right?
2.) if it has rear defroster, when its turned on?
3.) when you turn on the blower motor (any speed)?

Happens when any of those are electronics turned on. sukukitom you're correct, it's obd 1 with no codes. And I do technically have a tach, just not currently accessible. I have prodemand and lots of tools as well because I'm an apprentice, just no experience with tbi or pcm controlled vehicles. (I try to stick to pre-'87 hot rods).
Edited by backyardmechanic93, Oct 11 2017, 06:34 PM.
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backyardmechanic93
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freegeo
Oct 11 2017, 08:51 AM
Quote:
 
diode module


In your 1st post you said you replaced the diode module. Where was that located? Was it for the idle up signal?

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Yes, I replaced the diode module with a known good one. It was behind the glove box left side, taped to the wiring harness. It's for the idle-up as far as I know.
Edited by backyardmechanic93, Oct 11 2017, 06:37 PM.
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freegeo
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Yes that is it. Sounds like maybe the signal to the pcm is not consistent. The procedure may help you figure it out.
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suzukitom
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Tom

I don't think the test procedure above can be followed without a Tech 1 scan tool, since his car is OBD1. And the idle up initial response seems to be consistent, but the main symptom is the up and down cycling... I think that a bad diode, signal wire or internal fault in the PCM would cause no idle up response at all, rather than a slow and cyclical idle up and down response.

A surging idle usually means that sensors may be sending conflicting information about the current idle speed to the PCM. The ISC is the main component that manages idle speed after the engine is warmed up.

I'm out of further suggestions without knowing what your actual rpm values were during the lights off/lights on scenario.
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geogonfa
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:gp suzukitom...things you can eliminate due to what was posted: ISC and the Diode pack...
since supposedly the car idles normal without the accessories on...or so we assume...
without having an actual Tach gauge hooked up we can't be sure...

I have a few more Questions though...

1.) When you start the car from being cold engine does the idle start high and come down to normal in a few minutes? (and this is why it's good to have a tach), What is the RPM's

2.) When you come down to a stop sign or stoplight, does it idle low then pick up without the accessories on? or does it slowly drop to normal idle? and what is that RPM?

3.) When was the last time you have done a compression test? if not recently, I recommend one to make sure the engine is healthy...

http://geometroforum.com/topic/2574993/1/
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backyardmechanic93
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geogonfa
Oct 12 2017, 07:56 AM
:gp suzukitom...things you can eliminate due to what was posted: ISC and the Diode pack...
since supposedly the car idles normal without the accessories on...or so we assume...
without having an actual Tach gauge hooked up we can't be sure...

I have a few more Questions though...

1.) When you start the car from being cold engine does the idle start high and come down to normal in a few minutes? (and this is why it's good to have a tach), What is the RPM's

2.) When you come down to a stop sign or stoplight, does it idle low then pick up without the accessories on? or does it slowly drop to normal idle? and what is that RPM?

3.) When was the last time you have done a compression test? if not recently, I recommend one to make sure the engine is healthy...

http://geometroforum.com/topic/2574993/1/
1. Yes, fast idle works fine, then drops slowly like it should (slow idle seems to be just fine, without putting a tach on)
2. No idle issues without accessories on, just drops off and idles slow like normal when stopping. Wish I knew the exact rpm, but seems to be unchanged since '95.
3. Have not done one for a year or so, but was around 170ish across all 3 at the time. I know it's probably getting a little low and I'm planning a rebuild withing the next 3 years. I believe it's still pretty even across all cylinders though, doesn't shake or run rough at all.
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freegeo
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You may not be able to do the procedure with out the tech 1, but all you would need to do is check 1 of the wires at a time going into the diode and watch if the signal going to the pcm varies any. If the diode and wire check out then it would have to be something with the pcm /ecm or wiring to the isc.
Edited by freegeo, Oct 13 2017, 06:19 PM.
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