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Old 08-25-2013, 10:20 PM   #1
bradford177
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Default SL2 idle rough - but different than others

Okay,
So I have been on this site for about 3 weeks scouring it for info on rough idle... have tried just about everything listed.
98 SL2 automatic.. 208k miles. Overall runs great.. but idles rough at stops and almost dies if I turn on the AC.
Cleaned the throttle body, IAC, cleaned and tested the MAP sensor, replaced plugs with copper AC Delcos, new wires, gas treatment.
It was real bad at first, but every thing I did made it a little better, then I replaced the battery and it seemed to fix it!..even with the A/C on, for about a week. Then it started idling rough again. So I take the neg. off the battery and it makes it better again for a little while.
No SES light, no codes being thrown.. I am getting lost here. The o2 sensor wire was melted lightly, but again, no code. Seems to run just fine on the freeway and acceleration seems good.

Any Ideas?

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Old 08-25-2013, 10:34 PM   #2
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Default Re: SL2 idle rough - but different than others

Have you checked/replaced the ECTS?

...
1997 - SL2, barebones, 161k, Auto - Rebuild complete and.. burnt a valve 20k later Arrgh!
Purchased 114k 10/07
Added OEM CC

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Old 08-25-2013, 10:38 PM   #3
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Default Re: SL2 idle rough - but different than others

Is the alternator original? Sounds like the battery isn't getting charged up. Wait until it is idling rough, then check the voltage at the battery terminals with the engine running. Charge the battery with a trickle charger all night, if the problem goes away that also points to alternator (and provides a temporary fix).

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Old 08-25-2013, 10:47 PM   #4
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Default Re: SL2 idle rough - but different than others

ects? no.. what is that..I thought it might be coil packs, but don't think it would be since I can temp fix it.


I had the alternator tested at auto zone.. it checked out.. I thought of that as well.

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Old 08-25-2013, 11:13 PM   #5
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Default Re: SL2 idle rough - but different than others

Engine coolant temp sensor. That'd be my first guess had you not mentioned the battery - definitely worth checking into/replacing with a brass tipped one.

Not coil packs, ICM. It's the thing under the coils that fires them, older ICMs are very sensitive to voltage, I assume the same is true of newer ones but to a lesser extent.

Parts stores test cold alternators by spinning them and checking voltage/amperage outputs. That can identify a totally failed alternator but not much else IMO. Try either trickle charging the battery overnight or reading voltage at the terminals during a rough idle to confirm that the alternator is either good or not. You don't want voltage to be below 13.3v at idle, opening throttle slightly should bring it closer to 14 in a hurry.

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Old 08-26-2013, 09:03 AM   #6
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Default Re: SL2 idle rough - but different than others

I am wondering about the coolant sensor... I had read that if the fan comes on it is still good... but now that you guys bring it up...
My wife mentioned that it does not do it as bad in the morning when cold...and I noticed the temp gauge is reading a bit higher than it use to.

So I will try replacing that tomorrow.

I actually had the alt tested in the car while warm and running.. and watched him do the test.. it was reading 13.7 to 14.1 at rough idle.. then went up when accelerated.

Is there a way to test the ignition module?.. or is that one of those spend your hard earned cash and cross your fingers...lol

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Old 08-26-2013, 09:45 AM   #7
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Default Re: SL2 idle rough - but different than others

If the ECTS hasn't been changed, assume that's the problem. Alternator sounds fine. The battery thing might have been some odd, unrelated phenomenon.

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Old 08-26-2013, 12:28 PM   #8
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Default Re: SL2 idle rough - but different than others

Read this; http://www.saturnfans.com/forums/sho...d.php?t=194136

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Old 08-26-2013, 07:35 PM   #9
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Default Re: SL2 idle rough - but different than others

I bought the sensor today.. will get it in tomorrow and update..
I read another post that said if the fan kicks on, then the sensor is good.. but now I am thinking different.

Also, it makes sense.. I unplug the battery, which resets the computer.. so until it re-reads all the sensors.. it runs good in closed loop.

So anxious to see if this fixes it... let you guys know.

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Old 08-27-2013, 12:32 AM   #10
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Default Re: SL2 idle rough - but different than others

well we will see... I was able to replace the temp sensor tonight, but the old one was not an original.. but I suppose a replacement can go bad as well. it did run good afterwards but it always idles good after I take the neg off the battery.. so we shall see after a couple of days.

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Old 08-27-2013, 02:19 AM   #11
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Default Re: SL2 idle rough - but different than others

Quote:
Originally Posted by bradford177 View Post
well we will see... I was able to replace the temp sensor tonight, but the old one was not an original.. but I suppose a replacement can go bad as well. it did run good afterwards but it always idles good after I take the neg off the battery.. so we shall see after a couple of days.
It's possible the sensor was still out of spec. The fan kicking on indicates that the ECTS circuit is functioning, but that doesn't determine if the ECTS is accurate or not.

What is your actual idle RPM? Has it changed at all with any of the previous tune up issues?

The reason I ask is that you may have a normal idle within spec, but a bad engine mount (or several) simply transmitting more vibration and making it feel rough. The primary culprit is usually the upper mount on the passenger side of the car. If you can slip a pen or your pinky finger in there, the mount is sufficiently collapsed and the car will shake more at idle.

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Old 08-27-2013, 09:57 AM   #12
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Default Re: SL2 idle rough - but different than others

I don't think it is the mount.. I looked at that.. but the RPM will actually be lower and fluctuate when it is idling bad.. then when you kick on the AC, it drops to 400-500 and almost stalls.

I tried the A/C after I warmed up the car last night and it held steady, but again, it would do that after I disconnected the battery and re-attached.. so I will give it a couple of days and see what happens...

As far as a temp.. the temp would usually hold between 1/4 and 1/2 until traffic or hills, then jump up to above 1/2.. fan would kick on some where between 1/2 and 3/4, and bring it back down to just below 1/2.. don't know if that is normal, but that is what it was doing.

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Old 08-27-2013, 01:57 PM   #13
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Default Re: SL2 idle rough - but different than others

With all that's said and done, eliminating the coolant sensor was the main reason for past posts. This recent explanation (above) tells all. The coolant sensor is operating correctly as mentioned and more so with what rpm's are seen during cold starts and when warm. The only thing standing out is the lower rpm when a/c is used and continuing rough idle. Aggravating conditions is manually resetting the pcm by battery disconnects. While this does no harm, you may not be aware that resetting the pcm resets it back to factory default as if it just left the assembly line. To put it briefly, the pcm must relearn all its emissions requirements all over again and that takes a few days of driving before the pcm can learn customized parameters. This also includes xmission shift points so resetting the pcm is not advised at this point unless you are patient and willing to repeat this as many times until solving a minor issue. Resetting the pcm only adds to troubleshooting when leaving it as is and dealing with the sole problem at hand might be a wiser choice.

Cold engine idle should be around 1200 rpm, quickly dropping back to 800-900 rpm after 10 minutes of driving. Engaging a/c with idle dropping may be the idle air control valve sticking or failing as its used in all engine phases, from cold start to a fully loaded engine running a/c at night with blower going, lights on and defroster on. All loads on the engine shouldn't alter idle rpm by more than 50.

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Old 08-28-2013, 09:30 AM   #14
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Default Re: SL2 idle rough - but different than others

I get what you are saying about the computer.. but I wanted to see if resetting it would yield the same results.. and it did. So something is telling the computer to idle rough after a time.

I cleaned the IAC real good.. now, wouldn't that make the car idle rough no matter if I reset the computer or not?

My wife drove it a short distance yesterday and said it ran better than all the other times.. so I got my fingers crossed.. she will be driving it up to work today which is a 100miles round trip... it usually starts to act up a bit after that drive, so we shall see if the temp sensor has something to do with this.

Thanks for constant input, and I will keep everyone posted. If anyone has any other ideas.. feel free to add them in there.

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Old 08-28-2013, 09:48 AM   #15
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Default Re: SL2 idle rough - but different than others

Cleaning iacv's only takes care of one issue, a mechanical problem. It doesn't fix a broken electrical connection and would be masked by repeated pcm resets. in effect, you're adding the pcm readjustment from resets while attempting to see if cleaning the iacv helps. A dead iacv will show itself immediately as either increasing rpm or lowering rpm; the iacv uses two motors to extend or retract the valve to control idle speeds from cold start up to loads like a/c and all electrical loads running at night in winter weather. Just leaving the pcm alone will allow a before and after test on anything you're trying to correct.

A better method to test the iacv; with a fully warmed engine idling normally, 800-900 rpm, block off the small hole directly in front of the throttle plate - the engine is supposed to drop down to 500-600 rpm because you blocked off air flow thru the bypass air port that's controlled by the iacv. The pcm will detect this rpm drop and immediately retract the iacv but with your finger blocking the hole, nothing happens until you remove your finger. The rpm jumps up immediately as more air is allowed in thru this bypass port and the pcm responds again, immediately to extend the iacv to reduce rpm back to idle. This blocking and unblocking tests the iacv to retract and extend from commands given by the pcm in response to rpm changes.

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Old 08-28-2013, 04:05 PM   #16
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Default Re: SL2 idle rough - but different than others

Very good.. thank you for that test method. If my problem persist in the next day or so, I will try that.

I did clean out the IAC without resetting the computer in the beginning of all this mess. It did not change the problem, but that does not mean the IAC is still not bad. So I will try that test.

Thanks

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