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Old 12-12-2007, 11:15 AM   #1
quasar
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1991 SC
1997 SW2
Default High Idle and rough running

I am wondering if anyone else has noticed what I recently experienced in both of my S-Series vehicles. The cars were not idleing smoothly and sometimes ran fast and then slow at idle. I know one of the usual problems is the ECTS, but in both cases what really solved the problem was the battery. In both cases the batteries, although starting the cars fine and appearing to run everything on the car okay, were three years old or older. As soon as they were replaced everything went back to specs. The engines started faster and the RPM stayed steady at 950 without hesitation. I am wondering if some of the problems other are having could be from older batteries that may not be providing enough amps to properly run the computer and everything else that is needed with todays cars. Anyone else experience this sudden improvement with new batteries?
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Old 12-12-2007, 03:42 PM   #2
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1995 SL2
Default Re: High Idle and rough running

Improvement in starting, yes I would expect that, with a new battery replacing an old, tired battery. However, if the old battery is adequate to start the car, it should be sufficient to power other OEM gear (including the computer) in the car once the car starts - assuming of course that there is a working alternator. Or course a vehicle tricked out with about a kilowatt or so of audio, etc, might present a different story.

A possible scenario is that the act of replacing the battery also caused a somewhat corroded contact to move, breaking through the corrosion, and restoring a working connection, at least for a while. Might have been ECTS or EGR, or maybe something else.

ECTS and connector should be checked and replaced as appropriate on all S-cars prior the the OEM change.
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Old 12-12-2007, 08:20 PM   #3
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1991 SC
1997 SW2
Default Re: High Idle and rough running

I stuck the old battery back in the 91 just before storage. The engine began to idle rough again. Put a new one in and it runs just fine. I will admit I know nothing about mechanics but still find this an interesting situation. Don't have ampmeter or other testing tools that may explain why.
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Old 12-12-2007, 08:24 PM   #4
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Default Re: High Idle and rough running

I remember when the battery in my Colt finally gave it up, the alternator voltage at idle was lower. I suppose a seriously effed battery can pull more from the alternator, and at idle, burden it, thus making the idle slower/rougher.
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Old 12-12-2007, 08:35 PM   #5
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Default Re: High Idle and rough running

I agree with the alternator theory. Keep an eye on the voltage you are getting with the engine running. If you have a voltmeter hook it up to the new battery and turn the engine on, you should probably read 13-14 volts. If not you may have a weak alternator.
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Old 12-14-2007, 03:48 AM   #6
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2000 L-Series 3.0L Wagon
1997 SC2
Default Re: High Idle and rough running

When in doubt, Take it to an auto parts store. They will test the battery and charging system for free.
...
15 S-series put back on the road.
Now working on a 2000 LW2 v6 auto. Timing belt, 2 intake valves and more getting discovered.
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