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Old 04-15-2009, 07:29 PM   #1
ifried85
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Default 1999 Saturn SC1 rough idle/vibration

Hi I have a 1999 Saturn SC1, automatic transmission with 108,000 miles on it. I have been having a problem with rough idle/vibration. The car typically is at about 1200 rpms in park when the car is warm and around 900 rpms when in drive. I have noticed that the vibration becomes worse when the car is in park or neutral but it is still there in drive. Also when the AC is on the vibration becomes even worse. The top and bottom motor mounts have been replaced recently. Also my egr valve failed recently and was also replaced. Does anyone have any ideas for what could be causing this problem? I would appreciate any help. My mechanic was not sure what to do in order to fix the problem.

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Old 04-15-2009, 08:10 PM   #2
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2003 L-Series 3.0L Sedan
Default Re: 1999 Saturn SC1 rough idle/vibration

Have you replaced the original engine coolant temperature sensor (ECTS)? Use the Forum Jump below to find the How-to library's videos, pictures, and procedures about it. It usually fails with rough high idling from constant rich mixtures. The replacement is the brass one from any auto store stocking it and will bring the idle back down as well as improve fuel consumption.

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Old 04-15-2009, 10:01 PM   #3
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Default Re: 1999 Saturn SC1 rough idle/vibration

Did you use OEM replacement mounts or the cheap smiley ones? Can make a difference. If you've gone that far also consider replacing the torque mount above the transmission and also the lower transmission mount. Although if $ is an issue I'd consider doing the lower transmission mount first. The one that attaches under the battery is pretty well protected and doesn't go bad as quick. When replacing them, the last thing you want to do is loosen then re-tighten the lower engine mount nut where the bolt attaches to the cradle.

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Old 04-16-2009, 10:33 AM   #4
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Default Re: 1999 Saturn SC1 rough idle/vibration

I am going to try and replace the ECTS tomorrow. My temperature gauge does not even go up to the quarter mark so hopefully this is the problem.

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Old 04-16-2009, 11:44 AM   #5
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Default Re: 1999 Saturn SC1 rough idle/vibration

Keep this in mind after replacing the ects; once the engine warms up but still doesn't show past the quarter mark on the temp gauge, replace the thermostat as it tends to fail slightly open. This causes slower warm-ups and reduces fuel economy. Once the thermostat (195F) is replaced, the temp gauge needle should be between the quarter and half mark (3/8ths).

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Old 04-16-2009, 01:31 PM   #6
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Default Re: 1999 Saturn SC1 rough idle/vibration

I am having the same problem with the rough idle in my 01 SC1. Does your RPM seem to fluctuate, in idle, like mine does, too?

Also, the thermometer on my car has NEVER gone past 1/4, so I'm going to look into this as well.

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Old 04-17-2009, 03:18 PM   #7
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Default Re: 1999 Saturn SC1 rough idle/vibration

So I replaced the ects and it has made a huge difference. The temperature gauge went up a little over the quarter mark. I only had a chance to drive the car for a few miles so hopefully once I make a longer trip it will reach the 3/8 mark. The RPM in park has dropped from 1200 to 800 and in drive the RPM has dropped from 900 to 700. However I am still having some vibration problems. My car is still vibrating but it is not as bad as it used to be except for when I put the AC on. As soon as the AC is put back on the vibration becomes significantly worse and the rpms go up by 100 and stay there. Any ideas on how to fix this. Could this be from the engine coolant thermostat or is this something else?

stytch my rpm's never fluctuated they stayed consistently high.

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Old 04-17-2009, 04:31 PM   #8
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Default Re: 1999 Saturn SC1 rough idle/vibration

With the ects taken care of you can attend to the other issues that may be affecting engine vibration. You may have to;
  • clean or replace the spark plugs
  • clean the ignition module and coils of corrosion
  • clean/degrease the throttle body
  • replace the thermostat - the original one may be running cooler than normal. It shouldn't take more than 10-minutes of driving to bring the temperature needle to the 3/8ths mark.

The idle speed will increase normally (automatically) when a/c is selected due to compressor load but not significantly. The increase is usually only enough to overcome compressor load, basically returning the idle rpm back to normal when it would normally drop if the PCM wasn't programmed to do so. Perhaps cleaning the throttle body will help reduce the increase as there are no adjustments for this. The PCM detects the a/c being selected and is programmed to compensate for the extra load by adjusting the idle air control valve (IACV in the throttle body) to restore rpm. The iacv already works properly because that's what's adjusting the cold idle speed (1200rpm) back down to normal idle (800-900rpm). The ects reports the coolant temps to the PCM in order for the PCM to control engine idle speeds at any temperature and load conditions.

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