Because 1998 Saturns suck. I have one, so I should know ;) Honestly, it's either your throttle, a clogged fuel line or a misfiring spark plug. It is common for the upper motor mount to wear out and cause excessive vibration especially at low RPMs. It is easy to replace yourself at a cost of about $90. It is in plain view and located on the belt and accessories side of the engine. To replace, first support the engine from underneath with a jack and block of wood on the oil pan. Second, remove the bolts on the motor mount and install new mount and the bolts supplied. Third, make sure you torque the bolts to the proper specs. That's it! It took me about 45 minutes. Immediate improvement in noise and vibration.
Idle is computer controlled by the pcm (computer)-not adjustable.
Fast idle motor is not working. This is allowing the idle to fall below the threshold of what it should be when the A/C is on.
Possible bad motor mount
The upper engine mount is collapsed. On the passenger side by the coolant reservoir look at the mount If you see 1/4 inch or more gap the mount is collapsed and will cause the whole car to vibrate
Bad idle, lack of power. Vehicle may shake or vibrate at idle.
Most cars typically idle between 400 to 1000 rpm
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The engine is running C O C K
your airflow is uneven check the airflow sensor wire/ sensor
I would look for a vacuum leak.
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