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.
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
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.
Idle is computer controlled by the pcm (computer)-not adjustable.
Possible bad motor mount
Bad idle, lack of power. Vehicle may shake or vibrate at idle.
your airflow is uneven check the airflow sensor wire/ sensor
Most cars typically idle between 400 to 1000 rpm
The engine is running C O C K
"How do you move the idle tensioner for a 1998 3 litre dodge?"
where is the idle control motor for a 1998 ford escort
The idle air control valve on a Saturn SC1 is found on the intake manifold. This valve controls how much air enters and engine and allows how high the motor can idle.
I would look for a vacuum leak.