There could be many reasons, a vacuum leak is one possibility.
A miss or A collapsed engine mount or A imbalance caused by a failing component, harmonic balancers and torque converters are common.
Engine miss.
It is idling. This is hard for an engine to do smoothly.
You should check the timing and plugs.
Bad wire
Your engine probably has a vacuum leak
check spark plugs and wires
The engine
probably the governor going bad
A 2002 Saturn SL with a 1.9L engine may experience a miss while idling due to several potential issues, including faulty spark plugs or ignition coils, which can cause incomplete combustion. Additionally, problems with the fuel system, such as a clogged fuel filter or failing fuel pump, can lead to inadequate fuel delivery. Vacuum leaks in the intake system or issues with the idle air control valve can also disrupt the engine's air-fuel mixture, leading to rough idling. Finally, a dirty or malfunctioning mass air flow sensor could impact engine performance as well.
The #1 cause of cold air from the heater when the car is stopped, or the engine is idling, low engine coolant levels.
A car that is stopped with the engine running.