Idling your car while waiting can be detrimental to the overall health of your car or it can have little effect, largely depending on your vehicle's design. Here are some of the possible results of idling while operating your vehicle, and a few factors that might lead to differing results.
Idling with the AC on can keep you cool but overheat the engine. The AC heat exchanger (condenser) is in front of engine radiator and can therefore heat air before it has a chance to flow through the radiator for maximum cooling effect. The radiator coolant can overheat, causing the engine to overheat, too.
If the automatic transmission is in drive, it causes heat buildup in the fluid, which is cooled in a heat exchanger built into the radiator, further increasing the cooling load. High ambient air temperatures along with idling while the transmission is in drive and the AC is operating can put maximum load on cooling system, and may overheat the engine.
Low Oil Pressure
Idling your engine might result in low oil pressure from the slow speed of the engine. Low oil pressure can result in insufficient lubrication of internal engine parts, leading to excessive wear. Not all engines will experience lower oil pressure at idle, though. High mileage engines are more susceptible to damage from low pressure.
Rear Wheel Drive or Front Wheel Drive
The orientation of your engine your car's wheels can have real effects on cooling system capacity at idle. Rear wheel drive autos have engines in line with their vehicles and usually their radiator cooling fans are directly driven from engine. If you're idling, the fan isn't turning as fast as normal, and not moving as much air, which could cause the radiator to overheat.
Front wheel drive automobiles have engine installed crossways to the car and therefore require an electric fan to move air through the radiator. Electric fans are controlled by a thermostat mounted in contact with the radiator coolant—they activate when the preset temperature is reached.
The thermostat will turn off the fan when coolant temperature is lowered enough to meet the preset range. If the fan is constantly running while idling engine, you look out for possible overheating. Observing the temperature gauge when idling a vehicle is a sensible precaution.
Extra Emissions Pollute Environment
In most engines, idling produces a higher amount of harmful emissions. Prolonged idling will add to the pollutant load in the environment. Idling for too long can cause clogging of the catalytic converter in the exhaust system and also contribute to other problems in the overall emissions control system.
Air Cooled Engine
Some automobiles have air cooled engines—they need a higher engine speed at idle to keep the fan moving air enough to cool effectively. Their higher idle speed is preset in manufacturing for this reason.
Hybrids combine an electric motor with a gas engine, and the design of their control systems usually shuts down the gas engine when vehicle is stopped, restarting it from electric power the next time accelerator pedal is pushed. This kind of system works well for stop and go traffic, and doesn't produce harmful emissions while not moving.
You Can Choose
If you find yourself in stop and go traffic or waiting in long lines make some choices. Turn off air conditioning and reduce the heat load on engine, shift vehicle into neutral or park to reduce transmission heat buildup, or just turn off engine and restart when you need to move again.
If it's warm out, open your windows to allow airflow to help keep things cool. In colder weather, keep windows closed to retain interior heat as best you can.
If you choose to idle your car, keep eye on gauges to know what coolant temperature and oil pressures are and make adjustments accordingly. Practicing common sense in these situations will go a long way in preserving your car, your health, and the environment.