Idling is necessary to preserve a turbo-charger.
I have a 1992 Volvo 940 sedan. It's equipped with a turbo charger to boost the power of the four cylinder engine. The owner's manual says that the engine should be allowed to idle at least two minutes before shut-off. The problems is the amount of gas and time I spend waiting for the turbo to cool down, even for a short drive! Your comments please.
RAY: Isn't that a pain, James? Who wants to sit in the driveway for two minutes while the turbo cools down?
TOM: The problem is, if you don't let it spin down before shutting off the engine, the heat of the turbo basically "bakes" the oil inside it. That clogs up the oil passages, and before you know it, you're out $1,500 for a new turbo.
RAY: And the harder you drive the car, the longer you should let it idle before turning it off. I'd say two minutes is probably about the maximum, and you only need to sit there for two minutes if you've been driving the car hard or driving on the highway.
TOM: For gentle driving on short trips, 30 seconds to a minute is probably fine.
RAY: But even that's a long time when you're just sitting there! So we recommend you join the book of the month club. That way, you can knock off a couple of pages of the monthly selection whenever you're waiting for the turbo to cool down, and at least make good use of the time.