The two times I idled my car for more than an hour and a half, I was blowing out soot thousands of miles later. Usually showed up when I had to pick up speed from a red light - lots of BLACK smoke and little power for an instant, then fine. Probably happened 4-5 times.

When faced with the choice of NOT being able to start the car or leaving it idle all night, I would leave it idle all night.

Big rigs, i.e. Cummins, etc suggest raising the idle for prolonged periods to keep the coolant, etc at "operating" temperature.

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Brian Toscano