How bad is this for the engine or any other components?

A friend left my car running for 7 hours. How bad is that for the engine or any other components?


Answer:
Consider what was going on with your car for those seven hours.
The engine was operating. It was igniting gasoline and air, parts were rotating, heat was being transferred, and various engine systems were at their equilibrium states.
If your car now starts and runs as it did before you left it running, it is likely OK. It is likely that no severe damage was done to engine components such as bearings. Certainly there was several hours-worth of “normal wear and tear” added to the engine — taken off its life. But that should not matter much.
Today’s engines are tested under much more difficult conditions than sitting idling.
Your car and its engine passed the test of being able to do many of the normal things involved in long driving. Your engine’s cooling system maintained the engine within its normal operating range. If not, it would have over-heated and perhaps been damaged and shut down. Or, if running too cool, it might have caused some sludge to build up in the oil.
Let the engine cool down and check the oil and coolant levels.
If they are OK, it is likely that your car is OK.

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