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@idle_hours_ike ·

Left the car idling for about 14 hours by accident — what actually gets damaged, and what should I check now?

Came out this morning and the engine was running. Best guess is it had been going since about six the previous evening, so somewhere around 14 hours. Petrol, five years old, nothing unusual about it.

It was parked outside, not in a garage, and it was not especially cold. Fuel gauge has dropped noticeably but there was enough in it that it did not run dry.

The internet is split between "engines run for days on end in some jobs, it is fine" and long lists of expensive-sounding consequences. Both cannot be right for my situation.

What I want to know:

  1. Is there anything actually damaged, or is this just wasted fuel?
  2. Is there a difference between 14 hours of idling and 14 hours of driving, from the engine's point of view?
  3. Is there anything I should check or change now rather than at the next service?
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  • @conduit_connie · 11h ago
    @conduit_connie ·

    The one thing genuinely worth understanding for question 2 is carbon and unburnt fuel.

    At idle the combustion is cooler and less complete than under load. Over many hours that deposits carbon in the exhaust path and lets a small amount of unburnt fuel past the rings into the oil, thinning it slightly. Neither is a problem in a single event — this is a cumulative thing, the reason short-trip-only cars get sooty and taxis used to foul plugs.

    The fix is free and takes twenty minutes: take it for a proper drive. Get it fully up to temperature and give it some sustained load — a dual carriageway rather than a crawl through town. That gets the exhaust hot enough to burn off what was deposited and drives the fuel dilution back out of the oil. Do it today rather than next week.

    That single drive is most of the answer to question 3.

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  • @tapped_thread_tal · 2d ago · 2 replies
    @tapped_thread_tal ·

    Short version: almost certainly no damage, and the people saying "engines run for days" are right — but for a reason that also explains why the worried list is not pure nonsense.

    Idling is mechanically gentle. Low revs, low load, low cylinder pressures. In terms of wear, 14 hours of idling is worth a small fraction of 14 hours of driving. Plant and generator engines run continuously for far longer than this by design.

    What makes extended idling different from driving is heat and airflow, not wear. At idle the engine makes just enough heat to stay warm but the exhaust runs cooler than it does under load, and there is no air moving through the engine bay except what the fan pulls. That is the seed of truth in the scary lists.

    For a single 14-hour event on a healthy five-year-old car, the honest answer to question 1 is: you bought a tank of nothing. Roughly half a litre to a litre an hour, so probably 7-12 litres.

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    • @idle_hours_ike · 11h ago
      @idle_hours_ike OP ·

      That matches the gauge. I was expecting worse from the drop and it lines up with about a litre an hour.

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  • @split_the_week_wes · 9h ago
    @split_the_week_wes ·

    Things worth actually looking at, in the order I would do them:

    1. Oil level and what it looks like on the dipstick. You are checking for a level that has gone up and oil that smells of petrol — that is the fuel dilution above. A slight rise is not an emergency; a big one means an early oil change.
    2. Coolant level, once it is completely cold. Fourteen hours of running with no airflow leans on the cooling system, and if there was a small weep you did not know about, this is when it shows itself.
    3. Anything on the dashboard. A car that overheated or ran lean for hours would usually say so.

    If all three are normal, treat it as a fuel bill and move on. If the oil is over the max mark and smells wrong, bring the service forward. That is the whole decision tree.

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