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What might happen if you drive aggressively without an oil filler cap(where you pour new oil into the engine) for a little while eg a an hour or few times. Any damages or just poor peformance?

Thanks

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    You get to clean the underside of the hood / bonnet and the engine bay.
    – Solar Mike
    Sep 16 at 14:05
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Really, neither damages or poor performance will occur for "just" not having the cap. What will happen is you've left a big hole in the top of your engine where three things happen. First, you'll lose oil, as the hole will allow it to escape. Second, you allow the ingress of dirt and contaminants. Third, since there's a big hole, gasses in the crankcase will vent to atmosphere, which isn't good for the environment.

There won't be any engine damage until the engine runs out of oil. Then, all bets are off. Oh, I guess at that point, performance will be down as well.

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This depends very much on the engine.

If it is a gas/petrol engine with a PCV (Positive Crankcase Ventilation) system that uses metered air to ventilate fumes from the crankcase, then leaving the oil filler cap off will allow unmetered air into the intake system, causing the fuel mixture to run weak. This can cause serious damage to the engine especially when driven hard.

On a healthy engine the fumes shouldn’t escape from the filler cap, because the PCV should still be sucking them into the engine to be burnt. Air should be getting pulled in the oil filler due to the low pressure in the crankcase. In a worn engine, however, more fumes may be in the crankcase than the PCV can cope with and so fumes will exit the filler.

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