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I don't want to say my car (Peugeot 406 1.8 16V) is noisy (don't get me wrong) but in normal driving, engine sound can be heard inside the cabin during acceleration. However, when I drive in downhill roads and stop before stoplight, the engine is so quiet idling that I can't even tell whether it is on or off!

This is quite weird to me because if the same thing would happen in uphill driving, it would make much more sense because in uphill driving oil gets hot and internal engine parts will expand and tolerances will be near zero. This is why most engines run quieter in uphill driving but I don't understand why my car runs extremely quiet after downhill driving!!

Can someone explain this phenomenon in some scientific or technical terms?

LFY MP7.3
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1 Answers1

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If engine component clearances were dependant on either going up, or down hill, engine failure would be classed as regular maintenance. Oil and component temperature is maintained fairly constant to avoid this effect.

With regard to the noise difference, going downhill the engine isn't loaded and often goes into overrun stopping injection of fuel, therefore no/less noise. Going uphill it'll be louder as your loading it.

Hugh Daniel
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