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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?

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  • But those prior to previous? Also good questions get answers...
    – Solar Mike
    Commented May 11, 2021 at 6:28
  • This question has two answers that others consider good but you have failed to accept either one. See mechanics.stackexchange.com/q/82567/10976
    – Solar Mike
    Commented May 11, 2021 at 7:10
  • @Solar Mike I upvoted the first answer in the question about break-in methods but in fact none of the answers had addressed the main point which is the difference between the methods. Yes, good questions will get answers but the previous question about Fluid II & III was not a bad one! They requested more information about the fluids in comments and I offered them spec sheets but no one replied after that!!
    – LFY MP7.3
    Commented May 11, 2021 at 7:37
  • So, perhaps the idea was that you could read the spec sheets to work out the differences.
    – Solar Mike
    Commented May 11, 2021 at 10:03
  • I knew there are differences between them but I don't know if they are interchangeable or not.
    – LFY MP7.3
    Commented May 11, 2021 at 11:08

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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.

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  • You are right about overrun but this quietness I'm talking about will continue for 15 minutes after driving downhill. I mean even after the road gets flat or uphill and injection of fuel starts again, the car continues to be extremely quiet for 15 minutes. This is also evident in idle operation. The engine idles quieter after downhill driving rather than after driving on a level road.
    – LFY MP7.3
    Commented May 16, 2021 at 9:43
  • Mechanical engine noise or exhaust noise?
    – Hugh Daniel
    Commented May 16, 2021 at 10:04
  • I'm referring to interior noise overall whether it comes from exhaust or other mechanical components.
    – LFY MP7.3
    Commented May 16, 2021 at 10:06
  • As we seem to have diverted from mechanical clearance noise to very general interior NVH I unfortunately can't help any further. I wouldn't worry about it at all, sounds very benign!
    – Hugh Daniel
    Commented May 16, 2021 at 10:15
  • If it helps you, it is more mechanical noise rather than exhaust noise (not irritating noise just something that can be heard). My exhaust is bone stock and never noisy.
    – LFY MP7.3
    Commented May 16, 2021 at 10:16

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