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RPMs start to fluctuate (more like large drop and back to 700 RPM and again drop and back to 700 and so on, but overshoot is not so evident) when coming to a stop only in window between 35-50 km/h and in neutral, meaning clutch not pressed in.

Observations:

  • If the clutch pedal is pressed in the RPMs stop fluctuating immediately.
  • RPMs won't even start to fluctuate if I come to a stop with clutch pressed in all the time.
  • RPMs fluctuate regardless if I'm breaking or just cruising to a stop, meaning the brake system's vacuum shouldn't be a problem?
  • When idling or normal driving at constant speed, RPMs are stable.
  • RPM's fluctuate in this and only this window regardless of the outside and engine temperature.
  • There are no DTCs or any other problems with the car.

Service info:

  • Oil and oil filter changes are made on 10.000-13.000km interval w/ Castrol Edge 0w30 A5 and OEM or MANN filter
  • Cleaned Throttle body 11.12.2020 @333.000km (also changed the gasket)
  • Fuel filter 25.02.2022 (MANN) @367.000km
  • EGR clean (not thoroughly, only what I could manage to scrape out) 05.12.2021 @360.000
  • Air filter 23.03.2021 (OEM) @340.000km
  • MAP sensor (Bosch) 11.12.2020 @333.000km

This problem is present since I got the car (2019 @297.000 km). Any ideas?

Volvo s80 2.4D M66 2009 @381.000km

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  • If the clutch is not pressed in, are you in "drive" or neutral? Is this a good old fashioned gear box, or is it an automatic or some sort of computer controlled pseudo-manual that only gives the feel of manual? Aug 2 at 0:51
  • It's a manual gearbox, so by that I meant cruising in no gear and clutch not engaged
    – spaceman
    Aug 2 at 6:32

1 Answer 1

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I have no experience with Volvo diesels, but VW TDI diesels can have the same issue. They call it "the slow down shudder" and it sounds exactly as you describe. They fix it by slightly increasing the IQ (injection quantity), which is adjustable through a factory-grade scan tool.

For more information, search forums for "slow down shudder" at tdiclub.com or vwvortex.com.

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  • Well my first thought was fuel mixture and the fact that I had some fuel intake problems before (clogged pipes), maybe that's it. I think that thorough clean of fuel intake system would be the way to go - for the first step.
    – spaceman
    Aug 1 at 6:19
  • Also, the mind blowing is that this happens always and only in that speed window. Could this be connected somehow with some sensor?
    – spaceman
    Aug 1 at 7:41
  • @spaceman This is a diesel we're talking about, no? There is no fuel mixture and no fuel intake system. Your clogged pipes were air not fuel pipes, probably the runner following the EGR valve. You have a fuel direct injection system that's controlled by computer, so any fuel adjustment has to be done by scan tool. The forum search for "slow down shudder" at tdiclub will answer your questions on how and why it happens on a VW, and it's never just a sensor. Where you go from there is up to you. Volvo forum or a Volvo experienced mechanic, I'd guess.
    – MTA
    Aug 1 at 12:32
  • As fuel mixture I meant the amount of that in a combustion chamber. What do you mean there's no fuel intake system? Last time I checked, the plastic connectors of diesel return pipe from injectors had rust in them so I assumed there's water in my diesel? Furthermore I thought maybe there's similar problem with fuel pressure and temperature sensors in common rail - maybe they're filled with such particles? If that's the case then the amount of fuel as seen as ECU is not the same as the amount in the chamber.
    – spaceman
    Aug 1 at 13:32
  • Sorry, I'm not so familiar with naming's and for fuel intake system I thought of low and high pressure fuel pump, filter, injectors, sensors... Thanks for now, I'll check the VW forum as you suggested - no help from Volvo forums (yet).
    – spaceman
    Aug 1 at 13:33

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