S80: Engine RPM fluctuating at specific speed

[urban60]

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 systems 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

[passenger]

same problem, I gave up

Cleaned everything: intake manifold egr throttle etc etc. Once engine stalled, rps was dropping so low.

I ordered now some special diesel additive. One member of this forum has resolve this problem with that additive and I want to try. But until september I will not have it.

[monkeh]

Leakback from injectors? Does the problem happen when a/c is on? I remember you saying you had this problem, I have an engine hunt too, Only when a/c is off.

[passenger]

I sended all 5 for testing. Like new one.

Didn't notice about ac off/on. I will oay attention next 1-2 days. Thanks

[urban60]

Well the problem is still present if A/C is on but maybe not so evident, so I'm guessing something in the fuel mixture is off. I tried diesel additives, but that didn't resolve this fluctuation. It did resolve rough idle though.

[urban60]

What still surprises me is that this always and only happens in that speed window. Maybe this is linked to some faulty sensor? I don't know why it would happen only then, regardless of how long I'm in neutral, regardless of the temperature, regardless of anything in the end.

[passenger]

Originally Posted by passenger

same problem, I gave up

Cleaned everything: intake manifold egr throttle etc etc. Once engine stalled, rps was dropping so low.

I ordered now some special diesel additive. One member of this forum has resolve this problem with that additive and I want to try. But until september I will not have it.

Oh that's you man 😁😁😁

[FreshAir]

It may be useful to compare fuel pressure at idle between cars.

Just a thought but maybe it is dropping too low so making the combustion process less reliable (more cycle by cycle variation so more difficult to control).

Thinking out loud. The ECU is trying to control the engine revs.

It has rpm input which is likely to be good as without it nothing works.

The only control it has is injector pressure and duration??

So injector cleaner is good idea (you already did this) to get best spray pattern.

It is only an issue at lowest injector pressure/duration (controlling to lowest rpm level).

I don't know enough about the control system to suggest much more than this.

[passenger]

I was thinking to risk with 80€ and buy sensor fuel pressure, with is on common rail pipe.

But why you dont go in Maribor and let that guy check.l your are? It's luxus to have such a mech near you

[passenger]

This guy replaced it just in case

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