I recently had the timing belt and water pump changed on my Volvo s40 (2006 2.0D diesel). The mechanic also cleaned and adjusted the injectors. After the service I noticed that att some rpms the engine started to "self-oscillate" at a low frequency, maybe 2-3 Hz, enough to get the whole car shaking. It appeared mainly in gears 3 - 5 and between 1800 - 2200 rpms. I have tried to pinpoint the rpms where this occurs while running in neutral, but it seems to mostly appear when the engine is under load.
The second effect was that the fuel consumption started increasing. For years it has been around 7.4 liters per 100 km, and now it is at 9.3, so a 25% increase.
I went back to the mechanic and he looked through the timing belt and water pump change, but could not find anything wrong. After some more investigation he found that one of the glow plugs needed changing, so he did that. This had no effect on the main problem of the "self-oscillating" phenomenon and the increased fuel consumption.
I am no mechanic (obviously), and I am wondering what could be the cause of this problem. It is sort of subtle since the car has to be driven for it to appear, which makes it harder to explain to the mechanic without sounding like I am imagining things. Could it be an error in the injector adjustment? When the engine is under load I guess more fuel is needed for a certain rpm so maybe that could explain why it is hard to reproduce in neutral?
I am not sure what to say to the mechanic to get this problem fixed. Any insight would be very helpful.