X5 30d E53 Random Misfire when cruising
 

My E53 X5 has developed random misfire (like an engine hiccup). It most commonly occurs when cruising at at speeds between 100-120km per hour and seems more likely to occur when the road is slightly uphill and engine under slight load on cruise control. It will misfire several times over a few kms, if you slowdown everything goes back to normal. Sometimes it also occurs when you start out your journey usually at lower speeds, the motor will hiccup half a dozen times then it returns to normal. Sometimes I drive the car and it doesn't happen at all. I think the problem may have only started after the crankshaft sensor was replaced? The car sat in the garage for some time after this occurred so I don't know if it is really is related. It's not throwing any fault codes and is really just annoying while its doing it. On a 100km journey it may only do it 2 or 3 times (have a patch of 4 - 6 misfires all within a couple of minutes). Anyone else every had this issue ?

Hard define what a misfire is on a diesel.... do you feel a stumble, where the power drops off momentrarily?

What age, M57 or M57N?

What were the symptoms why the crank sensor was replaced? Was the new part original BMW?

Yes it feels like a stubble as if the power drops just for a second then kicks back in.
The sensor was replaced when the starter motor was also replaced, it was due to the
car not starting without extra boost, not sure if it had anything to do with that problem or if it was just replaced because the starter was being removed. The replacement part was an German after market (Gebe Brand)

...also its a 2005 M57N with 317,000 km on the clock.

Misfire on a diesel? There is no Ignition System on a diesel that would give a misfire, therefore the issue has to be fuel delivery, or the lack thereof. Intermittent low fuel pressure would be from a failing fuel pump or fuel pressure regulator.

On a gasoline engine, a misfire diagnosis starts with the ignition system -- plugs, coils, those kinds of things -- then moves to fuel and then to internal engine issues -- poor compression. Your diesel does not have ignition components, the entire combustion process comes from fuel and compression.

I don't think, but am not sure, that the diesel engine has a code for misfiring. In the gasoline engine, a cylinder fires on a command, then the next cylinder has to fire in a specific period of time else a misfire is declared on the previous cylinder. Let's say that the firing order is 1-5-3-4-2-6, when 5 fires then 3 must fire within a certain time else 5 did not fire well and a misfire is detected. The gasoline engine knows when the cylinders fire, and therefore when the next one should fire, but your diesel does not know when a cylinder fires so it would not know that the next one fired late. The point is, I'm not sure that the lack of codes is part of the symptom set or not.

Actually, it does (or at least the BMW M57N does) - it uses incremental rotational speed increases as measured by the various engine sensors (crank, cams etc.) to detect power output from each cylinder "firing". ;)