Diesel Filter: Dark Sludge - What It Means?

Hello.


'Today, I replaced the diesel filter on a 140hp common-rail A3 (with 80,000 km on the clock)... but the fluid that was inside didn't look very healthy.'


It looks kind of underwhelming for a diesel engine, doesn't it?

Okay, I just installed a new filter (shouldn't this also be replaced during the 60,000-mile service, right???)

Where does this come from?? Oil coming from the tandem pump? The engine oil level is slowly decreasing, and the oil doesn't smell excessively bad.


Can this be tested? I think there's a method that involves applying compressed air and checking for pressure loss.


Best regards,
Marc


I'm sorry to bother you again with something silly, but the forums where you can find helpful answers are few and far between...

edit:

The diesel fuel in the car is about 1.5 to 2 months old. Could it have undergone a permanent change if it was exposed to very cold temperatures for a while? (We had temperatures as low as -12°C about 3 weeks ago).

Hi,

What exactly do *you* dislike about the diesel? Since you're not providing specific details about what makes you hesitant, it's difficult to comment on it.

I notice two things:
1.) The diesel appears slightly cloudy. What was the temperature of the diesel when you took the sample, and how long had it been at that temperature? I suspect these are paraffin crystals. It's not necessarily a problem if the diesel temperature reaches the cloud point, it just doesn't look very appealing.

2.) It appears that your diesel filter is full of black particles. In the second picture, you can see a small accumulation of black particles on the bottom of the red container, to the right of the filter.
What exactly it is cannot be determined by simply looking at it; it needs to be analyzed. However, I bet these are degradation products of biodiesel. But there's nothing you can do about it.

hi

Well, what I don't like is that the diesel is so murky/dark... When you hold the cola bottle up to normal daylight, you can't see through it.


I was also thinking about the paraffin issue... the car is always parked outside. Does that mean the diesel has been below freezing for the past two weeks? However, I drove it 50 km before the filter change... I think the sensor reported something around 10°C after the change. Will it completely dissolve again?

What actually surprises me more is the black coating on the filter and in the pot... It felt kind of 'slimy.'


Is it possible for the tandem pump to push oil into the diesel?

Hi,

What happens if you store the diesel in the Coke bottle in a living room with a temperature above 20°C for a day? Will it become completely clear again?
If applicable: Normal paraffin crystals.
If the eye turns blue: Make it clear to the wife that this experiment will help secure her mobility .
The question is when exactly you refueled: Summer diesel becomes cloudy at slightly above 0°C, while a transitional product becomes cloudy around -10°C.
I actually don't find your diesel to be that bad. You could try putting it in the freezer: if it's summer diesel, you'll get a really nice, white paraffin precipitate at the bottom of the container.

The black particles in the red container: Are these individual, irregularly shaped particles with a diameter of a few millimeters that float freely when shaken, or is it a completely connected mass?
I suspect this is just normal biodiesel buildup. I would only worry about the tandem pump if your oil level drops significantly. Can the tandem pump actually pump oil into the diesel? I thought it only goes the other way: diesel into the oil, due to the pressure differences.

It doesn't necessarily have to be from the diesel filter that's in the red bowl; it's possible that it was already in there.


I've had the problem with summer diesel in the tractor twice during the winter... but in those cases, it was so extreme that the entire contents of the tank were like slightly warmed candle wax.


As I mentioned before, the diesel has been in the tank for 1-1.5 months. Depending on how the gas station handles things, it could still be summer diesel... or at least not a proper winter diesel. However, in PD engines, the diesel is heated up quite quickly, so it should have already reached normal temperature ranges by that day (about 1 hour before).


I know about the higher diesel pressure... however, there's significantly less pressure on the 'suction side'... I've also heard something similar from a VW mechanic... If it's really bad, you can probably clean the entire fuel system... VW has an instruction to bleed used pumps with compressed air after installation because otherwise diesel can get into the oil... I'll test that... if everything is okay then, I'll leave it like that for now... I'll open the filter again in the summer and check it... I don't want to risk wasting 250€ on a guess.


'Thank you.'

Hi,

To be absolutely clear: Warm up the cloudy diesel: Let it sit for a few hours at around 20°C. If the cloudiness disappears, it's definitely not engine oil, but just paraffin crystals.

Regards,
Sure, here is the translation of the text from German to English:

"Micha"

Let the bottle sit for 1-2 months. You might be surprised.

I've also seen decomposition products in diesel fuel that looked like a slimy, green substance – it was more like algae. The black particles in the fuel in my old car came from a disintegrating diesel filter housing. The part, made of black plastic, was completely faded on the inside and therefore almost light gray. You could peel off flakes from the plastic surface with your fingernail. The plastic itself was so brittle that pieces of it also broke off and floated around inside the housing. After emptying the housing into a jar, the fuel might have looked like what was described in the original post.

I was told that this was caused by the legally mandated biodiesel content in the diesel fuel. There was a modified diesel filter housing for vehicles that had been converted to run on biodiesel, and these conversions were carried out in the late 1990s. This modified part was available as an OEM part, but it was never installed on the assembly line.

hiho


I tinkered with something today... (as a change of pace, for once... I'll never get to work on anything with an oil engine again...)

According to Volkswagen's instructions, the tandem pump is okay, and the diesel that was recently added now looks normal... I suspect it was some kind of 'under-cooled' summer diesel.

The PDEs are oil-lubricated. Therefore, there is some oil contamination in the Kst (presumably a component). I would consider this normal, given the relatively low concentration and the fact that the component already has some mileage. Best regards.

hi

Seriously??? What exactly is being lubricated with pressurized oil in that? The pump piston has to seal against 2000 bar of diesel pressure, so I doubt any 'extra' oil is getting in there, and what else would there be to lubricate on that thing?


With a pressure range of 4-12 bar, the oil should theoretically have already mixed with the diesel fuel, or am I mistaken?



^^ I don't want to make anyone feel stupid, but I can't think of any way that could happen with the PIDs... at least not in the way you're suggesting (oil mixing with diesel).


Thank you.

Have you considered the suction lift? That's where the vacuum is likely located. It's possible that some oil could be drawn in through the plunger movement, especially in slightly older systems. However, the concentration is usually not a cause for concern. So, go ahead!

Hi,

Hutfahrer wrote:

I have also seen decomposition products in diesel fuel as a slimy, green substance - it was more like algae.

Decomposition products of diesel are not slimy. What is typically slimy are certain bacteria, fungi, and algae that tend to settle on the separation layers between water and diesel in filters/tanks, etc. There, they have everything they need: water and hydrocarbons.

Regards,
Sure, here is the translation of the text from German to English:

"Micha"