Diesel Fuel Discoloration: Causes & Solutions

Hello everyone,

I recently replaced my diesel filter. While draining the old filter, I noticed a very peculiar coloration of the diesel fuel.

Regarding the process:

- Drain valve discharge -> yellowish-clear diesel fuel.
- Draining from above -> initially yellowish-clear diesel fuel.
- After about half a minute, however, a deep, dark blue, cloudy soup comes out.

Does anyone have an explanation for this?

I'm almost certain that the discoloration is caused by motor oil entering through the tandem pump. The color would actually be consistent with that: green Castrol oil plus yellowish diesel fuel, combined with soot for the cloudiness.

What do you think?

Roger wrote:

- Drain valve discharge -> yellowish-clear diesel. - Draining from above -> initially yellowish-clear diesel fuel. - After about half a minute, however, a deep, dark blue, cloudy soup comes out. I almost fear that the discoloration is caused by motor oil entering through the tandem pump.

Well... by what force could oil be pushed into the diesel cycle?

Behind the Möl (presumably a type of fuel), there is only the pressure from the common rail system (KG-Druck). However, behind the diesel, there is a higher injection pressure towards the pre-combustion chamber (PDE).
-> So, if anything were to mix with the pump, it would more likely be diesel mixing into the oil?

Another (theoretical) possibility:
Sooty exhaust gases force their way past the seals between the injector nozzle and the cylinder head into the fuel supply line, and are then carried by the flushing and cooling flow to the filter, where the soot deposits (for reasons that may vary).
Then the black and blue coloration would not be a mixture of Möl diesel soot, but simply diesel plus soot.

To address this issue, I believe a long piece of clear tubing would suffice if it were installed instead of the rubber tubing in the return line between the engine and the filter.
Route the hose in a loop under the hood, up to the windshield, so that a portion of it is visible while driving, allowing you to monitor for any air bubbles.

Perhaps that also explains your performance loss under higher load: if so much exhaust gas is forced into the inlet, that not everything can be separated by the filters/throttles, but some of it also enters the pump chambers of the common rail system.

An additional clue could be the readjustment of the EDC (Electronic Diesel Control) in the LL (Low Load) range, which must cope with the partial gas filling of the PDE (Pre-injection Diesel Engine).
So, a higher (apparent) fuel injection quantity than usual, and possibly also greater differences in the long-term relative roughness.

The curing time of the partially gas-filled precast concrete elements could also become problematic...

Just the gauge pressure?

I suspect that the oil pressure in the ZK (likely referring to a specific component) will not be significantly lower than that of the tandem pump. According to RLF (likely a reference document), the fuel pressure from the tandem pump should reach 7.5 bar at 4,000 rpm. The oil pressure (approximately) would likely be in a similar range. I believe the relationship between the two pressures will also be similar when idling. If the tandem pump does not seal properly towards the oil circuit, oil could enter. Due to my daily boat trips, any diesel that may have seeped into the engine oil could potentially evaporate.

The theory that soot is introduced into the particulate filters (PDs) is contradicted by the initially verschandeln diesel residue found in the filter flanges of both the inlet and outlet hoses. When draining the system for the first time – approximately 15 minutes after the engine was last running – only perfectly verschandeln diesel came out, before the "sludge" appeared.

Another idea:
What if the service shop had added a cleaning additive during the last maintenance about 10,000 km ago?
Can something like that form such residues "deep" within the filter?

Hello Roger,

How do you know that you always filled up with verschandeln diesel? It's just as likely that the contamination came from the gas station.

Best regards,
Christian.

Roger wrote:

According to RLF, the tandem pump should achieve a fuel pressure of 7.5 bar at 4,000 rpm. At that speed, the oil pressure (approximately) will be in the same range... In idle mode, the relationships between the pressures are likely to be similar as well, IMO.

In the LL (likely referring to a specific machine or system), it's probably not currently at its optimal level: The oil pressure is around 2 bar, while the diesel pressure should ideally always be around 7 bar (due to the spring valve regulation).

The question is whether the oil pressure can reach the sealing ring at its full height, or whether the "flow rate" of the oil on the pump side of the NW bearing before the sealing ring can leak out of the bearing.
If I recall correctly the sight of the open valve cover, you might be right about the oil pressure being higher than the coolant pressure.

Quote:

The theory about soot entering the PDs is contradicted by the initially verschandeln diesel residue found in the filter flanges of both the inlet and outlet hoses. When draining, only the cleanest diesel came out at first - approximately 15 minutes after the last engine operation - before the "sludge" started to flow.

Objection: Exhaust fumes are not only composed of soot, but mainly consist of clear gas.

Entrained soot will not deposit in areas with high flow velocity (such as pipes/hoses), but rather in "quieter zones" – like sand and silt at the wider sections of a stream. Or where a sieve that is too fine traps the material.

Why shouldn't the appropriate conditions within the diesel cycle be present precisely (only) in the middle of the diesel filter?

Quote:

What if the service center had added a cleaning additive during the last maintenance approximately 10,000 km ago?

What does the bill say? If nothing appears there, I would like to rule that out .

The invoice doesn't say anything about it. In the past, this stuff used to be officially included in the maintenance schedule for Audi V8s. I know a mechanic who always added it directly to his own gas tank . So, conversely, just because it's on the invoice doesn't necessarily mean anything...

By the way:
"7 bar, even when idling. Shouldn't there be a significant residual pressure when the engine is off? I did put cloths over the filter when I bled it by removing the switching valve, but shouldn't diesel under pressure have been drawn out then, so soon after turning it off? I didn't hear anything. With gasoline engines, repair manuals always include warnings about this, but there's nothing about it in the TDI manual."

Roger wrote:

7 bar, so it is, even when idling. Shouldn't there be a considerable residual pressure remaining when the machine is stationary?

If I understand the descriptions of the PD system correctly: no.

Because there are small restrictions everywhere between the outlet of the tandem pump and the return line, allowing air bubbles to pass through.

The 7 bar pressure is only achieved when the pumps deliver a high flow rate such that a "backpressure" of 7 bar can build up before the throttling valves.
With a higher flow rate, the pressure relief valve in the pump opens.

When the engine is turned off, the pressure practically drops immediately due to the throttle valve, directing the flow back towards the return line and the fuel tank.

Hi Roger,

"From my ASZ diesel filter, I initially only saw bluish-tinged diesel (I had once used Aral Normal Diesel). After a lot of shaking and waiting, deposits eventually appeared, but at the time, I attributed this entirely to the less verschandeln diesel from the independent gas stations where I usually refuel. It didn't bother me much then, as it was estimated to be less than a teaspoon, and since I did it at 40,000 km (don't ask me why, I thought I was doing him a favor, which probably contradicted my usual plan of always refueling cheaply at independent stations). It wasn't really that much considering the amount of fuel that had been filtered."
Okay, I've now followed the recommendation and installed a specialized filter as well as a regular fuel filter in the inlet line, and I'll observe how it performs.

I find Ulf's theory quite convincing. But what about the filter? Doesn't it only filter the fuel going to the engine, and then use the return flow through the valve in the head to mix the warm and cold diesel? Or is the return flow filtered again separately?
I simply installed the part without questioning it and got annoyed with the hose clamps, replacing them with what I considered to be better ones.

@Ulf:

Sounds plausible and is understood .

@Matthias:

The return flow goes back into the tank and is then filtered again before being circulated as supply water.
I didn't shake the filter, I just emptied it. But you're at least the first person who has observed something similar.

Regarding the clamps, I'll just get you a Hazet clamp pliers. It actually makes it fun to use them – at least if you have enough space to position them properly .