new diesel filters

Hey fellow mechanics,

I recently looked at a VW Touran and noticed that new diesel filters are being used, which no longer have that strange connector system for the return line. Now, all four hoses are connected to corresponding connection pipes. It's clear they've had enough of the problem with air being sucked in.

It might be worth considering switching to the new type of filter when you do your next filter change. In my ALH engine, there's usually some air in the hose leading to the pump.

Another question in this context is: at what level of vacuum does diesel fuel start to vaporize, even at ambient temperature? In a vacuum, water boils at just a few degrees Celsius. If the pump creates a significant vacuum, shouldn't bubbles eventually form in the diesel fuel as well, even if the system is sealed? Could the issue with the bubbles be related to this vaporization?

Regards,

Eike.

'...it seems so new that no one can think of anything.'

Hi Eike,

Before you switch to the other filter, I would recommend checking whether the new filter has an integrated bypass for the return flow or not.
The plastic part that you insert into the filter is a bypass for the diesel return line. I'll have to look it up again to remember exactly how it works (senior moment).

This is a thermostat that, when the temperature is too low, redirects diesel from the return line back into the filter. Because the diesel coming from the pump is significantly warmer than the diesel coming from the tank, this prevents the filter from clogging in winter (there are still people who store their fuel for years).

I believe the threshold is around 35 degrees.


By the way: The diesel fuel/pump mainly gets warm due to the compression of the diesel. That's why there's a return cooler in PD engines and in automatic ALH engines (with a larger pump piston).

CU Gremlin.