No heat with sand deposits in coolant tank

[Jk57]

Have 2.0 TDI dsg blue motion 2013 with 80k miles. The heating stopped working and coolant light came up on the dashboard. Took it to be looked at and there was a burning smell coming from the engine bay like melting plastic. They replaced the water pump and flushed the heater core several times. They reported small sand like deposits that were causing a block. Heating now works fine but I'm worried what caused this. Anyone come across this before or has any ideas as to the reason? Thanks

 

[Jk57]

Here's a picture of the coolant tank showing the sand around the edge

 

[Schnell_R]

The Passat diesel heater core would clog up and stop working. It was determined that the solder used in the heater cores reacted to the coolant and would cause the core to fail. VW had a TSB on the issue some years ago.

 

[Schnell_R]

I think the sand is just happenstance, probably from the road.

 

[Jk57]

I think the sand is just happenstance, probably from the road.

Thanks but how can that enter the heater core and cause a block if it’s from an external source? It was found inside so it was thought to come from something within the engine.

 

[Schnell_R]

Thanks but how can that enter the heater core and cause a block if it’s from an external source? It was found inside so it was thought to come from something within the engine.

I see that the sand is on the outside from where the cap threads down...No way the sand came from the tank if the cap was in place. That's my thinking anyway.

 

[Schnell_R]

Not sure if the blocks are sand cast at the foundry? Only other explanation I could think of..