OPEL VECTRA 1.9 CDTI 120HP 2005, 385.000KM

Hello forum members,

I'm experiencing a rather peculiar issue with my vehicle's cooling system, and I'm hoping to gather some insights and suggestions from this community. Here are the details of the problem:

I've noticed the presence of a strange, volatile foam in my expansion tank. This foam doesn't have a pasty consistency; instead, it becomes much more visible when the coolant temperature is below 85 degrees Celsius. What's even more puzzling is that as the temperature continues to rise, this foam gradually disappears along with some small bubbles, and sometimes the coolant changes the colour pink to purple, and after some minutes turn back pink.

To investigate further, I tried idling the car with the heater on at maximum to lower the temperature. I observed that when the temperature is below 85 degrees Celsius while idling, this foam and some small bubbles form. However, in the same idling condition but without the heater on, at around 90 degrees Celsius of coolant temperature, there's no bubbling at all, and the foam diminishes significantly.

I also conducted tests by revving the engine in neutral. When the engine is cold (below 85 degrees Celsius), a significant amount of foam appears in the expansion tank. Yet, at around 90 degrees Celsius, whether I accelerate or not, there is either no foam or very little, depending on the situation.

One crucial point to note is that there is hardly any pressure in the coolant system. I can open the expansion tank cap even with the radiator fan running, and there's minimal risk of burning myself due to the low pressure in the system.

I want to emphasize that there is no overheating issue whatsoever.

This is indeed a perplexing case, and I'm reaching out to this community in the hope that someone might have some ideas or insights into what could be causing this unusual foaming phenomenon. Your input would be greatly appreciated.

Thank you in advance!

1. [Note: Here are one link to images and videos illustrating the issue for better understanding:]

https://drive.google.com/file/d/1WRUszqeuQZiqMXTL2xWyOsKZgUvE-nny/view?usp=drivesdk