Thick Coolant in Volvo D13 engine?

Oddly enough it can also go slushy if it was almost straight antifreeze without added water.
Also if the types of antifreeze are mixed it can cause thickening or gelling. Be sure to put in the type recommended by the manufacturer.
Generally I bought premixed then I didn’t have to worry about getting the proportions right.

 

First step is testing...
And if you're running subzero you should be setting your coolant between 60-70% for maximum protection in these -20 to -40 temps out west

This is true!

Do not exceed 70% coolant ...things start going backwards...


Do not mix green coolant with red ... It will destroy your cooling system.

If you suspect it's gotten mixed... It needs flushed out properly and refilled.