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I have all this brown/orangish mineral crud in my coolant expansion tank. I've flaked some of the more substantial deposits off as you can see in the last image below. Is it something I should be concerned about? I'm thinking about removing the tank and dissolving it in 6% vinegar for a bit and blast it off with a waterpik. If some of this crud flaked off and wound up down in the radiator fins could it cause problems? If crud like this is visible in the reservoir is it assumed to be elsewhere in the coolant system?

Expansion tank coolant float with visible mineral crud, slightly submerged. Expansion tank coolant float with visible mineral crud, above waterline. Expansion tank coolant float with visible mineral crud, above waterline different angle. Crud lifted out of coolant expansion tank with a coat hanger.

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IMO, if the engine is cooling as it should, I would not be concerned with it.

What you are seeing is fairly typical in cooling systems as they age. The buildup is either deposits from the water used to mix the coolant (that is why distilled water is recommended) or just the coolant mixture has attracted some contaminants over time.

It is possible there may be some buildup in the rest of the system. If you are concerned, you can perform a chemical flush of the cooling system to break down and remove deposits.

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I did some work on another system on my vehicle, replacing the Volvo Crankcase Breather Pipe Assembly - Genuine Volvo 30731068 as part of a PCV system repair. With the old part removed I started hacking away at the hoses, one of which is a coolant hose that runs trough the assembly and inserts where the thermostat housing is just to the left of the intake manifold.

PCV Coolant hose section attached to thermostat housing

Here's a small section of that hosing spliced open and spread apart to reveal the interior surface of the hose. That same brown orange crud I saw in my coolant reservoir was seen there as well. Coolant hose cut open to reveal crud on the inner surface

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