White powdery residue around the thermostat housing

[TA301]

This has me baffled. Never seen anything like it.

[5th T/A]

On my iPad the photo is not very clear, making it harder to determine what I am looking at.

A couple questions, what does the coolant look like when you remove the radiator cap. Is the level low? I wouldn’t be surprised if you have a coolant leak and old coolant causing corrosion. Old coolant can turn acidic. You can borrow a coolant pressure tester from most auto parts stores to help isolate the leak source, for free or a minimal rental fee. There is a test you can perform on the coolant determine if it’s bad, requiring a DVM and taking voltage readings of the coolant in the radiator. Lots of examples online, here is one >

https://www.familyhandyman.com/project/coolant-testing-with-a-multimeter?_cmp=stf

https://www.youtube.com/watch?v=sxQ2zHHZfoI

You can also use PH strips to test acidity, available at most auto parts stores.

[apfstg]

That’s hard water in my opinion
A radiator shop owner told me once to use distilled water to mix with antifreeze instead of tap water and I’ve always done that since

[roadking77]

To me water is water When I filled my radiator for the first time I read and was told to use distilled water, so thats what I did. I think apfstg is on to something and also I would imagine something is leaking. I would second 5th ta recommendation of using a pressure test. I just got one from Auto Zone. It was free but required a $200 deposit.

[tinpusher]

I think it’s a combination of several things. As mentioned before, distilled water should be use to keep the mineral content removed from the coolant system. Dissimilar metal’s, aluminum thermostat housing, cast iron manifold and a metal thermostat sandwiched between the two. The gasket is probably saturated causing moisture to combine with metals and oxygen causing that corrosion.  I have seen it on my car , especially when it’s not run for several months….. I don’t use distilled water either.
I’m thinking that the engine temperature helps keep the gasket from becoming saturated and if not run for a while, it gets saturated and wicks…. Just my opinion, not necessarily a “leak”

[langss]

I started using Distilled Water years ago on the advice of the radiator shop owner that simply said the water here in California is HARD. That's not exactly what he said, but this is a family friendly forum. So that's what I use. I just replaced the Radiator in my 92 Camaro and at less than $2.00 a gallon, it was the cheapest thing I put in that car. The tap water here is heavy with Calcium and Lime. It shows up on the shower head and the toilet. I don't even let the dog drink it. He gets bottled water from Costco