Does anyone know what causes red gunk to build up in the coolant reservoir / overflow tank?
About a year ago I noticed that the coolant in the reservoir looked bad - dark reddish brown - and there was some kind of gunk stuck to the inside walls of the tank.
I took it to two different mechanics, and both said that it's "normal" for the tank to look like that on the Windstar (2002, 3.8L). One of them drained some coolant out of the radiator and it looked perfect, and that there was no problem. Around the same time I replaced the lower intake manifold gaskets, and the coolant in the block did look normal (green).
The condition of the tank bothered me, so I replaced it with an OEM one about nine months ago, and had the cooling system flushed to make sure there was no gunk in it.
Now there is some gunk appearing in the new tank, and some red specks floating in it.
Does anyone know what can cause this?
If oil or transmission fluid was getting in the coolant, wouldn't the entire system show signs of contamination (not just the reservoir)?