Red gunk in coolant reservoir

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)?

A few thoughts...

The only red fluids I know of in the Windstar are the power steering and transmission fluids.

Of course, transmission fluid is the obvious suspect: There are transmission cooling passages in the radiator. Given how long this problem has been around, and that you didn't mention major problems elsewhere, if there's a leak - it must be a small one.

However, the transmission fluid in the cooler is at a fairly low pressure - I suspect that normal engine cooling system pressure would be higher than the transmission fluid pressure in its cooling lines. This brings up another side of this theory: Do you see signs of engine coolant in your transmission fluid? Any hint of rust on the transmission dip stick would be the first clue.

Since oil floats on water, I'm not surprised that you see little sign of oil in the engine coolant - except in the reservoir, which is the highest point in the system.

This is just a theory; others may have better theories!

Its hard to have a clear coolant bottle on a high mileage car. I have had some luck removing the reservoir, filling with a mixture of sand and water, shaking it around to get rid of some of that wall scum. Then an energetic flush with the garden hose.

Red? It may be rust. If the coolant is ever allowed to get too low in anti-freeze, rust can take over. I have heard of water pump impellers just rusting away. Acid is the problem. It helps to add a tablespoon or two of baking soda ever so often. Raises the Ph.

Thanks for the ideas, seems like it must be rust or transmission fluid. Just strange, because with either of those I'd expect some visible evidence of contamination in the rest of the system.

When time permits, I'll get a pressure test kit, disconnect the transmission lines, and see if coolant comes out of the trans cooler part of the radiator.

Just another idea. About 2 years ago I have replaced the water pump and have used the gasket included with the pump. Unfortunatly and to be sure that I have a good seal for my pump replacement job I have added RTV silicone (hight temp silicone orange color). About a year after the job done I have started to find in the tank this kind of orange gunk. It was a mistake to add this sealer to the gasket and I have to re-torque the water pump because it was leaking, but the exess of silicone went into the cooling system and I had to remove that many time from the tank.
Did you do a job like the water pump or like you said the manifold where you have used
this H.Temp silicone as a gasket?
Just in case....