name that sludge :)
 

So today I pulled my old thermostat out as part 3 of my ongoing cooling system saga. As some of you may know, I couldn't get the coolant to circulate and couldn't bleed the system. Additionally the cooling fan is not coming on. When I pulled the hoses I noticed that there was a bit of thick sludge in the system. Not a ton but a decent amount.

The sludge doesn't seem to cause an oil to rise to to top of the expansion tank, and doesn't give the coolant an oily look. There is no sign of water in the oil, the plugs look perfect and there is no white smoke from the exhaust

So, I'm wondering if it can be from someone topping off the coolant with the normal green stuff or maybe something else? I know the head gasket is a possibility but it seems to run well and I would think if it does have a leak its very minor. I'm trying to decide if I should just bolt everything up and flush it, or pull the head while I can afford the downtime. If I don't have to I'd prefer to wait until spring.

Any thoughts on the looks of this? Sign of head gasket? Or maybe two incompatible coolants?

Never seen that before..

also http://www.minitorque.com/forum/f26/...-radiator-957/

Why you topping it off with green coolant?!
Not the right stuff....
i would flush it...refil it with the coolant of choice, preferably mini/bmw ok'ed stuff, and see if you problems go away....
Always dilute coolant with distilled/de-mileralized water if it does not come premixed.
As to the reason gor the sludge....
Who knows...chemical reaction...
Cause or symptom, who knows...
A basic oil test can tell you if coolant is contaminating the oil....

The green coolant could have been added by the previous owner or a shop I am thinking. The coolant was more of a greenish yellow color and the date stamp on the thermostat was 8/07 and the build date of the car was 4/07 which leads me to believe that it must have been changed at the dealer, just don't see how they would use the wrong stuff. Perhaps it was tap water they mixed in that could cause it?