
Originally Posted by
Fivefingerdeathpunch
Backflush the heatercore and that's about everything covered.
The brown sludge was the dexcool coolant. Likely you never serviced it and it came in contact with lots of oxygen/air at some point and cause the reaction to take place. Dexcool and air in the cooling system do not like each other and thus the sludge.
I would change over to universal green/yellow coolant when you do this work.