I took my 99 B3000 pickup in for an oil change and the guys there didn't recommend a cooling system flush though the coolant in the reservoir was the color of the Mississippi. I had them do it anyway. I had this flushed just 7K miles ago. But how could the coolant get so dirty so fast? I don't know alot about engines, but I do know the cooling system is supposed to be a closed system. So if there were an open where dust and dirt can get in the system, then there would have been leakage, and or a low fluid level, but when the shop flushed the coolant, they said and even showed me that the amount of fluid they flushed out of the system was normal. The guy even said that it didn't look like there was anything wrong with the head gasket or any other seals.

So my question is, How could coolant get so dirty without a breach in the sytem AND with normal fluid levels? Am I looking at a serious problem with my cooling system that might mean alot of repairs are in my future?

 

For molecules, there's no such thing as sealed. You can wiki Dalton's Law of Partial Pressures for details. When your coolant is circulating, oxygen from the atmosphere gets into the system and rusts the iron on the walls of the cooling passages of your engine until the amount of iron oxide in your coolant is in equilibrium with the iron oxide (rust) on the walls of the coolant galleries, then the activity stays balanced, but the coolant looks muddy; perfectly normal. The reason for flushing is to clean sediment in the rad. The reason for replacing the coolant is that there is more than just iron in your system, the rad metal alloy is different, the metal gasket material is different etc. etc. so the engine manufacturers specify an antifreeze which contains additives best suited to the engine such that electrochemical corrosion induced by the presence of different metals in a common cooling liquid can't cause metal to migrate (mainly from your rad core) causing perforation leaks. The inhibitor effectiveness deteriorates with time though, so at intervals of around two years, the system should be drained, flushed, and refilled with coolant per the manufacturer's specified type and concentration.
The new coolant will very quickly re-establish the iron oxide equilibrium and again look just like Mississippi mud. Your mech. was right, if your engine is OK and you're not blowing compression into the rad, losing coolant, overheating etc. etc. then don't worry, it's absolutely normal.