Mud-colored, oily, or rusty coolant means you're past a simple drain and refill. A contaminated system carries restriction — oil film, rust, and scale all reduce heat transfer even after the underlying leak is fixed.
How to clean it right
Diagnose the source first, drain completely including the overflow tank, then run a dedicated cooling system cleaner with the heater on full hot so it reaches the heater core. Repeat until drain water runs clear.
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