Mechanics

LOSING COOLANT THROUGH OVERFLOW TANK

GMC Jimmy

My V6 Jimmy, 1999, is losing coolant through the overflow tanks overflow tube. After a short drive, the coolant backs up and overflows. I have a new 16 lb radiator cap. I've noticed the coolant is very dark, almost black. It loses approx. 1/2 gallon each time. No other leaks. The oil is clean and clear. What could be causing this?
Thanks,

Jim
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Jims440
May 8, 2007.



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