Coolant looking very black

Coolant reservoir leaking

Had the check engine light come on a few times, the code is something to do with the coolant system. The temperature gauge doesn't like to register. Replaced the temp sensor, flushed the system, and c… 

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2013 Ford focus second time to change the Pistons what's wrong

My engine light came on so I took it in. Now my coolant temp goes up and down when I'm driving. What could be wrong