Sounds like you have either a bad thermostat or a blown head gasket. Maybe a failed water pump. You can try installing a new thermostat. What does the hose running to it feel like? Hot? Does it feel like coolant is flowing through it? How does your oil look? Can you pull the spark plugs and look at ...

If that the case then I might have a blown head gasket. The top of the pistons are not uniformly dirty. Some area is cleaner than others. I took a few photos of the top of the piston but don't know how to attached to the post. Any instruction on how to replace the head gasket or a diagram of the &q...

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I believe it's better to drain the coolant indeed. If the problem is no (or not enough) cabin heat, try first the Prestone Flush treatment https://www.youtube.com/watch?v=69ymGTqHxRw Don't forget to test the thermostat as well www.youtube.com/watch?v=gQxQnuU22Ow If you decide to go with a new heater...