Hi there,
You should not mix coolants as this can cause problems in the longer term, you should first address the coolant loss issue, have a pressure test done as this will help locate any external leaks, when this is done and you are happy that the coolant loss is repaired, drain & flush all the coolant including the heater core and re fill with the recommended mix, the very short term mixed coolant wont be an issue just don't leave it to long.
Mark (mhpautos)
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Saturday, October 25th, 2008 AT 8:50 PM