You've probably blown the head gasket, but start with checking for diagnostic codes:
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Have you checked your oil for contamination? Start by looking under the oil cap for something like chocolate pudding or beads of water. You can pull the spark plugs to read the plugs and look if any of the pistons look a lot cleaner than the others. Finally, a compression test is easy to do and you can usually borrow the tool from a local parts store.