Get a read on the plugs. If you are burning oil in any substantial way, it will show. If coolant is leaking into one or more of the cylinders, that or those plugs will be cleaner than the others. Don't want to be pessimistic, but in the absence of any external leakage sounds like a head gasket. Try this..Start it up and let it idle, remove the cap from the expansion tank and when it's warmed up, punch the gas a few times. If you see bubbling or marked over flow, then you've got a leaking head gasket. Good luck..

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Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.