I'd do this for kicks. Drain coolant and replace the thermostat. Use 50/50 M-B or G-05 Coolant on the refill then drive the car with the pressure cap loose and see what happens. If you can get away without running it at pressure it will probably stop leaking. You might even try a lower pressure cap or replacing the cap too and see if it might be running at too high a pressure. I'm not saying this is a fix, but it might just save you from having to face doing a head gasket in the near future.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz