Today on my way to work temp gauge maxed out. I have been noticing low coolant light on and I add distilled water. This time the light wasn't on. Heater was blowing cool air. Stopped to add water at service station. Lots of pressure in overflow tank. As I was adding water I could hear it boiling inside which it wasn't doing before adding water. When I thought it was full, I turned on car without radiator cap. Water started flowing out of the overflow rapidly. I turned car off and added more water capped reservoir and continued the rest of the 2mi to work. As I was driving I could hear what sounded like boiling water coming from the passenger side fender. No steam though. When I got to work I felt the radiator hoses. Top radiator hose was very hot and hard with pressure. Hose at top of radiator to overflow was hot. Bottom hose on reservoir was cool. Reservoir itself felt lukewarm.

When I was adding water it looked like there may have been a small amount of oil but since I just bought this car recently I don't know when the last flush was. Service documentation States about 2 years ago.

I have noticed on occasion around the lower radiator hose On the passenger side on the ground there will be a small amount of coolant. I have seen a drop on the back side of the alternator too. Very small amount less than an ounce.

What could cause boiling in the reservoir? Thermostat?

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1987 300 SDL
2000 E320 Wagon