May just be the cap on the expansion tank aka radiator cap.

What color is the coolant now? If it is not MB or Zerex and especially if it is green then drain it all out of there, replace the thermostat like lsmalley recommends including the seal, then refill with 1/2 distilled water and 1/2 Zerex G05 or MB.

If it is green or you know it is not MB or Zerex you may want to flush the system after draining the old stuff out. I found a couple of pounds of citric acid on ebay for $10 delivered.

Drain the coolant and fill with citric acid mixed with 2.4 gallons of water to flush (run at operating temperature about 5 minutes). Then flush with water for an hour or so and flush/back-flush heater.