cincip ... 11/14/2012
1993 Honda Civic DX 4 Cyl 1.5L
Coolant loss apparently through reservoir cap.
While driving coolant system builds up pressure and leaks out of cap of overflow reservoir. The overflow reservoir and cap were replaced with no change. Could the thermostat be restricting the flow causing the fluid to boil? If the head gasket is leaking cylinder pressure would it show up during a compression test? Any other ideas what could be causing the increase in pressure?
You have air trapped in behind thermostat. Check upper radiator hose to engine block gooseneck connector to see if there is an air bleed off bolt or screw.