1990 240sx se Coolant Problem

[rtexas]

My car keeps boiling excessive coolant into the overflow reservoir. The car doesn't seem to overheat. The gauge rises to normal operating temp, just below the halfway mark and stays steady which indicates the thermostat is operating??. Circulation seems good in the radiator. I have flushed the radiator as well and bled the system as instructed in another post I found on this site. The car has a clutch fan and an electric fan that is wired to a manual switch under the dash. Evidently the previous owner had either added the electric fan or did some creative wiring. Never the less, the car runs at normal temp. When I turn on the electric fan, it raises the idle considerably. When I kill the engine after driving for a short while (15 - 20 minutes), the coolant fills up the reservoir and over-flows. I know there should be some coolant go into the reservoir, but not this much. After driving a couple of times, I have to add coolant. My first thought was it was a bad radiator cap but I have replaced it twice with no change. I read some post similar to this but with no definitive resolution. I have purchased a new thermostat but with the car not actually showing that it is overheating, I don't really think this is the issue. Any ideas?

[elwesso]

I would definitely replace the Tstat.. If its not opening al the way it will build up pressure fast.....

Just because the temp gauge is in the middle means very little!! theres basically 3 stages on the temp gauge- cold, warm, and meltdown.

[rtexas]

Well I've change the thermostat and now I have another problem. When I turn the car over, it blows coolant out of the radiator. It builds up pressure in the system as soon as the engine turns over.

[NISTECH]

You need to check for a blown head gasket or cracked head/block. There is a test kit you can get at most good autopart stores called a block tester. This tester checks for HC's in the cooling system. This really sounds like your getting combustion pressure in the cooling system.

[rtexas]

I was afraid you'd say that.