GMC Repair: 95 Chev Cavalier Z24 2.3L, coolant level, radiator cap


Question
Problem:  My 1995 Cavalier Z24 began to overheat and I only noticed this when I turned the heat on.  It blew heat, but the engine temperature rose?  The low coolant light also came on and my coolant had boiled over.

What I have done:  Suspected sticky thermostat.  Replaced.  Found a leaky water outlet fitting.  Replaced along with new "o-ring".  Problem still exists.  I can drive the car at normal operating temperature, but as soon as the heat is turned on, same problem.  Suspected plugged or restricted heater core.  Flushed heater core.  Coolant still boiling.  Confirmed that water pump is still pumping coolant through the system.  
Coolant level seems to fluctuate up and down as low coolant light goes on and off.
Cooling fan does come on, but duration I cannot tell you.   
The only 3 options I can see left are: Faulty rad cap?  Water pump moving coolant, but not quick enough?  Cooling fan relay not functioning properly?
I hope that it's not a head gasket or cracked block.  If it is there are no tell tale signs of coolant in oil, oil in coolant or smoke from exhaust.  Is there anything I'm missing?  

Answer
Hi Jeff. Does this overheating happen in town or on the highway? If this problem happens while you are traveling at highway speeds then we can eliminate fan collant fan relay.The relay only kicks in when heating up occurs while you are in the city or stuck in traffic, there is sufficient air flow thru the radiator at highway speeds and the fan usually doesn't come on. I can't imagine why this would only happen with the heater turned on, there is no connection for this to happen, if anything, the opposite should be true, it should help cool down the vehicle. The radiator cap should hold preasure, but again, if you are not loosing any coolant and you have a bad cap, it should not affect the heating up to the extreme you are experiancing. I could give you other possibilities, but none that i can think of right off hand that would be affected by turning on the heater. My instinct in this case would be a head gasket,you don't have to see coolant in the oil for a bad head gasket, and you don't have to see it in the exhaust if it is leaking preasure into the cooling system. I have seen many head gasket problems with this motor, also cracked and warped heads, check closer into theses and i would quess you'll find your problem. MANFRED