2002 Pontiac Bonneville

Engine Cooling problem
2002 Pontiac Bonneville 6 cyl Front Wheel Drive Automatic 88000 miles

Hi, I am having a problem with overheating. First off, let me apologize for the long windedness of the question. I wanted to make sure to include all relevant history and information. The temperature fluctuates from normal on up to redline. I have replaced the water pump (but the old one turned out to be fine), and the thermostat. I recently flushed the whole system to make sure there were no obstructions and I did not find any. I am getting intermittent heat from the heater; about 80% of the time is is blowing cold, about 20% of the time I actually get heat. I tried venting the air from the cooling system as well. It blew steam for a bit and then chugged out some coolant, then blew steam for a bit and then some more coolant, and then it seemed to do nothing but blow hot air (very little steam). I seem to be pulling in air from somewhere but I checked all of the hose connections and they seem tight and are not leaking coolant. Both input and output hoses from the radiator are hot. The coolant reservoir is full. There is a head gasket leak when the engine overheats that is venting steam out of the engine. As far as I can tell, the seals to the cylinders are fine. There is no visible evidence of oil in the coolant or coolant in the oil. Last night, after flushing, refilling and venting the system, I drove for approximately 20 miles on the highway and about 10 in the city. Everything seemed fine but then it started with the overheating again. I am unable to find a cooling system diagram for this vehicle. I thought that maybe the heater core was blocked from the addition of some Bars Stop Leak previously and was thinking that I could just bypass the heater core but it appears as though maybe there is a split system in place and that the engine is receiving coolant even if the heater core is blocked? Can anyone offer any ideas as to what the problem may be? Thank you in advance for any help you can offer.
May 15, 2008.

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