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The title says it all. One day, out the blue, I noticed that the heater wasn't blowing hot air, even though the temp gauge was higher than usual. (But not in the red.) All day, the gauge see-saws back and forth, often very quickly, between 150 and 200 degrees, with heat only occasionally. Today I took off the radiator cap to see if there was enough coolant in there, and as soon as it was off, coolant begins to gush in through the hose from the reservoir. Is it supposed to do that? I haven't worked on cars since my 20s, when they had carburetors and drive shafts and distributors and things like that. These new motors are too darn complicated! Any ideas? :dunno:

P.S.: I may have overfilled the reservoir. It says "Fill to the 'Cold' line, but I can never see any line!
 

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Is it a 3.4 or 3.8? I had a similar problem with mine. The gauge would just seesaw back and forth. The system should be contained with no loss of coolant. the overflow is there to allow for the fluid to expand and contract with temperature. First thing is to fill it and monitor the level. The heat is produced by the coolant which goes into a heat core and warms some air; if there is no coolant in that part of the car, then you will have no heat.
 

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Discussion Starter · #3 ·
Is it a 3.4 or 3.8? I had a similar problem with mine. The gauge would just seesaw back and forth. The system should be contained with no loss of coolant. the overflow is there to allow for the fluid to expand and contract with temperature. First thing is to fill it and monitor the level. The heat is produced by the coolant which goes into a heat core and warms some air; if there is no coolant in that part of the car, then you will have no heat.
Yes, it is a 3.4. And I filled the reservoir a week ago, and the level seems stable--until, as I said, I opened the radiator cap. Then it came rushing in and filled the rad to overflowing! I have no idea what the problem is....
 

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Have you flushed it recently? Mine had a bad head gasket and I guess that crap GM put in to stop leak (or previous owner) held it together for a while. Anyway I didn't have any white smoke at all just that seesaw in the temp gauge. After a while I think the stop leak stuff failed and I started getting flow back to the Heating Core (it has 2 steel pipes going to it and it is located in the firewall) then I would hear a sound of water rushing (making it to the heating core) after a while the engine started to stumble and I would get an ever slight puff of white smoke.

I am not saying its your head gasket (they have chemical kits to test for that which can be purchased and many places) but the heating core may be plugged thus you have no heat.

If you have not recently does a flush, do it. I keep the dexcool in it; no problem with it on any car I've owned (problem being those clay tablets). Or use the green, either way flush it if it has not been done in a while. Check oil for coolant and check coolant for oil.
 
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On a side note when my head gasket went enough to really notice, a similar thing would happen when my radiator; it would also surge some coolant out because it not only had the pressure of the water pump (the coolant system in general about 15psi) but it also had the engine pressure added. This can cause coolant to be pushed out into the overflow.
 

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I would get a coolant flush with a machine at a quick lube or similar shop. I then would replace the thermostat and gasket. These GM's come with dexcool which can be a problem child. The dealer puts stop-leak tablets in the radiator which over time will cause a problem. Also, mixing ANYTHING with dex-cool will cause a problem and/or sludge in the system. Which would cause a cooling/heating problem. Since the engine temp is fluctuating it could also be air in the system or possibly a faulty or failing coolant temp sensor. Just things to think about.
 

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Very true. When in doubt do the maintenance first!
 
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