Hey, sorry if this was already posted somewhere.

I have a 2003 Grand Prix GT 3.8L, 136,793KMs. It's my wife's daily driver.

Anyways, this winter I noticed that the heat wasn't blowing that well and that the engine temp was getting a little higher than usual; upon checking the coolant I noticed that the reservoir tank was bone dry! I topped it up and also took the cap off for the radiator hell of it and noticed some brown sludge (I read somewhere that antifreeze can decompose and turn that way when air gets to it, which happens if the coolant level is really low. I'm sort of hanging onto that and trying to not to think of a head gasket!). I'm hoping against hope that it's not a head gasket issue, I don't smell that telltale sweet coolant smell, the exhaust isn't really white and cloudy, and I recently had the oil changed and they didn't see anything that would indicate coolant mixing with oil.

Anyways, after I added more coolant the engine temp was normal for a day then went back to getting higher than normal again, I checked and the coolant reservoir wasn't dry but it was very low (without any signs of leakage in the driveway) so I topped it up again. I then thought of adding Stop Leak just in case but when I went to add it to the rad I only got about a table spoon in and it was full. SO, did I maybe add to much coolant? Since I've topped up the coolant the last time the levels have remained the same. I had the thermostat changed thinking that maybe it was stuck closed and not letting the coolant circulate; it's not normal for the rad to be full to the top when the car has been cool and not running for an entire day I thought? That did seem to fix the luke-warm interior heat issue within the vehicle.

Just as a point of reference, the engine temp always sat between the lowest point and the first major mark (I think that's 70 and 85 degrees, I can't remember now and I can't seem to find a picture haha); I've had the car for about 5 yrs. When it gets "hotter than normal" it will climb slightly above the first major mark when idling but when I start driving it will go down a bit. I was sitting in the driveway today and noticed that when it gets around 85-90 degrees I don't hear the fans coming on, maybe a blown fuse?

Sorry for the long post but I am trying to flush out resolutions other than "replace the head gasket" or "replace the manifold gasket" haha.

If anybody has any input I would really appreciate it.