GM-GMC: Floating Temp. Gage 1996 4.3L Blazer 4wd, coolant leaks, coolant level

Question
My Blazer temp. cage floats and air corresponds. Hot when temp. down and cold air when overheating. I think a cracked the blocked today as a loud thud was heard under hood. I pour in antifreeze and its going somewhere, but doesn't appear to be gushing out from below. Haven't checked oil. what was the problem and did I crack block you think.

Answer
It sounds to me like you have a coolant leak somewhere.  When the coolant level is low it bypasses the heater core, which results in poor or no heat and will cause your thermostat to open and close causing gauge fluctuations.  As for the noise you heard,  it is unlikely that your block cracked.  It would definately be leaking when you added coolant to your system.  Top up your coolant, check your oil, make sure it isn't contaminated with coolant, run you vehicle to operating temperature (with the radiator cap on) and have a good look around for coolant leaks.

hope this helps