'96 Tahoe, ~130K mi. In January, it began overheating. Changed the thermostat and cleared a clog out of the tube between the overflow reservior and the radiator. Once unclogged, engine continued to overheat, and the reservior was now more full. Put it in the shop where they replaced the radiator bypass valve (I believe that's what they called it).
Today, I noticed the check guages light on and saw that the temp was pegged on the red line. After stopping and letting it cool, I added a full gallon of antifreeze (all I had with me, and the radiator was still not full). I noticed the overflow reservior had plenty of coolant in it, actually well above the " HOT" line. I drove it about 2 miles before it began overheating again. Once stopped in a safe parking lot, I started it again and revved the engine to 2k RPM. The temp went up from about 230 (about 2/3 of the guage's range) to just below red line, then dropped back to 230. I kept running it as it went down and up through this range two more times. The last time, it heated up slightly more than the previous two times so I shut it off. I checked the overflow reservior and it was even more full than prior to the 2 mile drive. At this point, it was almost up to the vent line. I also noticed the color of the coolant changed from light pink to a darker, dirtier brown color.
I'm assuming that after the original overheating, air entered the system, and that the later fluctuations in temp were caused as the new coolant was sucked into the system replacing the air. However, I'm more concerned with why coolant is flowing out of the radiator to the overflow reservior, but not the other direction.
I vaguely recall the mechanic back in January mentioning a gasket or something similar that could be leaking, allowing air into the cooling system, consequently forcing coolant out of the radiator into the overflow. I believe this was addressed back in January. Any thoughts?